Biliproteins
By Hugo Scheer1*1
Dedicated to Professor Hans-Herloff Inhoffen on the occasion of his 75th birthday
Biliproteins, covalently bonded complexes of proteins and bile pigments, serve as light-harvest ing pigments in photosynthesis and light-sensory pigments of photosynthetic organisms. Re cent developments in the biochemistry and biophysics of these pigments are reviewed and an attempt is made to describe their functions of light-harvesting and of information transduction on a molecular level.
1. Introduction tion, it was first enriched and characterized by absorption spectroscopy as late as 1959181. Phytochrome is a photorever- Cyanobacteria, red algae, and cryptophytes contain large sibly-photochromic pigment. The position of the equilibrium quantities of blue and red pigments, which essentially deter between its two forms (R- and FR-form), the total concentra• mine the color of these organisms and may amount to 40% of tion, and other factors are fundamental in regulating the de• the protein111. Engelmann[2] was the first to relate these pig velopment of plants191. Another group of photoreversibly- ments to photosynthesis; Haxo[3\ Emerson[4\ and Gantt[S\ photochromic pigments, the phycochromes, were isolated among others, recognized their function as light-harvesting from various cyanobacteria1101, and at least one phyco- pigments, mainly of photosystem II. In 1928, Lemberg™ de chrome was also related to developmental processes like monstrated that they contain bile-pigment chromophores; in chromatic adaption11-11 -12). accord with their origin and composition, these chromopro- These three groups of pigments are referred to as bilipro• teins are thus called phycobiliproteins. teins (Table 1). This contribution deals with recent develop• Pigments of this structure, but having completely different ments in their biochemistry and biophysics. Since the last functions, exist in many other organisms. The most impor comprehensive survey in this field1 l3al several different as• tant compound of this group is phytochrome. This sensory pects have been reviewed*1-5'9 13 25-3101. It should be men• pigment of green plants was discovered in 1945 by action- tioned that besides these genuine biliproteins, there also exist 171 spectroscopy . Owing to its instability and low concentra- bile pigment-protein aggregates lacking a covalent bond be• tween both component parts. These include the physiologi• cally important complex of bilirubin, which is only spar• ingly soluble in water, with serum albumin1261, and an in• {*] Dr. H. Scheer 13 27 30J Botanisches Institut der Universität creasing number of invertebrate pigments* . A selec• Menzinger Strasse 67, D-8000 München 19 (Germany) tion of them is included in Table 1.
Angew. Chem. int. Ed. Engl. 20, 241-261 (1981) © Verlag Chemie GmbH, 6940 Weinheim, 1981 0570-0833/81/0303-0241 $ 02.50/0 241 Table 1. Natural occurrence and functions of biliproteins. The first three groups of pigments are the subject of this report.
Pigment Occurrence Function Chromophore structure Ref.
Phycobiliproteins Cyanobacteria Antenna pigments of photosynthesis Phycocyanobilin (ία), phyco- see Text (e. g. phycocyanin (PC) Red algae erythrobilin (2) etc. allophycocyanin (APC) Cryptophytes phycoerythrin (PE» Phytochrome Higher green plants, certain al Reaction-center pigments of photomorpho- Ob), (3) [a] see Text gae, fungi (?), mosses, red algae genesis (?) Phycochromes Cyanobacteria, red algae Possibly reaction center pigments of photo- da) (?) [1, 10-12] morphogenesis and chromatic adaption ("adaptachromes")
Bilirubin- Vertebrates Water-soluble transport form of bilirubin 1261 serum-albumin complexes Aplysioviolin Aplysia (sea hare) Defense excretion Monoester of (5) [29] Turboverdin Turbo cornutus (mussel) Protective coloration (?) (18-Ethyl]-[3-(2-hydroxyethyl)j- 130] (19), R = H Pterobilin Lepidoptera Protective coloration (?) IX^-Isomer of (19), R = H [271 Phorcabilins Lepidoptera Protective coloration (?) Extended derivatives of pterobil 127] in, similar to (31)
[a] The substituents R, R' und R" in (3) suggest that the double bond between C-4 and C-5 present in Pf is no longer noticeable in the absorption spectrum (see Section 2.3).
2. Structure of Chromophores
The two major chromophores of phycobiliproteins are (la) (phycocyanobilin) and (2) (phycoerythrobilin)1*1. (In the for mulas the bond to the protein is indicated.) (ία) is the blue chromophore of phycocyanins and allophycocyanins, (2) is the red chromophore of phycoerythrins. One or the other of them exists in each of the known phycobiliproteins, while R- phycocyanin (R-PC)r*] contains both. In addition, there occur several other chromophores with hitherto unknown structures, e. g. a phycourobiiin in the -γ-chain of B-phyco- erythrin (B-PE)1**11311, the red chromophore in the α-chain of
1321 phycoerythrocyanin , the third chromophore (P59o) of PC from a Hemiselmis species™ and the blue chromophore of PC from Chroomonas[34]. Strictly speaking, the structures (la) and (2) have so far been unequivocally determined on only a few phycobilipro teins. Generally, they are identified by chromatographic comparison of the cleaved chromophores and spectroscopic investigations on denaturated biliproteins (see Section 2.3).
(3)
The chromophore (lb) of the R-form of phytochrome (Pr) [*] The nomenclature of the bile-pigments has been modified several times in re is very similar to phycocyanobilin (la){35~3*]. Instead of the cent years. Four systems were and are used concomitantly. Trivial names and the 137,381 numbering system of H. Fischer, which correlates the bile pigments with the por ethyl-group it contains a vinyl-group at C-18 , but the phyrins, are mainly used in the older literature [for example (19) = biliverdin absolute configurations at C-2, C-3 and C-3' are identical1391. IXa]. The numbering system, still used today and shown in formulas (1) and The structure of the P -chromophore is still unclear, howev (23), results from the first attempt of a rational nomenclature. It allows no direct fr correlation between the C-atoms of macrocyclic tetrapyrroles, and the linear te- er, it is known that it contains a shorter conjugation-system trapyrroles derived therefrom. Apart from using a certain number of trivial-na 351 than the Pr chromophore* but the same ß-pyrrolic substit• mes, it is based on the completely saturated "bilan". An exchange-nomenclature 1371 is possible for both systems: A new substituent replacing the original one of a pa uents . Comparative investigations on free bile pigments rent compound is placed in square brackets [viz. [(18)-vinyl]-(la) = (!b)~\. The have currently led to two models which would account for nomenclature used in this article corresponds to the Vlth Memorandum of the the properties of the P -chromophore. It could be a geome• IUPAC Nomenclature Commission. Aside from a limited number of trivial-na fr 1401 mes [e.g. (19) = biliverdin] it is based on bilin which, in natural bile pigments, con trical (Z,£)-isomer or a substitution product of the Pr- tains the maximum number of non-cumulated double bonds [cf. R. Bonnett. in chromophore[411 (see Section 2.4). [24a], Vol. 1. 1978, p. 1: example: (23) = 2,3,7,8,12,13,17,18-octaethyl-2,3-dihy- dro-l,l9[2l/f,24/fl-bilindione]. The nomenclature recently proposed by the IUPAC (Pure Appl. Chem. 51, 225! (1979)) is similar. 2.1. Cleavage of Chromophores [*·] Abbreviations: PC = phycocyanin, APC = allophycocyanin, PE = phyco erythrin, Pr, Pfr = phytochrome in the R-or FR-form(see Sections 4.3 and 5.2). Prefixes indicate the parent organisms: C = cyanobacteria, R = red alga, & = Ban- For a long time elucidation of the structures of biliprotein- giales (an order of red alga), Κ = cryptophytes. chromophores was complicated by their covalent bonds to
242 Angew. Chem Int. Ed. Engl. 20, 241-261 (1981) the proteins, which preclude a cleavage without chemical The key reaction in the synthesis is the regioselective cou modification. Depending on the conditions of cleavage, dif pling of rings A and B, which is possible in good yields by ferent phycobilins result from one and the same pigment, so condensation of the monothioimide (7) with the pyrrole ylide that the nomenclature became very complex (cf. 113·22·24*). (8); condensation of the product (9) with the CD-fragment The best characterized products are the ethylidenebilins (10) yields (4b) (Scheme 1). For thermolabile substituents*541, (4a) and (5), which are formed as main products on treat sulfur-contractionI54a) can be used instead. By the first proce ment of the corresponding biliproteins with the chromo- dure the 18-vinylbilindione (4b) (— "Pr-bilin") was ob tained1381. It could be correlated with (lb) by treatment of the latter with HBr/TFA1551. The stereochemistry of (4a) and (5) at C-2 was established by chromic acid degradation to the imide (6) with known absolute (4jR)-configurationl56}, while that of (5) at C-16 was established by asymmetric synthesis and correlation with (4/U6Ä)-urobilin(46-571.
2.2. Degradation Reactions of Biliproteins
On oxidation with chromic acid, tetrapyrroles are de• graded to cyclic imides possessing (at least in principle) the same ß-pyrrolic substituents. This method, originally intro•
i4e>.R = C2H5 (5) duced by H. Fischer, has in the meantime been improved
(4b). R = C2H3 several times and standardized, and has especially been ap• plied to biliproteins by Rüdiger et β/.*13·531. The "hydrolytic" phores (la) and (2), respectively, in refluxing methanol1421 chromic acid degradation of phycocyanin (PC) at 100°C or long chain alcohols*431, as well as by treatment with HBr/ trifluoroacetic acid (TFA)1441. The cleavage proceeds by ste PC.PE reoselective elimination of the C-3-thio ether*451; in (2) and/ or (5) the asymmetric C-16 can epimerize at the same time*4647*. The structures, suggested originally from *H- NMR and MS data148"511 and from chromic acid degrada- tion(13 52 531, have been confirmed by total syntheses*46,54*.
