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Coordination Compounds in Biology R -ESONANCE--I -JU-Ne--1-9-99

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Coordination Compounds in Biology R -ESONANCE--I -JU-Ne--1-9-99 GENERAL I ARTICLE Coordination Compounds in Biology The Chemistry of Vitamin B12 and Model Compounds K Hussain Reddy Vitamin Bl2 is an important coordination compound in biology. It is an interesting biomolecule in the sense that no other vitamin contains a metal ion. This is the only naturally occurring organometallic compound found in biology. An intriguing aspect of vitamin Bl2 is the great stability of the metal-carbon bond. A great deal of new and K Hassain Reddy is interesting inorganic chemistry has been uncoveredwhlle Reader in Chemistry at Sri studying systems pertinent to BI2• In this article some Krishnadevaraya University where he has salient features of this unique molecule (B 12) and its model compounds are presented. been teaching since 1983. He teaches bioinorganic chemistry and structural Vitamin B12 is one of the naturally occurring coordination inorganic chemistry. His compounds in biology. Some of the other important examples research interests are in . are chlorophyll, haemoglobin, myoglobin and cytochromes. The the Chemistry of model compounds of common feature in these biomolecules is that a metal ion is biochemical relevance. enclosed in a macrocyclic ligand. Although vitamin B12 · is He did post-doctoral certainly the most complex non-polymeric compound found in research in the University nature, it is devoid of protein structure making its biological role ofCalifomia at San Diego during 1989-90. relatively easy to understand. Vitamin Bll is known to be present in plants, animals and also in bacteria. It plays an important role in the metabolism of nucleic acids and in protein synthesis. It is of critical importance in the reaction by which residues from carbohydrates, fats and proteins are used to produce energy in living cells. In humans, deficiency of vitamin Bll leads to In humans, pernicious anaemia. In this article, the structure and reactivity deficiency of of vitamin B12 and its analogues are described. Attempts to vitamin 8 leads model the complex biomolecule using simple coordinating 12 to pernicious compounds are discussed. anaemia. R-E-S-O-N-A-N-C-E--I -JU-n-e--1-9-99--------------~-------------------------------6-7 GENERAL I ARTICLE Box 1. Pernicious Anaemia Pernicious anaemia (relatively uncommon in tropical countries) is usually accompanied in mammals by the increased excretion ofmethylmanoic acid in urine. It occurs mainly between 45-65 years and affects females more frequently than males. One of the first signs of deficiency is the failure to form red blood cells hence the term anaemia. But the disease is not treated successfully by the methods that work for common iron deficiency, which explains the term 'pernicious' (serious) anemia. Today it is effectively controlled with hydroxocobalamin. It should be given parenterally in a dosage of 1000pg twice during the first week, then 1000 pg weekly for further 6 doses. The patient must have regular parenteral doses of hydroxocobalamin indefinitely (1000 pg intramuscularly every 3 months). for life. 1 For more on the life and contri­ butions of Dorothy Hodgkin, see Structure of Vitamin B12 Resonance, Vol. 3, No.6, 1998. The active component of liver extract was first separated and finally crystallized in 1948. In 1965, Dorothy Hodgkin Figure 1. Structural gloss­ determined the structure (Figure 1) crystallographicallyl. The ary of vitamin B12 deriva­ molecule is an octahedral cobalt (III) complex with a 15- tives membered 4-nitrogen ring ligand, called corrin, occupying the X=CN, Cyanocobalamin, equatorial plane. All the side chains of corrin are made of X= H 0, Aquocoba/amin 2 acetamide and! or propionamide groups. One of them is an (B12); , isopropanol phosphate residue attached to a ribose and finally X=CH3 Methylcoba/amin; X=5 '-Deoxyadenosy/, co­ terminated by 5, 6-dimethyl benzimidazole, which binds to the enzymeB12 " Co(lIl) ion . ....• ---.. HfJOC CONH2 x -~--------~---------------------~--------------R-E-S-O-N-A-NC--E-I--Ju-n-e--1-9-9-9 GENERAL I ARTICLE Figure 2. Structural com.. parison of corrin and por.. phyrin. Corrin Porphyrin The corrin ring resembles a porphyrin ring at first glance but is not fully conjugated and therefore quite different chemically. The structures of corrin and porphyrin are given in Figure 2. Besides th~ equatorial ligand, there are two axial ligands in most Bl2 derivatives. Derivatives of B 12 The various derivatives of B12 result most commonly from changes in the axial ligands bound to cobalt. Often it is conve­ nient to draw a greatly abbreviated structure for a B12 molecule using a rectangle for corrin ring (Figure 3) and a curve with arrow at the end to pendant axial base viz. a-5, 6-dimethyl benzimidazole nucleotide, which acts as the ligand from below the basal plane. When this group is the lower ligand, the molecules are usually referred to as cobalamins. Thus, if R = CN-, the