SERIAL REVIEW CAROTENOIDS 2 Genetics and molecular biology of carotenoid pigment biosynthesis GREGORY A. ARMSTRONG,’1 AND JOHN E. HEARSTt 5frtitute for Plant Sciences, Plant Genetics, Swiss Federal Institute of Technology, CH-8092 Zurich, Switzerland; and tDePai.tment of Chemistry, University of California, and Structural Biology Division, Lawrence Berkeley Laboratory, Berkeley, California 94720, USA The two major functions of carotenoids in photosyn- carotenoid biosynthesis from a molecular genetic thetic microorganisms and plants are the absorption of standpoint.-Armstrong, G. A., Hearst, J. E. Genet- energy for use in photosynthesis and the protection of ics and molecular biology of carotenoid pigment chlorophyll from photodamage. The synthesis of vari- biosynthesis. F14SEBJ. 10, 228-237 (1996) ous carotenoids, therefore, is a crucial metabolic proc- ess underlying these functions. In this second review, Key Words: phytoene lycopene cyclization cyclic xanthophylLs the nature of these biosynthetic pathways is discussed xanlhophyll glycosides’ 3-carotene provitamin A in detail. In their elucidation, molecular biological techniques as well as conventional enzymology have CAROTENOIDS REPRESENT ONE OF THE most fascinating, played key roles. The reasons for some of the ci.s-t Tans abundant, and widely distributed classes of natural pig- isomerizations in the pathway are obscure, however, ments. Photosynthetic organisms from anoxygenic photo- and much still needs to be learned about the regula- synthetic bacteria through cyanobacteria, algae, and tion of carotenoid biosynthesis. Recent important find- higher plants, as well as numerous nonphotosynthetic ings, as summarized in this review, have laid the bacteria and fungi, produce carotenoids (1). Among groundwork for such studies. higher plants, these pigments advertise themselves in flowers, fruits, and storage roots exemplified by the yel- -James Olson, Coordinating Editor low, orange, and red pigments of daffodils, carrots and to- matoes, respectively. In green plant tissues, carotenoids ABSTRACT The crucial roles of carotenoids and become evident only during the annual degradation of their metabolites in photooxidative protection and chlorophyll in the autumn. Both unmodified and metabo- photosynthesis, not to mention nutrition, vision, and lized dietary carotenoids also serve as natural colorants cellular differentiation, make them an important and in organisms that are not themselves carotenogenic, such complex class of biological pigments. Significant ad- as crustaceans, insects, fish, and birds (2). A few promi- vances within the last few years have enhanced our nent examples include the pigments found in egg yolks, understanding of the genetics and molecular biology lobster shells, salmon flesh, and flamingo plumage. of carotenoid biosynthesis in bacteria, fungi, algae, Wackenroder first proposed the name “carotene” in and plants. All of the genes involved in carotenoid 1831 in describing the pigment he had isolated and crys- biosynthesis from Rhodobacter capsulatus, an tallized from carrot roots. In 1837, Berzelius coined the anoxygenic photosynthetic bacterium, and from sev- term “xanthophyll” to denote chemically a yellow pigment eral species of Erwinia, nonphotosynthetic bacteria, he had extracted from senescent leaves. Tswett, recogniz- have been molecularly characterized. Recent studies ing the chemically related nature of the compounds have revealed that two early enzymes of carotenoid known as carotenes and xanthophylls, created the desig- biosynthesis, geranylgeranyl pyrophosphate synthase nation “carotenoids” in 1911 to encompass both classes of and phytoene synthase, are structurally and function- pigments (3). Today, “carotene” is used to refer to a hy- ally related in all carotenogemc organisms. In con- drocarbon carotenoid, and xanthophyll denotes a carotene trast, the subsequent conversion of phytoene, the derivatized with one or more oxygen-containing functional first C40 carotenoid, to p-carotene requires two de- groups. A century and a half after Wackenroder’s isola- saturases and one cyclase in oxygenic photosynthetic tion of the first carotene, 563 structurally distinct carote- organisms (cyanobacteria, algae, and higher plants) noids and their glycosides, not to mention isomers, had but only one structurally distinct desaturase and a been chemically characterized (4) and approximately 60 structurally distinct cyclase in other carotenogemc new structures have since been described. bacteria and in fungi. Studies of the enzymes that introduce oxygen-containing functional groups into carotenes to produce xanthophylls, the vast majority 1To whom correspondence and reprint requests should be addressed, of all carotenoids, are still in their infancy. This at:InstituteforPlantSciences,Plant Genetics,Swiss Federal Institute of review summarizes the most recent developments in Technology (ETH), CH-8092 Zurich, Switzerland. 