Η Η
(6a) (6b)
yields the three imides (6), (11) and (12), while that of phy-
37] coerythrin (PE) and Pr* affords the imides (6), (11) and (13), the latter of which readily decomposes under the reac tion conditions. On oxidation at ambient temperatures, no (6) is formed, and (11) can only be extracted in about 5096 yield, thus indicating protein bonding to the respective rings1135. Under these conditions, the degradation of the tetrapyr- role is accompanied by oxidation of the thio ether to sulfone
Η
(U)
{5 ] (14), which eliminates —S02—C2H5 to give (6) * in the presence of aqueous ammonia. This reaction sequence not Scheme 1. only demonstrates the protein-bond of ring A, it is also ste-
Angew. Chem. Int. Ed. Engl. 20, 241-261 (1981) 243 reoselective145 58 591 and leads, with the well-known 4R,E- reaction, in this case however between rings Β and C, is the configuration of the imide (6a)[5(i\ to the 2Ä,3Ä;3jR-configu- diazo reaction1685. This reaction, important in the medical ration of (la), (lb), (2) and (3). A possible second binding analysis of bilirubin1691, can be applied to the higher oxidized site1'31 is still controversial122'60"651. bilindiones present in the biliproteins, only after a pretreat-
,70) During chromic acid degradation the information con• ment . The protein first has to be unfolded by addition of cerning α-pyrrolic and methine-substituents is lost and, e.g., urea, and then the central methine bridge of the chromo hydrolysis of the ß-pyrrolic substituents cannot be excluded. phores has to be reduced by NaBH4. The complete reaction The milder degradation procedure with Chromate yields im• sequence can be carried out under very mild conditions ides of the terminal rings, whereas the inner ones yield pyr- (4°C, pH = 7). The fragments (16) and (17) were obtained roledicarbaldehydes113-531. Thus, it is possible to differentiate, from PC. (17) confirms again the binding of ring A to the e.g., between isomeric biliverdins, and to assign the IX a- protein; both of the last mentioned reactions also indicate the type substitution to (l)-(5). A recently developed milder existence of a second bond at ring C, at least in PC of Spiruli- na Platensis, used in these studies. The proportion of free, ex- tractable (16) is increased after hydrolytical pre-treatment; (15) is only obtainable in this way.
2.3. Spectroscopy of Denatured Pigments
A sensitive, non-destructive method for the analysis of bil- iprotein chromophores is UV-VIS absorption spectroscopy. In the native pigments the spectral properties are strongly dependent on the protein environment (see Section 3), but the non-covalent interactions responsible for these effects are decoupled by complete unfolding using urea, guanidinium
COOCH3 COOCH3 chloride, or heat. Although still covalently bound to the pro tein, the chromophores can be correlated with free bile pig (15) ments of known structure. The identification is improved by measuring the free bases as well as the cations, anions, and method of oxidation enables regioselective cleavage of bili- metal-complexes; for an exact characterization the most suit protein chromophores like (la) at the methine bridge next to able are cations and zinc-complexes135-36 617 ,*72f. The method ring A, to give formyltripyrrinones. The method is specific has also been successfully applied for the quantitative deter for A-dihydrobilindionesi6667J. Thus, tripyrrinone (15) was mination of the number of chromophores (Table 3)i6,72J; and obtained in this way from PC. Another selective cleavage by acid-base titration the pK-values of protonation and de- protonation are available as additional important paramet-
ers[35.36]
COOCH3 COOCH, C02R C02R
(18) 09)
Denaturated PE is spectroscopically very similar to (18); it contains the rhodin-chromophore (2)[7i]. Denaturated PC
and Pr absorb at longer wavelengths than (18), but at shorter wavelengths than bilindiones such as (19). The spectroscopic similarities to (23) indicate the 2,3-dihydrobilindione conju gation system for the chromophores of both pigments. The 18-vinyl group of (lb) leads to a small red-shift of the ab sorption as compared to pigments containing (la){35\ At
pH<5.2 denaturated Pfr has an absorption maximum at 610 nm; thus, in contrast to native phytochrome the denaturated
Pfr absorbs at shorter wavelengths than denaturated Pr under
οη the same conditions (λ™χ = 660 nm). Therefore, Pfr has a
244 Angew. Chem. Int. Ed. Engl. 20. 241-261 (198/) 351 shorter conjugation system than Pr* . Analogous results tal synthesis of (£,Z,Z)-bilindiones. In comparison to the have been obtained when proteolytic digestion is used in (Z,Z, 2)-educts, the absorption maxima are displaced to stead for the uncoupling of the chromophore1551. The A-dihy- shorter wavelengths, e. g. to regions expected for a Pfr-chro drobilindione structure of the chromophores (la) and (lb) mophore model. This shift has been rationalized in terms of and the thioether-bond have recently been proved, too, by a simultaneous change of the configuration at the double NMR spectroscopy of the respective bilipeptides*38'651. This bond between C-4 and C-5 in (20) and of the conformation method also afforded independent proof of the presence of of the neighboring single bond between C-5 and C-6. By this the 18-vinyl group in (lb)m. mechanism the ^-configurated double bond is partially un coupled from the remaining ττ-system*911. For the thermal re- isomerization AG0 =-20 kJ/mol and Δ//+=105 kJ/mol 2A. Reactivity of Chromophores were determined*921. Non-symmetrically substituted bilin
401 The different spectra of both forms of denaturated phy diones yield two (£,Z,Z)-isomers* . A regioselective isomer ization is observed with less symmetrical educts*931. tochrome (Pfr and Pr) demonstrate that the chromophores have different molecular structures. Conformational1731 As a consequence of the hydrated ring A—and additional
1741 changes, protonation-deprotonation and the like, can thus ly of the 18-vinyl group in the case of Pr [(lb)]—the chromo be excluded as the sole reactions during phytochrome-trans- phores of biliproteins are perturbed from symmetry. Rama- formation, and photochemical reactions gain a special inter chandran calculations indicate a preferred isomerization of 4 est. The primary reaction of the phytochrome system is the the A -bond next to the hydrogenated ring for steric reasons, photochemical transformation of P into P , or vice versa. r fr ο The reaction has several spectroscopically well defined inter mediates, which are different for the forward and back reac tions, respectively. The structures of the intermediates are still unknown* but the results of flash*75 781 and of low-tem perature spectroscopy177,79 821 as well as of dehydration ex periments182 841 have shown, that only the first step(s) are photochemical one(s), followed by dark-reactions (cf. 1181). Owing to the formal analogy with the reactions of rhodop- sin1851, an analogous nomenclature1841 has been used. C02CH3 In the context of these results, photochemical reactions of
861 free bile pigments* and especially of bilindiones related to 941 thus products of the violin spectral type are expected* . This
the P -chromophore (lb) were investigated. From the crite 90bl r assumption has recently been confirmed experimentally * .
ria, determined by the natural system, it should be possible 15 4 Δ -£, but not A -£-configurated 2,3 dihydrobilindiones are to obtain some evidence regarding the structure of P , the fr accessible by total synthesis, the former having a rhodin-type
reaction pathway, and the primary signal of the phyto 90bl spectrum due to uncoupling of ring D* . A violin-type
chrome system. There exist two well known photochemical 4 spectrum is then expected for the hypothetical, less stable Δ - reactions for the conversion P ->P which meet most criteria: r fr £-isomer. 1. geometrical isomerization at the double-bond between C-4 and C-5 or C-15 and C-16; 2. oxidative substitution at the C- 5 methine bridge. It has been known for some time that di- pyrromethenones form stable geometric (Z,£)-isomers which are in photochemical equilibrium187 88J. Photochemical stud ies on pterobilin ("biliverdin ΙΧ-γ") gave first indications, that corresponding isomers also exist in the case of bilin
1271 (22) diones . The isomerization reactions have been investi gated systematically by Falk et al with partial structures and integral bile pigments. They could show that bilindiones isomerize at one or at both terminal methine bridges and they have isolated and characterized the resulting products, like (20)[40-88·891. Starting with an ^-configurated formylpyr- The hypothetical (£,Z,Z)-isomer of (lb) [cf. (22)] would romethenone Gossauer et a/.*901 recently also achieved the to-
be spectroscopically comparable with the Pfr-chromophore. The reaction mechanism is as yet only difficultly reconcila ble with the properties of phytochrome. The photoisomeriza- tion of bilindiones does not proceed directly, but rather via intermediate rabinoid products formed in a dark reaction. Due to their low electron density at C-10*95], the bilindiones reversibly add nucleophiles*89b-96-991 and the yellow pigments thus formed are the substrates proper for isomerization*89bl. In accord with their absorption maximum at 450 nm, blue light is especially effective for isomerization*8951, whereas the
action-spectrum of the Pr->Pfr transformation has its maxi COOR COOR mum at 660 nm in the red spectral region, the absorption-
Angew. Chem. Int. Ed. Engl. 20. 241-261 (1981) 245 maximum of the Pr-chromophore. The mechanism of reac hydrogenated ring on these reactions is very pronounced. (23) tion is, however, very dependent on the substrate structure, reacts much more rapidly than the fully unsaturated ana and direct photochemical reactions basically seem to be pos logue and other bilindiones, and the reaction is regioselective sible in all cases in which the geometry of the tetrapyrroie at C-5. Although the spectroscopic characteristics of (24) and
1091 skeleton differs from the flat uniform helix. Examples are N- (25) correspond to denaturated Pfr, like other purpurins* alkylated bilindiones1931 and non-(Z,Z,Z)-isomers1921. It still they regenerate (23) in only small yields and under drastic remains open, how the asymmetry of the natural chromo conditions. phores and their protein environment can affect the course of With regard to the reversion, especially the products (26) reaction. and (27), derived from anaerobic photooxidation, seem at
The second model reaction for the Pr->Pfr transformation tractive, because they correspond in their spectroscopic char
is a photochemical oxidation of the Pr-chromophore (lb). acteristics to denatured Plr and because they can regenerate
m Denaturated Pr is oxidized by strong oxidizing agents (Fe , (23), at least thermally. These reactions, too, are specific for CeIV) to products, spectroscopically similar to denaturated the A-dihydrobilindione (23) and are regioselective at the C-
(351 Pfr . The phytochrome transformation had already been 5 methine bridge; corresponding reactions at C-15 or with correlated with redox-reactions1100"1021; and the spectra indi fully unsaturated bilindiones were only observed as rare ex- cate a photochemical oxidation of the chromophore during ceptions11061. The dimerization of (23) to (26) is reminescent