molecule is cyanocobalamin. Cyanocobalamin is also properly called vi tamin Bl2• The presence of cyanide is an artefact of the isolation procedure and is of no biological significance. The cyanide complex is not active as the coenzyme. Other common cobalamins Figure 3. Abbreviated are methylcobalamin (R = CH3), aquocobalamin (R = H20), structure of vitamin 8 hydroxycobalamin (R = OH-), and 5' -deoxyadenosyl cobalamin 12 (R = 5'-deoxyadenosine). This latter compound is also often derivative. referred to as coenzyme B12• R Upon hydrolysis the benzimidazole side chain can be cleaved at the" phosphate to give a class of derivatives referred to as cobinamides. In cobinamides unless otherwise specified, a molecule of water has replaced the benzimidazole as the lower axial ligand. Just as with the cobalamins, the name ofcobinamide derivatives depends on the nature of the upper axial ligand. For R-E-S-O-N-A-N-C-E-I--Ju-n-e--J-9-99--------------~------------------------------- GENERAL I ARTICLE Scheme 1, Oxidation states of vitamin B 12' _ e- '-..... Co1/ / ~ example, methyl cobinamide, aquocobinamide. In some cases another ligand (usually cyanide) occupies the lower axial position. In such a situation one would refer to the molecule as dicyanocobinamide or aquocyano-cobinamide for R = CN- or H20, respectively. Finally, Co (III) can be reduced chemically or electrolytically to give derivatives with Co in lower oxidate states. The one electron reduction product is referred to as vitamin B12r and the two electron reduction produCt is referred to as vitamin Bl2s' (r stands for reduced and s for super-reduced). The oxidation states are given in Scheme 1. The most common reducing agents used to produce Bl2s include sodium borohydride, zinc dust in ammonium chloride and chromous ions (pH 9-10). It is possible to generate the inter­ mediate Bl2r species chemically with zinc in acetic acid or ascorbic acid or chromous ions (pH 5). 7 B12r is a low spin d complex. It contains one unpaired electron (in the 3d/ orbital) and is the only paramagnetic B12 derivative. 8 It is a S-coordin~te complex. The B12s form is a low spin d complex. It is diamagnetic, a potent nucleophile and an important intermediate in the synthesis of alkyl cobalamin derivatives. The coenzyme Bl2 is believed to be formed via nucleophilic (A The coenzyme 8 attack on adenosine triphosphate TP). In the presence of 12 diazomethane, the B form is converted to methylcobalamin, is believed to be 12s formed via Therefore an alkyl radical, a carbocation and a carbanion can all nucleophilic attack be produced (in vitro) in Bl2 chemistry. on adenosine R - Co(nI) ~ R + Co(II) (B12r) triphosphate R - Co(nI) ~ R+ + Co(I) (B12s) (ATP). R - Co(nI) ~ R- + Co(III) (B12a) __------.AA~AAA-------- 10 v V VV V v RESONANCE I June 1999 GENERAL I ARTICLE Co(tfen) Scheme 2. Alkyl transfer reaction Co(tfen)-N, N'-ethylene bis(4~4,4-tr"'uoro-1-methy/-3-oxobutylidene­ Box 2 iminato)coba/t(11) complex. CHJHzO)Co(acen)=N, N'-ethylene bis (1-methyl-3-oxobutylidene­ Alkyl transfer reactions imina to) methyl (aquo) cobalt (/II) complex. involving cobalt alkyls have recei ved considerable amount of attention due to In both bacteria and in liver, coenzyme B12 is most abundant, but a similar alkylation that smaller amounts of methylcobalamin also exist. Bacteria present occurs with mercury. in intestine synthesise these derivatives and supply to our body. Concern about environ­ mental pollution of mer­ Nature of the Carbon-Cobalt Bond cury virtually originated The successful determination of the molecular structure of from the incidence of Minamata disease during coenzyme Bl2 provided an unexpected surprise in that it contained 1953-1960 in Japan which an apparently stable carbon-cobalt bond. This is the only resulted in many deaths naturally occurring organometallic compound. An intriguing and babies with genetic aspect of the Bl2 system is the great stability of the carbon-cobalt defects. Undoubtedly a bond. Although alkyl-cobalt bond in alkyl cobalamins is major factor contributing surprisingly stable, it can be cleaved and transferred into a to the attention focused on number ofother chemical species. During the reaction involving this reaction is the current model systems in Scheme 2, the initial Co(ll) species is oxidized concern about environ­ to Co(III), while the original alkyl Co(III) species is reduced to mental mercury pollution. It has been demonstrated Co(II). The initial Co(H) species usually has an equatorial ligand that methylation ofmercury with more electron withdrawing groups. Therefore, this is an in B12 rich organisms is organometallic example of electron transfer reaction mediated responsible for this by group transfer. Some important reactions of methylcobalamin environmental problem. and aquocobalamin are presented in Scheme 3. R-E-S-O-N-A-N-C-E- .-I-J-u-ne---19-9-9---------------~------------------------------------n- GENERAL I ARTICLE Scheme 3. Reactions of R vitamin 8 12 derivatives [in 1 2, reactions and R = H20 and in the rest R =CHJ.
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