228 Vol. 10 February 1996 0892-6638/96/001 0-0228/SO 1.50.© FASEB SERIAL REVIEW STRUCTURES OF CAROTENOIDS Eubactena, fungi, Most carotenoids contain a linear C4o hydrocarbon back- higher plants bone that includes between 3 and 15 conjugated double H2O bonds (1, 5, 6). The number of double bonds largely de- d2 DMAPP ‘PP termines the spectral properties of a given carotenoid, which typically absorbs light between 400 and 500 nm. A lPP critical step in the formation of the first C10 acyclic hy- drocarbon carotenoid, phytoene, is the tail-to-tail conden- H2O sation of two molecules of the C20 intermediate GPP geranylgeranyl pyrophosphate (GGPP).2 This molecule GGPP synthase arises from the head-to-tail condensations of four C5 iso- ‘PP prene units derived from the general isoprenoid biosyn- thetic pathway (Fig. 1). Some organisms produce A4.,#{176}#{174} -0 Sterols partially degraded pigments known as apocarotenoids or FPP norcarotenoids, and a few bacteria synthesize C45 or C50 IPP carotenoids by further isoprene additions to the C40 back- bone (1, 5, 6). C30 carotenoids synthesized by the tail-to- Chiorophylls -0’ Quinones tail condensation of two molecules of the C15 isoprenoid G,bereIIins intermediate farnesyl pyrophosphate (FPP), rather than GGPP GGPP, also occur in some bacteria (7). In general, how- GGPP - ever, phytoene serves as the classical precursor for other carotenoids. Most organisms, particularly higher plants, Phytoene c2o .t.#{174} synthase algae, and fungi, synthesize 15, 15’-cis-phytoene, al- though some microbes produce mixtures of isomers that L1 U CU3 can include all-trans-and 9-ciz-phytoene (5, 6, 8). A series of desaturation and cyclization reactions con- PPPP 4, verts phytoene into cyclic carotenes, such as n-carotene (Fig. 2). The successive introduction of conjugated dou- Phytoene ble bonds during this process lengthens the chromophore, producing colored carotenoids beginning with c-carotene. Figure 1. Biosynthesis of phytoene from the general isoprenoid biosyn- Lycopene, cyclic carotenes, and xanthophylls usually ex- thetic pathway. Intermediates through GGPP serve as precursors for other ist in the all-trans configuration, indicating that at least important compounds. The conversion of two molecules of GGPP to one isomerization step must occur (1, 5, 6). The cis to phytoene, which occurs predominantly as the 15, 15’-cis isomer but is all-trans conversion does not, however, appear to require shown here as the all-trans isomer for convenience, is the first reaction a distinct isomerase activity (9-12). unique to carotenoid biosynthesis. Prephytoene pyrophosphate (PPPP) is Carotenoid biosynthesis as a whole can be roughly an unstable intermediate. GGPP synthases are structurally conserved in both prokaryotic and eukaryotic carotenogenic organisms, but can differ thought of as an inverted tree, with the trunk representing in their substrate specificities (dotted line). Phytoene synthases from the early reactions common to all organisms and the variousorganisms arealsostructurallyconserved.Genetic lociassociated branches symbolizing the remarkable variety of xantho- with specific enzymatic functions are listed in Table 1. phylls that arise by the species-specific introduction of oxygen functionalities into acyclic (Fig. 3) and cyclic a varietyof carotenes (Fig. 4). These oxygen functionalities, repre- pigments, carotenoids perform essential biologi- cal functions. Universally, colored carotenoids provide pho- sented by hydroxy, methoxy, oxo, epoxy, carboxy, and al- tooxidative protection against the effects of singlet oxygen dehydic groups, thus provide most of the basis for the and radicals generated in the presence of light and endo- structural diversity observed among the carotenoids (1, genous photosensitizers such as chlorophylls, heme, and 4). protoporphyrin IX (13). During photosynthesis carotenoids can transfer absorbed radiant energy to chlorophyll mole- FUNCTIONS OF CAROTENOIDS cules in a light-harvesting function, dissipate excess energy via the xanthophyll cycle in higher plants and certain algae, In addition to their obvious role as visually attractive natural and quench excited-state chlorophylls directly (14, 15). Re- cently, the structural role of carotenoids as the molecular glue of certain photosynthetic pigment-protein complexes has be- come evident (16, 17). p-Carotene and structurally related 2Abbreviations: DMAPP, dimethylallyl pyrophosphate; FPP, farnesyl compounds serve as the precursors for vitamin A, retinal, and pyrophosphate; GPP, geranylpyrophosphate; GGPP, geranylgeranylpy- rophosphate;