Pfr-formation. This possibility was studied using (23) as a of the reversible photodimerization of pyrimidines in nucleic
[ιΌΊ] model for Pr. Comparison of (23) and (19) allows one to fur acids (cf. e.g. ). In phytochrome, the presence of two ther analyze, whether a hydrogenated ring has similarly pro chromophores would be a precondition, which seems unlike nounced effects on the reactivity of bile pigments, as it has in ly from the known data*14*18-21-551. The formation of the py- the cyclic tetrapyrroles (cf. e.g. n°31). The products of pho- ridiniobilindione (27) demonstrates, however, that in princi tooxidation of (23) are summarized in Scheme 2. In presence ple other partners may take place in the reaction besides a of oxygen, the purpurins (24) and (25) are formed in a regio- second bilindione1411. Probably, the reaction proceeds in two
1661 selective, self-sensitized singlet-02 reaction . steps. The educt is first oxidized in one or two one-electron (25) is likewise accessible by a smooth dark-reaction1191, steps. Such a series of oxidations has been established elec- which is also suitable as a degradation reaction for bilipro trochemically with bilindiones1'081 and is supported by obser teins1671. Spectroscopically similar products were observed vation of long-lived cation radicals during oxidation of Zn- during the dark reaction of (19) with singlet-0211041; the typ (23)ll09]. In the second step, the nucleophilic addition of py ical spectrum with two bands1661, however, makes the pro ridine at C-5 takes place. Obviously, the reactivity of the in posed endoperoxide structure11041 unlikely and rather sug termediates is reversed. The (27) that is formed, formally gests the formation of purpurins as well. The influence of the possesses the conjugation system of the educt (23); but its ab-
246 Angew. Chem. Int. Ed Engl. 20. 241-261 (1081) sorption spectrum is shifted to shorter wavelengths, because between remarkable limits [e.g. 80 nm for (la)] and thus per the A5-bonding is twisted by the steric hindrance between the mit a "fine tuning" of the energy transfer. The question as to rings A, Β and the bulky C-5 substituent*4,U01. the origin of these optimizations, which may be paraphrased
Regarding the latter as a model reaction would require Pr under the heading "molecular ecology", is essential for an to contain both an oxidant and a nucleophiie. According to understanding of the function of biliproteins. Geometrical present day knowledge, amino acid residues are the only transformations, protonation-deprotonation, and a restricted candidates, because phytochrome is transformed in solution flexibility of chromophores seem to be some of the essential and contains only carbohydratestml besides the chromo factors. phore. For oxidation, cystine residues are possible candi dates, for addition tryptophan, tyrosine, serine, cysteine and 3.2. Geometry of Chromophores the like. There do in fact exist certain indications to this ef
fect; thus, Pfr contains one to two accessible SH-residues The first detailed analysis of the geometry of free bile pig ,I2n3 [li4 more than pri ^ and (23) reacts with e.g. derivatives of ments stems from Moscowitz et al. \ postulating a cyclohe- tryptophan to give violins*!061; but here again a decision is lical porphyrin-like structure in solution for urobilins [(28); not yet possible. for sake of clarity, the substituents have been omitted], with the sign of the twist determined by the absolute configura 3. Chromophore-Protein Interactions tion of the a-pyrrolic centers C-4 and C-16. Cyclohelical
3.1. Molecular Ecology jo*
The chromophores of native and denaturated biliproteins -NH differ so conspicuously in their properties (Table 2) that a re Η (28) lationship between the two may seem unlikely. Phycobilipro teins can be denaturated reversibly and in excellent yields with urea or guanidinium chloride*221. Since the covaient bonds between chromophore and protein are retained, the different properties of the chromophores in native and dena tured pigments, respectively, are exclusively due to non-co- structures have also been established for bilindiones, like (19), both by X-ray analysis*1151161 as well as by Ή-ΝΜΙΙ*1,7,1181 measurements and measurements of sol
,,7n91 Tablc 2. Molecular ecology of the biliproteins. Comparison of the properties of vent-induced circular dichroism (SICD)* in solution. native bilindiones, with those of denatured pigments and free bilindiones of simi In the formally achiral bilindiones of the biliverdin type, this lar structures. results in a uniform population of two enantiomeric confor Free bilindiones or dena Native biliproteins mations (29a) and (29b) which differ in their sign of rotation tured biliproteins and are in equilibrium with each other (AH * = 42 kJ/mol
[l20] UV/V1S absorption Broad bands, intense Narrow bands. weak for a derivative of (19) ). The equilibrium may be shifted 1171 spectra (e»35000) (a) in the near (ε« 15000) [a] in the near sufficiently by dissolution in chiral solvents* or by adsorp (see Fig. 1) UV, weak (15000) |a] in UV. intense (ε^ 100000) tion to the chiral biopolymer serum-albumin*1211 to allow the the visible range [a] in the visible range Photochemistry Mainly radiationless deacti High quantum yields for observation of typical Cotton effects of inherently dissym vation, low quantum yields fluourescence (> 0.6 in metric chromophores. In support of this, purpurins like (25) (10 ' 3) for fluorescence phycobiliproteins) or pho containing planar chromophores11221 give no SICD-effect*1231. or photochemical reactions tochemical reactions (% 0.15 in phytochrome) Chemical Poor, ready formation of Good; little to no reaction stability metal complexes, facile nu- with these reagents cleophilic addition or re duction at C-10, sensitive towards photooxidation
[a] All extinction coefficients are given with respect to one chromophore. They have to be multiplied by the number of chromophores (Table 3) to obtain the ex tinction coefficients of the respective biliprotein. Somewhat varying ε-values are cited in the literature, due to different methods of determination.
valent interactions with the native protein. These interac coo® coo® tions are essential in optimizing the chromophores for their (29a) function of photoreception. This includes (a) an increase in the extinction-coefficient by nearly one order of magnitude, The £,Z,Z-isomeric bilindiones are SICD-negative1901, too, leading to an increased light-absorption, (b) the suppression thus supporting the suggested a/?//-,iy/i-,j>7i-conformation of radiationless deactivation to diminish energy losses in fa [Formula (20)Y9iK vor of fluorescence (phycobiliproteins) or of photochemical The state of equilibrium between the two helical forms reactions (phytochrome), and (c) the chemical stabilization may also be shifted by asymmetric centers within the mole of chromophores (Table 2). In addition, also the positions of cule. The high rotational values of optically active urobilins the absorption maxima of certain chromophores are variable are due to the asymmetric centers C-4 and C-16 having the
Angew. Chem. Int. Ed. Engl. 20, 241-261 (1981) 247 s same absolute configuration by which one form is preferen• phore. In the case of cyclohelical conformations ß„v is tially populated for steric reasons11141. Since the absolute con• small and thus similar to that for a porphyrin, whereas in ex• figuration of the helix can be derived from the sign of rota• tended conformations it is large and thus similar to that for a tion of helical chromophores, the absolute configuration of polyene. This is one essentially consistent result obtained the assymmetric centers is thus also indirectly aCCeSSi- from MO-calculations by several groups180-91'I2K131 1341 and is ^ö. 57.94b. Π 4] blg confirmed by the spectral data of free bilins of known con•
s In the biliprotein-chromophores (1)—(3), ring A has at formation. ß„v = 0.25 in cyclohelical (19) (vide supra), but is least three asymmetric centers. The different steric hindrance increased to 6.4 in isophorcabilin (31), which by reason of its additional intramolecular bridges can only assume extended conformations*2701.
of the two forms has been estimated by Ramachandran cal A value of β™ = 0.15 was measured recently for a 21,24- culations on (30) as a model of denaturated PC and Pr to re sult in an energy difference of Δ(?° = 4—8 kJ/mol(941. Ac methanobilindione, which necessarily possesses the all- (3211 cordingly, denaturated PC exhibits a rather strong optical ac 5y«,Z-conformation . The variation in the β™ of bilin 5 ι 241 diones due to formation of cations, anions and Zn-complexes tivity (Θ6Ο = 133000 ' ). In the chromophore (2) of PE, the influence of the asymmetric centers of ring A is opposite to is comparatively small, and the absorption in the near UV, in 361 the influence of the asymmetric C-16; accordingly the optical particular, remains essentially unchanged* . activity of denaturated PE is much lower11441. The hitherto described effects can only be rationalized in terms of a preferential cyclohelical conformation of free bile pigments of the biliverdin type. This conformation is, howev er, not rigid but rather flexible, and it also exists in equilib rium with more extended conformations. The broad ab sorption bands, which are unstructured even at low tempera tures1191251, refer to this situation, as well as the exceptions in planar pigments like (25)[66J. Recently, the problem was in vestigated in detail by the Mülheim group. It was possible, by fluorescence spectroscopy, to differentiate at least two CO Protein COO® conformer populations in biliverdin*1191. The form fluores• cing at longer wavelengths was identified as a cyclic confor• mation, since only this one showed an SICD effect; the form emitting at shorter wavelengths was associated with a more extended conformation because of its higher ratio of absorp• tion of the visible and near UV-band, β™ (vide infra). The equilibrium between both forms is dependent on tempera ture and solvent; the open form is preferred in viscous H- donor solvents11261, especially in lipid-vesicles11271. Strong in tramolecular Η-bridges similar to those of bilirubin*1281 were also implicated as the origin of the formation of isomers in biliprotein cleavage products. These Η-bridges were only ob served in the case of the free acids*431. The denaturated biliproteins are so similar in their proper ties to free bile pigments of corresponding structure [(18) for
PE, (23) for Pr and PC], that they also are expected to assume 1 1 1 i ι ' 1 preferentially cyclohelical conformations. The modified 700 600 500 4 00 700 600 500 400 spectroscopic properties of native biliproteins (e. g. PC, Fig. •— λ [η m] *— λ [η m J 1) originate mainly from their chromophores being rigidly Fig. 1. Spectrum of denatured (a) and native phycocyanin (b) from Spirulina pla- fixed by the protein in an extended conformation119*1251. The tensis (same concentrations in (a) and (b)). The denaturation by urea (8 M) is re versible; similar changes are observed upon thermal denaturation. The formulas intensity ratio of the visible with respect to the near UV- depict the type of conformation for the two forms: cyclohelical for (a) and ex band is mainly determined by the geometry of the chromo tended (in a sterically unhindered conformation) for (b).
248 Angew. Chem. Int. Ed. Engl. 20, 241-261 (1981) In denaturated PC at pH 7.5, Q™ = 0.43; it is increased to 4.1 in native PC, with both bands changing their intensity in opposite directions (see Fig. I)11251. Similar effects are ob
I55] served with Pr . Thus, the chromophores of both native pigments are present in an extended conformation. There is an interesting aspect of these findings: the extended confor 192 941 oo - — ON ci I . τ- *Λ> «o ones and the necessary energy has to be provided by the CZ*I ' , ~ Os protein. The energies of folding of several phycocyanins are indeed much lower than in other proteins of comparable 11351 size , i.e. the difference is possibly due to the strained 4J t> ι 00 0£> i chromophores11291. In native ΡΕ, g*v increases to a compar able extent as found in PC, as compared with the denatu a£ ca ct rated pigment11391441. To our knowledge, the relation be tween the absorption and geometry of chromophores of PE ο ν a> 4> has only once been studied theoretically1144*1, and extended 4> 4> 1» t. pigments corresponding to bilindione (31) possessing the te- * £"8 er υ υ QS υ ο α: α; α: υ ο ο trahydro-conjugation system of rhodin (18) have so far not been reported. On the basis of calculations, however, it is I I I I I I likely that is also a certain indicator of the geometry of molecules, which would suggest extended chromophores for native PE, too. The high ellipticities of the long-wavelength CD-bands of Γ- ON ON ON Ο native PC as well as of PE would indicate an inherently dis symmetric chromophore, and therefore a more or less uni form twist of the extended chromophore. Generally, the Si £ bands have a positive sign. P is a remarkable exception in r if having a negative CD-band despite it presumably1371 having 3 < the same absolute configuration of the asymmetric centers as 1135 1 Pfr at ring A " . At least in this case, an exciton-coupling would seem to be unlikely since phytochrome carries only G2.ca.cao β Ö β β ö Ö β 3 Ο one chromophore. The picture is more complex with Pfr, the active form of phytochrome. On the one hand, of native Pff is only half that in native Pr (see e.g. and, on the other, the ab sorption maximum of the native pigment is extremely red- CQ shifted. Denaturated Pfr absorbs in its protonated form I I I I 5 I (pH = 2) at 610 nm1351, corresponding to a maximum for the free base at about 570 nmi35,7,). The absorption maximum of native Pfr (Xmax = 730 nm) is shifted to the red by more than X X 160 nm ( = 3850 cm"1). While the lower value of is still compatible with a rather extended conformation, the pro ς» a a «3 * d d ^ xxx nounced red-shift cannot be explained by a conformational f"> τ}· * 3.3. Charge and Chromophore-Chromophore Interactions In addition to geometrical transformations, electric 522 \w-o> wο» rο» ΙΛ Μ ΙΛ ΙΛ VI ο ο E £> ON ! r- NO r- NO charges in the vicinity of the chromophore are possibly im -< > >Λ m ΙΛ 3). Although they frequently have the same molecular struc• tu ai ω ω w CL< Ο, CU a- °r ture [e.g. (2) in C-PE] and by and large the same geometries, Sa SS 6 tC x> ώ * Angew. Chem. Int. Ed. Engl. 20. 241-261 (1981) 249 too, they differ spectroscopically, chemically and functional• pigments196 155 ~1571. Both were recognized as essential, but a ly, due to the different environments of their chromophores quantitative estimation is difficult, since chemical manipula (see Section 3.4) and their correspondingly different chromo- tions will generally influence both factors. Thus, e.g., the N- phore-protein interactions. For example, APC-B absorbs at methylation of a dipyrromethenone will inhibit intermolecu- 670 nmI,48b>, the s-chromophore of C-PC at 590 nmi36'140 1421; lar proton transfer by suppression of dimerization in unpolar in spite of their absorption difference of 80 nm ( — 2020 solvents11561, but will at the same time restrict the conforma- cm-1), however, they possess the identical chromophore tive mobility. Similar reasoning applies to iV-protona- (la). Chemical differences are evidenced, e.g., by a different tion196 1551. The influence of viscosity which essentially de stability towards reduction11291 or unfolding11251, spectroscopi• creases vibrational relaxations has hitherto mainly11581 been cally several absorption bands can be resolved, especially at investigated at lower temperatures196*1321551. Bilin low temperatures1125 136 ,391, in the fluorescence1139 1431 and diones1961321551 and especially conformatively more rigid CD-spectra*144 147]. These differential effects are physiologi• pigments like isophorcabilin (31) show a moderate fluores cally essential (see Section 5.1) for an optimal energy-trans• cence at 77 Ki,55cl. fer and they were investigated for the first time in some de• In this respect, the rather strong fluorescence of isophorca- tail by fluorescence-measurements11401. The site-specific rubin (32) is noteworthy11651. The ring-Α,Β fragment of (32) chromophore interactions are obviously already fixed in the is identical to that of a common 10,23-dihydrobiIindione sub-units, since the CD- and absorption spectra are un• ("bilirubin"), and its fluorescence should therefore only be changed during aggregation to the (aß)-monomers. In a C- weak186 166l The CD-fragment is rather rigid, however, and PE the spectrum was identical with the sum of the subunit is incapable of allowing intramolecular Η-transfer, so the spectra11441. In contrast, further aggregation of the monomers fluorescence has thus been related to the latter11651. Hitherto, causes shifts to longer wavelengths of the visible bands. An no direct approach has been tested to quantitatively separate additional long-wavelength band appears in the differ• the contributions of proton-transfer processes; one possibility ence1148 154), low-temperature*136-,37J and CD-spectra1145" would be a systematic investigation of 2H-isotope ef 1471521. Its displacement is small in PC and PE (10—15 fects11671. nm = 250—450 cm-1), but rather pronounced in APC. Mon• omeric APC absorbs at 620 nm, the trimer at 650—670 nm, at the same time the absorption is almost doubled11491. Recently, evidence has also accumulated for chromo- phore-chromophore interactions which have been related to HjCOOC- displacements of absorption maxima. S-shaped CD-bands were observed in C-PE11441, B-PE1311, K-PC1341 and an allo• phycocyanin11521. This type of band may be caused by exci- ton splitting, but a decision is difficult if several chromo• phores are present. The long-wavelength shift of the absorp• 11471 11491 tion-band of the trimeric APC (αβ)3 was recently in terpreted as a chromophore-chromophore interaction of "in termediate strength" ( = CD-inactive) between different monomers in the aggregate11501. In biliproteins the radiationless deactivation is strongly de creased. The vibrational relaxation can be effectively de creased by a rigid fixation of the chromophores. Indications 3.4. Increased Fluorescence and Chemical Stability of this are the decreased bandwidths of the absorption spec- trai66.125] whicfc is also evident in the fluorescence-excitation- The second conspicuous feature of native biliproteins is spectra1139 1431, the large negative temperature coefficient of their photochemical behavior. Phycobiliproteins fluoresce the fluorescence1171391, and the small phonon-coupling to the with quantum yields near to one (see ll42]), phytochrome has protein11381. quantum yields of 0.13 and 0.17 for the photoreversible An effective instrument for repressing proton-transfer- transformation between the two forms1171. Free bilindiones reactions would be the transfer of chromophores into a hy and denaturated biliproteins show only a very low fluores drophobic environment, e. g. into the interior of proteins. An ί156 1581 cein 9,130,132.. 55-157) and phosphorescence ' ; the extended conformation of the molecule would further inhibit "natural" a//-5);w,Z-bilindiones166105*159-1621 and even the dis intramolecular transfer. The lack of typical reactions of bil tinctly more reactive A-dihydrobilindiones166*1051 are also indiones, e.g. the formation of Zn-complexes[1681 in bilipro photochemically rather inert. The major pathway of deexci- teins, could be evidence of this1221691 as well as the quenching tation of free bile pigments or denaturated biliproteins is of fluorescence with benzoquinone1170,3221. therefore radiationless deactivation. Two mechanisms are On the other hand, there are several recent results which plausible: vibration-induced deactivation11631 and proton- put the chromophores into a hydrophilic or at least water ac transfer or even only perturbations of hydrogen-bond poten- cessible position near the surface of the biliproteins: Thus tial-curvesi164J. Both are able to cooperate in a complex man there are reports of redox-reactions of chromophores at elec ner, since, e.g., an intramolecular hydrogen bond is able to trode-surfaces11711, in solution199 100 129 1721 and at mem- hinder torsional movements, but at the same time can open a branesf1731, of the reversible addition of thioles and dithionite new channel of radiationsless deactivation (see iJ63i64i). (or rather sulfoxylate11741) to the central methine Both mechanisms have been investigated on partial-struc bridge11291721, of chromophore-chromophore interactions tures11591 and—hitherto less systematically—on integral bile during the aggregation of APC11511, of low-temperature pho- 250 Angew. Chem. Int. Ed. Engl. 20. 241-261 (1981) tochemistry related to proton transfer11381 and of an H2l80- organisms; a more distant relationship exists between corre• exchange of both lactam oxygens at C-t and C-1911751. In the sponding subunits of different types of pigments and be• case of the reaction of PC, APC and PE with dithionite, it tween the different subunits of the same pigment. Also the could be demonstrated that this reagent is in thermody pigments of red algae and cyanobacteria seem to be more namic equilibrium with the chromophores. At least in this closely related with each other than than with the pigments case, the decreased reactivity of the chromophores in the na of cryptophytes. tive pigment is thus determined thermodynamically and not The protein moiety of phytochrome is distinctly different kinetically11291. Possibly, the conformational changes are one from that of the phycobiliproteins. It is considerably larger, it direct cause of the increased stability. probably possesses a dumbbell-shaped structure, and Pfr in Perhaps the quantum yield of fluorescence is the most sen particular has a tendency to associate with membranes. Im• sitive parameter of the state of phycobiliproteins. During munochemical and spectroscopical investigations demon• controlled protein unfolding it already clearly decreases be strate the distinct similarity of phytochrome(s) from different fore the absorption- or CD-spectra noticeably change11441761. organisms. So far there is no direct indication of a phylogen• Under partial denaturating conditions, the photochemical etic relationship with the phycobiliproteins. The similar behavior of phycobiliproteins changes in a complementary structure of the chromophores, their similar binding to the fashion. In presence of allylthiourea11781, the photochemical apoprotein, comparable non-covalent interactions, and final• stability of PC is decreased11771. In presence of 0.5 M guanid- ly the occurrence of phycochromes are indirect indications inium chloride11791 or on diminution of the pH-value to thereof, but they don't exclude a converging development. 3.811801, solutions of PC and APC become photoreversibly- photochromic. The latter result is especially of interest with regard to the phycochromes110121 and possibly to phyto 4.1. Phycobiliproteins: Monomers chrome, too, because they show similar difference-spectra. The most data available on primary structures are those for phycocyanins. Aside from the determination of Ν-termi 4. Proteins nal sequences (see115-205,2061) and of chromophore binding re gions162"65,207-2111 of several organisms, the sequences of PC Phycobiliproteins represent a period of evolution of about isolated from Mastigocladus laminosus[62\ a thermophilic 3.5 billion years and they exist in procaryotes as well as in cyanobacterium, and from Cyanidium caldarium162^ a mono eucaryotes, in very different biotopes. They are thus of inter cellular thermoacidophilic red alga, have recently been es est for comparative investigations which currently culminate tablished. The complete analyses of the ß-chain of PC iso• 62 631 in a complete sequencing of two PC-molecules* * . The lated from Synechococcus spec. 6301 (formerly called Anacys- most important data of biliproteins that have been character {64] tis nidulans) 7 a monocellular mesophilic cyanobacterium, ized spectroscopically and by their origin, are summarized in and of the ß-chain of APC from M. laminosus have also been Table 3. The phycobiliproteins are globular and soluble in carried out12051. The homologies of the respective a- or ß- water, those isolated from red algae and cyanobacteria show chains of the two complete structures are remarkable (80 or pronounced aggregation. Analysis of the N-terminal se 78%) considering the different taxonomic positions and bio• quences, immunochemical investigations and hybridization topes of the organisms. experiments indicate a common phylogenetic tree for these However both are thermophilic organisms and the se• pigments (Fig. 2). The closest relationship exists between quence, known up to 80%, of the marine cyanobacterium Ag- corresponding subunits of one type of protein from different menellum quadruplicatwn shows only a homology of about 30% (L. Fox, private communication). A high homology is also found on comparing the PC ß-chains with that of S. oc-C-PE p-C-PE spec. The a- and ß-chains of PC from each of the two organ• {2«[2)) (3-<*ϋΊ) isms show much lower homologies (25% in PC from M. lami• nosus)12^. The relationship between the same subunits of pigments of a different spectral-type is also obvious in the primary structures. Thus, the ß-chains of PC and APC of M. laminosus are homologous to 25%, and the relationship be• comes even more distinct on comparing the secondary struc• tures (as calculated by the method of Chou and Fass- manf»s\ The homology is not uniform, but particularly high in the chromophore regions. An earlier survey indicated a very high homology of the N-terminal regions within the corre• sponding subunits of pigments of different origin and of dif• ferent spectral-type1151. The low homology of these regions of PC and APC of M. laminosus found by the "Zurich-group", is, however, the first example of an exception to this rule, or it indicates an exceptional position of APC. The homology of the chromophore binding regions is supported by compari• Fig. 2. Phylogenetic tree of biliproteins and their a- and ß-subunits from cyano• bacteria. C-PC = Phycocyanin, C-PE=phycoerythrin, APC = allophycocyanin son of sequences of chromopeptides isolated from different (after [2051). 12091 162 phycoerythrins and cyanins «.207,210.210 Thus? flve dif. Angew. Chem. Int. Ed. Engl 20, 241-261 (1981) 251 ferent amino acid sequences have been identified in the re• typical for globular proteins, with APC containing a higher gion of the binding cysteins in PE from Pseudanabaena proportion of hydrophobic amino acids than PC and PE. W1173, containing five chromophores. PE from Phormidium The crystal packing*32,193 1941 and the investigations in solu• persicum, containing six chromophores, possesses six different tion*2221 show an oblong shape (2.5—3.5 nm diameter, 5—6 binding regions, five of which correspond unequivocally to nm in length) of the (aß)-monomer of PC; a size which has the five regions of the first named pigment12091. The se• also been deduced for other biliproteins*223 2251 from elec• quences of peptides characteristic of the individual chromo• tron-microscopical measurements. According to circular phores can be correlated with the spectra of the chromo• dichroism measurements on several pigments, the α-helix phores in these pigments1'391. Hitherto, little was known content is about 60% in the a- and 40% in the ß-chain*1441471. about the phylogenetic origin of the increasing number of The secondary structure of PC and APC of M. laminosus has chromophores on the individual peptide chains on going been estimated from the sequence. Here, too, the a- and ß- from APC to PC to PE. Interestingly, however, a comparison chains show differences, and there are indications of a differ• of PC and APC from M. laminosus shows that the additional ent flexibility of the peptide backbone in the environment of chromophore binding site on the ß-chain of PC is formed by the different chromophores*2051. insertion of a short peptide in the APC sequence*2051. As the second essential method, the immunochemistry Studies on the protein moieties of the biliproteins comple• confirms the relationships between the phycobiliproteins. ment the data on the chromophore binding mode as derived The pigments of the same spectroscopic type, 1486,149 187,227 2291 £(32.72,14Kb. 149,187,227 229] APC* ' Or ρ Qr from investigations of chromophores (see Section 2). With pEii98.227,228 .23i] 2131 .23o i i lated immunochemical^ ir only one exception* all of the known "chromopeptides" are c ose y re 13562 65 205 2I61 12121 respective of their origin from the procaryotic cyanobacteria contain cysteine . The early proposition , and the eucaryotic red algae. In contrast, the pigments of the that this amino acid takes part in chromophore binding was different spectroscopic types do not undergo cross reactions, first verified for a B-PE peptide from Porphyridium cruen- l20 even if they are produced from the same organism. On this tum *\ and it was later recognized that cysteine functions as basis, R-PC*721 and phycoerythrocyanin*32,2301, which contain the binding amino acid for all chromophores in several C- blue chromophores (la) as well as red chromophores (2), phycocyanins and phycoerythrins162 63'209'21K216). These re• were classified as phycocyanins, and APC-B as a true allo- sults are supported by a comparison of the number of phycocyanin*148b). The subunits of individual C-PE*2311 and bonded cysteines and the number of chromophores in pig• also those of APC*1491—which are difficult to differentiate by ments of several red algae and cyanobacteria*2171. It follows, other methods—are immunochemically not related, in con together with the proof of the thioether bond to ring A of the trast to the close relationship of corresponding subunits of chromophores [Section 2.2, formulas (1), (2)], that the blue pigments from other different organisms. (la) and red (2) chromophores are bound to the protein via cysteine residues. The yellow urobilinoid chromophores of The cryptophytan-pigments, too, assume an immuno 2271 B-PE and R-PE are supposed to have even a second cysteine- chemically special position. First investigations* revealed chromophore bond to ring D*22-2,7J. no relationship with pigments from cyanobacteria and red algae; this has been supported for PC*2281, whereas PC as well Pigments of cryptophytes have hitherto been the subject of as PE from two different cryptophytes cross-react with PE fewer investigations; the finding of a single, free cysteine-SH from the red alga Prophyridium cruentum, but not with C- in one chromopeptide is indicative of a special binding posi• PE*1981. Accordingly, both pigments of cryptophytes seem to tion*2151. be related to PE from red algae. There is less agreement on other and possibly additional Until now no defined immunological determinants are chromophore protein bonds. In particular, the linking of ser• known. The missing cross-reaction of APC and PC has been ine with one of the propionic acid side chains is plausi• taken as an indication of the chromophore not being a deter ble*22 209,213,215|. There are no such indications from the se• minant, since both contain (la)[2il]. The different spectra of quenced phycocyanins and allophycocyanins, but such a PC and APC, however, point to different states of this chro connection is discussed as being the only binding site of a K- mophore in the native pigments which may lead to signifi PC*2151 and of a R-PE*221, and it was identified in addition to cant differences in immunochemically relevant criteria, e.g. the cysteine bond in one C-PE chromophore*2091. The prob• conformation and charge. There are also indications of the lem is complicated by the possibility of both artifactual hy• chromophore being accessible from the outside (Section 3.4). drolysis of esters and the formation of new bonds during pro• Immunochemical methods are principally suitable for a nu tein degradation (see *221). Crespi and Smith1214] postulated a meric taxonomy. With C-PE, isolated from seven different lactim-ester binding to aspartate to account for the easy split• types of cyanobacteria, it could be demonstrated by a quanti ting of the thioether bond, but investigations of models show tative study that there are determinants specific of the spe that this activation is not necessary*54^58·2181. Also, a binding cies, as well as determinants specific of the spectral type, and to glutamate is reported for a cryptophytan chromopep• that the results depend upon the method used (phage test or tide*2151. precipitation test)*2301. Only little is known so far about the secondary- and tertia• ry structures. Although PC and PE in particular can be read• 4.2. Phycobiliproteins: Quaternary Structure ily isolated and crystallize well, there exists as yet no high-re• solution X-ray analyses*32,193,194-219'2201. This may be ex• The quaternary structure and higher aggregates have been plained by an unfavorable packing, leading to partial obliter• investigated more thoroughly than the secondary and tertia ation of reflexes due to interference from neighboring mole• ry structures. They are decisive for the biological function of cules*2211. The amino acid sequences and the solubilities are the phycobiliproteins. Most of the pigments possess an 252 Angew. Chem. Int. Ed. Engl 20, 241-261 (1981) (a,ß)„-structure, with w-varying between one and higher than somes*223 2251, as the light harvesting superstructures of these 12 depending on their state of aggregation and their origin organisms*51. Dependent on the conditions of isolation, high (vide infra) (Table 3). In some pigments, yet a third subunit er aggregates*23,2391 and hetero aggregates*223,224,2401 have also Ι,53 ,93 ,941 has been found, as in B-PE and R-PE (α6β67) ' , in been found, which represent more or less intact fragments of ί1821 51 12251 APC I (α3β37) and in PC from Chroomonas spec, with an phycobilisomes* and even aggregates thereof . Electron 1891 2481 αα'β2 structure* . microscopic investigations by Mörschel et Λ/.* revealed The molecular weights vary between 9200*1891 and that the hexamers of PC are made up of two torus-shaped 232] 20500* for the α-subunits and between 16000 and 23500 (a,ß)3-trimers, one on top of the other, and that in B-PE the for the ß-chain, with the α-subunits of the cryptophytan bili 7-chain occupies the central cavity of the cylinder that is proteins at the lower limits. The -γ-chains are considerably formed. heavier. The subunits are classified by definition according In the pH range close to the isoelectric point (χ 5.3) the to their molecular weights (a < ß). In the case of a different equilibrium is mainly shifted in favor of the hexamer. In di number of chromophores, the ß-subunit always contains lute solutions at pH = 6 to pH = 5.4, an equilibrium between more than the α-subunit. An alternative classification is pos monomer and hexamer has been established for several phy sible immunochemically. It has been proven especially useful cocyanins with an equilibrium constant of about 1030 in fa 239,2411 with APC*1491, which has subunits carrying one chromophore vor of the hexamer* . 2421 each and which may have very similar molecular weights111. Also, trimer-hexamer equilibria* and—at higher pH 2431 2441 The subunits can be separated preparatively by ion-ex values—monomer-tetramer* and monomer-dimer* change chromatography, after being partially1139'1961 or com equilibria have been studied by ultracentrifugation. At low pletely162 64,,47,149,223 2341 unfolded. The separately renatu- concentrations, C-PE exists in a monomer-dimer equili 1391 rated subunits preferentially aggregate to dimers. In a mix brium* . The aggregation is favored at elevated tempera tures*2381, at higher ionic strengths*2451, by arenes*2461, and by ture, the native pigments (αβ)Λ can be reconstituted in good 2471 yields. Smooth hybridization in 40—60% yield has been low concentrations of guanidinium chloride* ; it is de 2451 239,2491 found12331 with complementary C-PC subunits isolated from creased by chaotropic salts* and H/D exchange* . monocellular and filamentous cyanobacteria, respectively; These results, which indicate a high degree of hydrophobic and there are even APC hybrids from the subunits of cyano interactions in aggregate formation, have been compared 2381 bacteria and the red alga, Cyanidium caldarium[lA9]. A limit with the formation of detergent micelles* . The role of or ing factor for the yields is certainly the sensitivity of the dered water structures was recently studied in association ex 2501 chromophore in the denaturated pigments. periments with tetraaikylammonium salts* . Since cyano bacteria also occur in extreme biotopes, the phycobilipro The controlled denaturation of monomer pigments has teins are suitable objects for the study of such adaptations. been investigated in the case of PC11251 and PEI144]. In each C-PC from a psychrophilic organism*2511 possesses similar ag case sequential effects could be observed. The fluorescence gregation properties as the pigments from mesophilic organ decreases first1144,1761, followed by absorption changes of the isms, whereas phycocyanins from halophilic*2521 and thermo chromophores (in C-PC in a stepwise fashion11251), and final philic*2531 organisms are clearly different. The molecular ly the protein structure (observed by CD) melts1144J. causes of these adaptions have also been investigated by ca- Corresponding to the strong coupling between chromo lorimetric measurements of the protein folding*1351 and se phore and protein the quantum yield of fluorescence and the quencing studies*621. absorption spectra are the most sensitive indicators of the In contrast to the pigments of the red algae and of the cya state of the protein (see Sections 3.3 and 3.4), e.g. partial pro nobacteria, cryptophytan biliproteins show no significant ag teolysis or denaturation. gregation. This is reflected in the absence of phycobilisomes The thermodynamics of the unfolding of proteins has been in cryptophytes. investigated on phycocyanins from several biotopes. Starting with undefined aggregates, AG0' for pigments from meso-, psychro- and halophilic organisms is in the range of 10—22 1351 kJ/mol* for complete unfolding by 8 M urea. These values 43. Phytochrome are considerably lower than for globular proteins of similar 235,2361 size and free of disulfide bonds* . Comparable values Although ubiquituous in green plants, phytochrome has have been found only for pigments of thermophilic organ been purified from only a few species sufficiently enough to isms*1351. A possible factor is the energy necessary for allow the chemical characterization of the protein. 1291 "stretching" the chromophores of the native pigments* Monomeric phytochrome from oats and rye has a molecu (see Section 3.2). The refolding kinetics of PC from Spirulina lar weight of about 120000*200,2011 and it probably has a 203,2041 L ,4 18 211 platensis is multiphasic, with a rapid first phase (τι/2 = 110 dumbbell shape* (for reviews see * - - ). The hither msec) accessible by fluorescence as well as by absorption to best characterized pigment from oats is readily cleaved by measurements*2371. endogenous proteases (at the incision?), to yield a still photo- Phycobiliproteins from cyanobacteria and red algae show chemically active fragment with a molecular weight of (with exception of the already complex-monomers APC-I, Β 60000*255 2571. This fragment is generally referred to as ΡΕ and R-PE (Table 3)) a distinct aggregation, which was "small" phytochrome; for some time it was regarded as the mainly investigated systematically by Berns et α/.*231 on C- native monomer. This partial degradation changes the ab PC. Starting from monomers, preferentially tri- and hexam- sorption spectra, the photochemical quantum yield*2621, the ers are formed*2381. These are the basic building blocks both energy transfer from tryptophan to the chromophore*2631, and for the crystalline pigments*32,194,2191 as well as for phycobili the immunological properties*2641. "Large" phytochrome Angew. Chem. Int. Ed. Engl. 20, 24t~26t (t98t) 253 forms aggregates, whose dimeric structure is confirmed by tentially photoreversibly-photochromic pigments of these or gel-filtration*203,256-2581, sedimentation'20'-254-255*, and electron ganisms are called phycochromes, irrespective of any known 12545 l101 microscopy . Additional forms of Pfr of even higher mo function (see ). To characterize the function of these pho lecular weight have been reported, whose nature is still un toreceptors, which in no case is as yet certain, Bogora Thus, indications of a preferential interaction between Pfr relations between phycobiliproteins and phytochrome. and cholesterol12681 were obtained, and Smith discovered a 12691 preferential assoziation of Pfr with dextran blue which may also be useful for the purification of phytochrome by 5. Biological Functions sorption in the Pfr-form on dextran blue-agarose and subse f269a} 5.1. Phycobiliproteins quent desorption after irradiation with far-red light . P,r isolated after in vivo transformation, contains a larger frac tion of high molecular weight components (2*400000)*2561. In Phycobiliproteins are antenna or light-harvesting pig presence of bivalent ions, it binds relatively unspecifically to ments of cyanobacteria, red algae, and cryptophytes. The ab 270 27, sorption bands of the most abundant pigments of this group, particulate fractions ("pelletability", see ( J) pr arKi pfr 12721 the phycoerythrins (PE) and phycocyanins (PC), are found are immunochemically indistinguishable , and isoelectric 1131 in the green to orange spectral range. The light-harvesting focussing also gives identical results' . Significant differ pigments of green plants, chlorophyll a and b, absorb only ences were found, however, on titration of readily accessible 11131 slightly in this range, thus guaranteeing biliprotein produc amino acids . Pfr contains one accessible cysteine and one ing organisms an ecological advantage in deep water and un histidine more than Pr, and the modification of two tyrosines der a canopy of green plant. inhibits the photochemical reactions. The organization of biliproteins and the mechanism of en ergy transfer is very similar in cyanobacteria and red algae, 4.4. Phycochromes but the cryptophytes differ considerably. In the former, the phycobiliproteins are localized in particles visible by electron Originally, the term phycochromes was coined—in analo microscopy, called phycobilisomes, which are located at the gy to phytochrome—for the light sensory pigments in cyano outer surface of the thylacoid membranes*5-224 2761. The phy bacteria (for a recent survey see ll0)). Today, however, all po cobilisomes of different species have distinctly different sizes 254 Angew. Chem. Int. Ed Engl. 20. 241-261 (1981) and fine-structures, the ones most thoroughly investigated to ble. Perhaps the aforementioned colorless proteins are neces• date being those from the two red algae, Porphyridium cruen- sary as structural elements12801. In support of this, phycobili• tum*5 2791 and Rhodella violacea[22y224] (see Fig. 3), and from somes partially dissociated into a crude APC fraction and a the cyanobacterium Synechococcus sp. 6301 (Anacystis nidu- PC-PE-complex can be reconstitutedl283bl. Since the tripartite lans)[21*\ Besides the light-harvesting pigments proper, e.g. units of R. violacea contain no colorless proteins, these rather PC and PE, they contain small amounts of several different seem to take part in the APC-PC-coupling or in the mem• allophycocyanins'148b1821, which are essential for the transfer brane-binding12231. A first experimental indication of this is of energy to chlorophyll. In addition, small amounts of color• the recent identification of a heavy, colorless protein (molec• less proteins have been described which are possibly impor• ular weight about 80000), in both isolated phycobilisomes tant for the organization of phycobilisomes1223,278,2801. and thylacoid membranes freed of biiiproteins'283bl. Comparatively less is known about phycobilisomes of cya• nobacteria, but the results indicate similar structural princi• ples as in those of R. violacea. By electron microscopy, Wild- man and Bowen{277] have identified phycobilisomes in all of the 27 tested cyanobacteria, in some of them with excellent resolution, but until recently the isolation was more difficult than in red algae. A complete separation from other pig• ments was achieved by immunochromatography, but the elu- tion of the phycobilisomes purified in this manner was only possible under dissociating conditions12801. Phycobilisomes have been isolated from Nostoc species115 ,a-2841 from Syne• Fig. 3. Model of a phycobilisome of the monocellular red algae, Rhodella viola- chococcus spec. 6031 ("Anacystisnidulans"){27*] and from other cea. The "core" is made up from three APC hexamers, the rods fixed to it ("tri• species1285 2861. The phycobilisomes of S. spec. 6301 appear in partite units") consist of one PC hexamer and two B-PE monomers, in this order when looking from the core (after E. Mörschel, W. Wehrmeyer, Ber. Dtsch. Bot. the electron microscope as aggregates of rods similar to those Ges. 92. 393 (1979)). of R. violacea (Fig. 3). Here, too, APC seems to be located at the coupling position, since the energy transfer from PC to APC is interrupted by the dissociation12781. Recently, the Within the phycobilisomes the different pigments are electron-microscopic characterization of phycobilisomes of densely packed in such order, that the absorption maximum two other cyanobacteria was achieved by using zwitterionic increases from the "outside" to the "interior" of the phyco- 51 detergents which are reported to inhibit the aggregation dur• bilisome. Gantt et a/.' developed a model for phycobili• ing purification12251. These investigations confirm the general somes from Porphyridium cruentum, which reminds one of an structural principle of Figure 3, with variations in the num• onion cut in half: layers composed of PE surround inner ber of the central APC-disks, the number and the length of layers of PC, which surrounds an APC-core. This arrange• 12781 ment was derived from dissociation experiments in buffers of the "branches" , and also by their arrangement in two {279 2 2] low ionic strength, whereby the pigments were released se• (Fig. 3) or three dimensions (P. cruentum * ). The phyco• quentially151. It was convincingly proved by fluorescence bilisomes of chromatically adapting cyanobacteria show 12871 spectroscopy12811 and immuno-electron-microscopy1281,2821. small but distinct differences in size and arrangement , as 12801 Using the latter method, APC could be shown to be localized well as in the pattern of the colorless proteins . The on that side of the phycobilisome facing towards the mem• "branches" of Tolypothrix tenuis increase in length in green 12881 brane12821. light . In this process, not only additional PE-units seem The architecture of phycobilisomes of the monocellular to be added, but also part of the PC is removed. red alga Rhodella violacea (see Fig. 3) is basically similar but Phycobilisomes are extraordinary efficient antennas which perhaps even more impressive, due to their flatness1223,2241. absorb light by a high effective cross-section, and transfer the Six conspicuous rod-shaped stacks are attached to the APC- excitation energy to the reaction-centers. This energy trans• 12241 11 core . These rods can be isolated by careful dissocia• fer has quantum yields up to 10096 «.143.289,2901 AND IS ONLY tion12231. By electron-microscopic inspection, they appear as decoupled under starving conditions (light12011, N(2921). The stacks of three small double-disks ("tripartite units"). The transfer mainly occurs to photosystem II (PS II), as was origi• double-disks have the dimensions and fine structure of hex- nally concluded from the bichromatic action-spectra'3,41. americ C-PC (αβ)6 or of the fundamentally similar B-PE Heterocysts contain no PS II and have been regarded as be• 12931 3231 (a6ßey). The former are supposed to be identical with the C- ing free of phycobilisomes (see, however, ' ). Recently, PC-hexamers which have been observed during the in-vitro Katoh and Gantt[294] isolated photosynthetic vesicles with the association or during the crystallization (homoaggregates, see phycobilisomes still bound, showing PS II activity. This ac• Section 4.3). A further dissociation of the rods and an analy• tivity decreases in parallel with the dissociation of the phyco• sis of the fragments revealed that two adjacent double-disks bilisomes (induced by a decrease in ionic strength). The cou• consist of PE, the final one of PC, j. e. they form a heteroag- pling between the phycobilisomes and the reaction centers is gregate of two different biliproteins'2231. Such heteroaggre- variable and depends on the physiological conditions12951. gates of PC and PE were recently also isolated from P. cruen- For Anabaena variabilis, grown heterotrophically in dark• iwm-phycobilisomes and from cyanobacteria1240,283" 2841. ness, however, an energy transfer to PS I was reported, and They show an excellent energy-transfer from PE to PC (re• recently in PS I samples from Chlorogloea fritschii{29(y\ fluo- versible dissociation, see '283*1). A reconstitution of phycobili• rescence-spectroscopic evidence was obtained for the occur• somes starting with the isolated biliproteins is not (yet) possi• rence of APC. APC fluoresces similarly to certain chloro- Angew. Chem. Im. Ed. Engl 20, 241-261 (1981) 255 phylls12941, but direct support comes from the results cited in phores of APC, the biliprotein present in least amount, are ref. <3231. the only ones to fluoresce, and all the remaining chromo One reason for the efficient energy transfer is the organi phores serve as sensitizers, with a transfer-probability of "at zation of the phycobilisomes. This ensures that the energeti least 99%" in phycobilisomes of P. cruentuml290K cally favored "downhill" transport of excitation-energy from Cryptophytes possess biliproteins as antenna-pigments, the "high-energy PE" to the "low-energy APC" is directed at too, but they are distinct from those of cyanobacteria and red the same time from the PE periphery to the APC-core of the algae (for a recent review, see ,298]). Cryptophytan bilipro phycobilisome. Possibly, a recently described "foot struc teins are localized on the inner side of the thylacoid mem ture"12791 serves for further transfer into the membrane and brane12991. Hitherto, the search for phycobilisomes has met for the fixation of the phycobilisomes12791. A second factor is without success, although Wehrmeyer et Λ/.13001 recently ob the intense absorptions of the biliproteins (%105 tained electron-microscopic evidence of particulate struc cm2 χ mol""1) which cover almost the entire spectral region tures which may contain biliproteins. The energy transfer in from about 500 to 670 nm (cf. 15]), combined with a high de cryptophytes must also be different, since no APC has so far gree of fluorescence (= low radiationless deactivation) of the been detected, and generally they only contain either PC or chromophores11421 and distances of 3.5—6 nm between the PE. As in the biliproteins of cyanobacteria and of red al- 1139 142 2971 chromophores < . These distances are optimal in a gae134-297-3001, only the chromophore lowest in energy fluores disordered structure for the energy transfer between suitable ces. In accord with the exciton-coupling between the chro chromophores by the Förster-mechanism, because they are mophores postulated by Jung et al.[34\ Kobayashi et al.[29n re below the critical radius. As has recently been described for ported an extremely fast transient (energy-transfer) (^ 8 ps) C-PE, the quantum yield of fluorescence increases by aggrega• in PC of Chroomonas. The transfer from K-PC to chloro 11391 tion of isolated subunits to the monomer . It increases phyll a by the Förster-mechanism appears unproblematical, even more during aggregation of the monomers to tri- and especially in pigments like PC-645 from Chroomon- 114 1 11421 [ y hexamers " , or to the quasi-hexamer B-PE . At the same «89.3001 and a5P4, pQ_$4x from Hemiselmis virescens * \ ab• 1139 ,42 297 time the fluorescence-polarization · ΐ and the time sorbing and emitting at rather long wave-lengths. In contrast, 12971 constant of the energy transfer decreases . the emission-band of PE is located in a region of minimal ab• Due to the rapid migration of energy and the greater prob sorption of chlorophyll a, so that either only incomplete en• ability of a "downhill"-transport of energy, the fluorescence ergy-transfer is possible, or another pigment has to be posi• 11391 in each aggregate is emitted almost exclusively from the tioned between them13011. Indeed, cryptophytes contain possi• chromophores, absorbing at the longest wavelengths, which ble candidates, e. g. chlorophyll c. were thus classified as "f'-chromophores1'401. The '^"-chro According to the data summarized above, the morphology mophores absorbing at shorter wavelengths act as sensitizers, of the antennas and the structure of the biliproteins would and only exceptionally show a detectable fluorescence11391. seem to be closely related. Each biliprotein of the cyanobac• During the gradual association of phycobilisomes, this trans teria covers only a comparably small portion of the spec• fer chain is extended successively to yield "Forster-cas- trum. Only APC has properties favorable for energy transfer cades" Due to their architecture, each pair of neighboring to chlorophyll; an energy transfer of PE to chlorophyll re• pigments provides for an optimal overlap of the emission quires the aggregation of several pigments. This disadvan• band of the donor and the absorption band of the acceptor tage is compensated for by the flawless architecture of the (Table 4). This process was demonstrated for biliproteins with phycobilisomes, which enables a vectorial energy flux direct different chromophores1140'142,2971, of heteroaggregates1223 253J, to the reaction-center of PS 11. In the cryptophytan phyco• and of whole phycobilisomes1143,289,2901. In the intact phyco cyanins, such energy transfer-chains are already realized bilisome, the result of this process is such, that the chromo- within the individual biliproteins. Chroomonas-PC, for ex• ample, contains chromophores absorbing in the region from 573 to 652 nm132*1891, so a higher degree of organization would seem unnecessary. Finally, in the red algae both strat• egies are combined. They contain phycobilisomes, as well as Tablc 4. Schematic representation of energy transfer within a phycobilisome like the one shown in Fig. 3 (only one tripartite unit and one APC are considered), the pigments R-PC, R-PE and B-PE, each of which contain and of the absorption and emission spectra of the phycobiliproteins (averaged different chromophores already joined to an energy transfer- absorption maxima, the emission maxima of "s"-chromophores have been esti mated from the absorption maxima by assuming a Stokes-shift of 15 nm). chain of moderate length. In addition to the well documented light-harvesting func• Pigment Chromophore Absorption Fluorescence [nm] [nm] tions, other functions of phycobiliproteins are discussed. Un• der conditions of deficient Nni and Si302i, phycobiliproteins P-BE PUB [a] 500 ^^j. 515 are degraded as protein-reserves (?). In cyanobacteria in par• "s" (2) 545 ^r^Z 560 "Γ (2) 570 ^-^^ 585 ticular they constitute a considerable fraction of the total protein, and basically they are of less importance for photo• C-PC "s" (la) 590 - ^ .. 605 "f (la) 620 635 synthesis than the reaction centers. They also may have func• tions in light protection, since mutants free of biliproteins are APC (fa) 650 665 more sensitive to light than the wild-types. Chla„ 686 Finally, there is substantial evidence for phycobiliproteins, 1 also having or having had functions in the electron trans• photochemistry fer1303,30^. A corresponding function in vivo has as yet not [a] PUB - phycourobilin chromophore. been detected, and may have been lost during evolution. 256 Angew. Chem. Int. Ed. Engl 20. 241-261 (1981) Electrodes covered with biliproteins show photopoten- this is not the case'1131, they regard this result as support of tials'171,3051; moreover, PC catalyzes electron-transfer through the conformational change of the protein, discussed by synthetic lipid-membranes'173,3041. In both cases the action Smith™ as a primary signal. Finally, a third hypothesis pos spectra and absorption spectra are similar. PC undergoes tulates a reversible redox-reaction between protein and chro specific interactions with Fe3+, but not with Fe2*, and it mophore with the formation of a new chromophore-protein shows an asymmetric effect on the electron transfer of bond'1061. Each of these hypotheses is able to explain modifi charged synthetic membranes loaded asymmetrically with cation of membrane-properties, directly by redox and/or that pigment1306'. The redox potentials of free bile pigments protonation-deprotonation processes, and indirectly by con have been calculated theoretically'95,3071, and the chromo formational changes by uncovering receptor binding sites, phores of native biliproteins have been shown to be much but a differentiation is not yet possible. more stable towards redox reagents than those of the denatu rated pigments135,1291. 5.3. Phycochromes and Phycobiliprotein Biosynthesis The biosynthesis of the phycobiliprotein-chromophores 5.2. Phytochrome follows fundamentally similar pathways as does the forma The function of phytochrome is that of a light-sensory pig tion of the more thoroughly investigated mammalian bile ment of green plants. In the transformation from pigments. δ-Aminolevulinic acid is condensed to a cyclic te- heterotrophic etiolated growth in higher plants, e. g. in seed trapyrrole, most probably protoporphyrin, which is subse lings below the surface, to autotrophic, photosynthetic quently opened oxidatively resulting in formation of a bile 311 growth, phytochrome mainly functions as a sensor of light as pigment with loss of the former C-5 as carbon monoxide' 3131 such, in the "high-energy reactions" phytochrome functions . The ring-opening process is formally and mechanistical ly similar'3141 to the heme-oxygenation'3151, but chemical evi as a sensor of light-intensity, and finally due to the Pr and Pfr equilibrium being dependent on the spectral distribution of dence would indicate that the ring-opening of a Mg-porphy- the light especially within the photosynthetically important rin via 7,8-dihydroporphinatomagnesium may also be possi 1611 red spectral region, it renders possible a "color-vision" in ble' . It is also not clear, whether the apoprotein is bound green plants. Whereas a large variety of physiological and to the chromophore after (and not before) the ring-opening, some structural aspects of phytochrome have been investi and whether it adds to the 3-ethylidene group of bilindione gated in considerable detail19,1416 lfq, knowledge of the (4) (and not to the vinyl-groups of a precursor). A hint as to 3121 mechanism of information-transfer and -transduction is as the alternatives, not put in brackets here, is the finding' yet only fragmentary. The models are difficult to evaluate in that Cyanidium caldarium excretes (4) as well as addition- this context and shall only be outlined here. For further in product(s) of (4) in the dark'S4t 3,6J. Another indirect indica formation, the reader is referred to recent re tion is the facile and reversible addition of nucleophiles to 54 5 5 9, , 4,16b l7, tH 2! 303 309 views' ' - ' - I (4) and ^t ^ «- ^ ancj very recently Troxler et al (private communication, 1980) have demonstrated the uptake of Phytochrome is synthesized in its physiologically inactive heme and its conversion into phycocyanin in cyanobacterial form, Pr; the beginning of each physiological reaction se protoplasts. quence is the photochemical transformation into Pfr. The for The biliprotein-synthesis in most of the cyanobacteria and mation of Pfr is fundamentally and nearly completely reversi ble by light >740 nm; this is not true, however, for all steps at least in some red algae is regulated by light. It is of special of the subsequent reaction-sequence. The in vivo decomposi interest that the antenna-pigments adjust to the quality of light available ("chromatic adaption"). In prevailing red tion of Pfr or Pr-receptor complexes is a simple example (cf. ii4.»^ light, especially the blue phycocyanins are formed, whereas eg Depending on the duration of the reaction (τΓβν) until an irreversible step is reached, the physiological answer in green light, e. g. under a canopy of leaves or in deeper is reversible (or annulled) only for a certain time. Since the waters, the red phycoerythrins are preferentially formed'1,11 2X73171 . This effect has been investigated mainly time (τΓβν) of escape from reversibility varies among the di 1 2 3m verse physiological answers by several orders of magnitude, with Tolypothrix tenuis * and with Freymella displosi- the phytochrome answers have been classified into modula phonl3i9'320h Based on the action spectra, the chromatic adap tion has been explained in terms of photochromic receptor tion (large Trev-value="reversible") and differentiation proc pigment-systems, functionally termed adaptochromes, or esses (small TREV-value = "irreversible") (for recent discus sions, see |1K,30X1). A variety of biochemical and physiological chromes. There are also photomorphogenetic effects in results critically summarized by Marmel309] have indicated, cyanobacteria and red algae which are connected with such at least for the modulation reactions, reversible changes of receptors. Little is known about receptor pigments (see Sec membrane properties to be operative. tion 4.4), and virtually nothing about their mechanism of ac tion. Based on the hitherto known differences between Pr and Pfr (see Section 4.4) and on the photochemistry of model-sys tems (see Section 3.3), several hypotheses have been pro 6. Concluding Remarks posed for the primary reactions. Song et α/.12631 postulated a light-induced photoisomerization'401 by proton-transfer, by For a long time phycobiliproteins have been the victims of which a receptor site formerly covered by the chromophore "mammalian chauvism" and, owing to their covalent protein becomes accessible. Hunt and Prattlu3] argue, that chemical bonds, have been much less investigated than the other tetra- modifications of amino acids newly accessible by this trans pyrrolic pigments of photosynthesis, the chlorophylls. In formation should interfere with the photoreversibility. As contrast to the latter, however, the phycobiliproteins have Angew. Chem. Int. Ed. Engl. 20. 241-261 (1981) 257 the great advantage of not being integrated into membranes. [15] A. N. Glazer, Mol. Celt. Biochem. 18, 125 (1977). This property and the discovery of the central function of an• [16] a) 7*. W. Goodwin: Chemistry and Biochemistry of Plant Pigments. 2nd Edit. Academic Press, New York 1976; b) H. Smith, R. E. Kendrick in other biliprotein, phytochrome, during the development of [16a], p. 378. plants, recently led to a boom in biliprotein research. No oth• 117] Κ Μ. Hartmann, W. Haupt in W. Hoppe, W. Lohmann, Η. Mark, Η. Ziegl er: Biophysik. Springer. Berlin 1978, p. 449. er photosynthetic light-harvesting system is as well investi• [18] a) W. Rüdiger, Struct. Bonding (Berlin) 40, 101 (1980); b) W. Rüdiger, H. gated as the phycobilisomes, and—with the exception of rho- Scheer in W. Shropshire, Jr., Η. Mohr: Encyclopedia of Plant Physiology. dopsin—no light-sensory pigment is as well characterized as New Series. Springer, Berlin, in press. [19] H. Scheer in [13d], p. 25. phytochrome. [20] R. T. Troxler in P. D. Berk, N. /. Berlin: Chemistry and Physiology of Bile In this report an attempt has been made to correlate the Pigments. U. S. Dept. of Health, DHEW Publ. No. (NIH) 77-1100, Wash properties of the isolated chromophores with the function of ington, D. C. 1977, p. 431. [21] W. R. Briggs, Η. V. Rice, Annu. Rev. Plant Physiol. 23, 293 (1972). the pigments in vivo. The correlation is somewhat subjective [22] P. OXarra, C O'hEocha in [16a], p. 328. and partly fragmentary, but has been borne out for the most [23] a) D. S. Berns in S. N. Timasheff, G. D. Fasman: Subunits in Biological part in recent years. Some of the obvious gaps in the case of Systems. Dekker, New York 1971, Part A, p. 105; b) R. McColl, D. S. Berns, Trends Biochem. Sei. 4, 44 (1979). phytochrome are the structure of Pfr and the closely related [24] a) D. Dolphin: The Porphyrins. Academic Press, New York; b) A. Bennett, questions regarding the nature of the primary signal and the H. W. Siegelman in [24a], Vol. VI, 1979, p. 493. role of specific interactions between phytochrome and cer• [25] B. J. Chapman in [11a], p. 162. [26] G. Blauer, Struct. Bonding (Berlin) 18, 69 (1974). tain receptor membranes and organelles. An advancement in [27] a) M. Choussy, Μ Barbier, Helv. Chim. Acta 58, 2651 (1975); b) M. Bois- this area should certainly stimulate further investigation of Choussy, M. Barbier, Heterocycles 9, 677 (1978). [28] W. H. Bannister, J. V. Bannister, H. Micaleff, Comp. Biochem. Physiol. 35, the related phycochromes as well. 237 (1970). Many of the numerous questions arising from the func• [29] a) W. Rüdiger, Naturwissenschaften 57, 331 (1970); b) Hoppe-Seylers Z. tionally and morphologically impressive model of the phyco• Physiol. Chem. 348. 129 (1967). [30] T. Ogata, N. Fusetani, K. Yamaguchi, Comp. Biochem. Physiol. Β 63, 239 bilisomes will only be possible to answer by the collaboration (1979). of biophysicists and biochemists. Details on the chromo- [31] A. N. Glazer. C. S. Hixson, J. Biol. Chem. 252, 32 (1977). phore-protein interactions and the interrelationships be• [32] D. A. Bryant, A. N. Glazer, F. A. Eiserling, Arch. Microbiol. 110, 61 (1976). tween the biliprotein-chromophores and the chlorophylls [33] A. N. Glazer, G. Cohen-Bazire, Arch. Microbiol. 104, 29 (1975). within the photosynthetic membrane are essential for an un• [34] J. Jung, P.S. Song, R. J. Paxton, M. S. Edelstein, R. Swanson, Ε. E. Hazen, Jr., Biochemistry 19, 24 (1980). derstanding of the energy transfer on a molecular basis. The [35] S. Grombein, W. Rüdiger, Η. Zimmermann, Hoppe-Seylers Ζ. 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COMMUNICATIONS Communications are brief preliminary reports of research work in all areas of chemistry which, on account of its fundamental significance, novelty, or general applicability, should be of interest to a broad sr^rum;;of chemists. Authors of communications are requested to state reasons of this kind justifying (I) (2) (3) publication on submission of their manuscript. The f,bI same reasons should be clearly apparent from the structure , and also to problems of contemporary corrin 131 manuscript. In cases where the editorial staff decide, biosynthesis . after due consultation with independent referees* The tautomerization of octaethylporphyrinogen (4) under that these conditions are not met, manuscripts will strict exclusion of oxygen produces nickel complexes of type 1 fal be returned to the authors with the request to submit (3), as described earlier . This transformation proceeds fas them for publication in a specialist journal catering ter, and gives different products, if instead of triethylamine for scientists working in the field concerned the guanidine derivative l,5,7-triazabicyclo[4.4.0]dec-5-ene (TBD)l4i is used as the base for tautomerization. Under the conditions shown in Scheme 1, (4) forms a mixture of nickel complexes, which apart from didehydrogenated compo- nents{5al consists mainly of the diastereomers (5) (see Table Interconversion of the Chromophore Systems 1). After anaerobic column chromatography on silica gel, a of Porphyrinogen and 1 1 total of seven diastereomers were discernible by high pres 2,3,7,8,12,13-Hexahydroporphyrin ^ sure liquid chromatography (HPLC); three of them could be By Jon Eigill Johansen, Virginia Piermattie, Christof Angst, separated preparatively by HPLC on silica gel and crystal Eva Diener, Christoph Kratky, and Albert Eschenmoserl*] lized, the major components being tctcc-(5)[Sb] and tctct- Dedicated to Professor Hans Herloff Inhoffen on the (5){5c] . The assignment of configuration for tctct-(5) followed l occasion of his 75th birthday from the molecular symmetry evident in the H-NMR spec trum, as well as from the spontaneous didehydrogenation in Our search for a non-oxidative isomerization of porphy• air to the known'1*-*1 Ni2+-isobacteriochlorinate tct-(6) and a rinogens (1) to the corphinoid ligand system of 2,3,7,8,12,13- Ni2+-bacteriochlorinate {ttc-(7), Table 1). The correspond hexahydroporphyrin (2) led at first to structures of type ing didehydrogenation of tctcc-(5) gives as the main product ,,a,bl fij , in which the cyclic conjugation of the chromophore tct-(6){t) and a new (therefore not the reconfiguration11*-*1) double bonds is broken, and not to (2). We have now been isobacteriochlorinate, which must have the configuration tcc- able to convert a porphyrinogen into the ligand system (2), (6) . X-ray structure analysis of tctcc-(5) (Fig. 1) again reveals about which only little is known so far121. This structure is of the specific macro-ring deformation, which was previously interest in relation to the problem of the origin of the corrin (*] Prof. Dr. A. Eschenmoser, Dr. J. E. Johansen, Dr. V. Piermattie, Dipl. Naturwiss. ΕΤΗ Ch. Angst, Dipl. Naturwiss. ΕΤΗ Ε. Diener Laboratorium für Organische Chemie Eidgenössische Technische Hochschule ETH-Zentrum Universitätstrassc 16, CH-8092 Zürich (Switzerland) Dr. Ch. Kratky Institut iür Physikalische Chemie der Universität Heinrichstrasse 28, A-8010 Graz (Austria) (**] This work was supported by the Swiss National Science Foundation, the Austrian National Science Foundation and the Austrian Academy of Fig. 1. Crystal structure of tctcc-(5). Projection oblique to the ligand plane, with Sciences. We thank Professor Η. H. Inhoffen for the donation of octaethyl- ring D in foreground. Positions of the hydrogen atoms are calculated, vibrational porphyrin. Dr. £. Zass for assistance in completing the manuscript, and Dr. ellipsoids of non-hydrogen atoms with 50% probability (cf. also Table 1 and [tc]. J. Schreiber for his advice on the use of HPLC. Fig. 3). Angew. Chem. Int. Ed. Engl. 20 (1981) No. 3 © Verlag Chemie, GmbH, 6940 Weinheim, 1981 0570-0833/81/0303-0261 $ 02.50/0 261