Table

Distribution of Carotenoids in Nature

Living Organism

Higher plants

Algae

Bacteria

Fungi

Animals

Some Relevant Information

The same carotenoids are found in chloroplasts as integrants of the photosynthetic apparatus:

a- and P-carotene, lutein, zeaxanthin, violaxanthin, and neoxanthin Extraplastidial in oily drops in some gymnosperm leaves: rhodoxanthin in Cupressaceae and semi-P-carotenone in Taxaceae family In reproductive tissues: liliaxanthin in white lily, crocetin in Crocus sp. stigmas In flowers, highly oxygenated carotenoids, P-carotenes, and species-specific carotenoids

(e.g., eschscholzxanthin in poppies) In fruits high variability and species-specific carotenoids: capsanthin and capsorubin in Capsicum sp.; lycopene in tomato; 5,6- or 5,8-epoxycarotenoids in carambola; apocarotenoids in Citrus spp. High variability, carotenoids are specific of each class but the pattern for Chlorophyta is similar to those in higher plants Red alga Rhodophyta: a- and P-carotenes and their hydroxylated derivatives Pyrrophyta: pteridin, dinoxanthin, and fucoxanthin

Chrysophyta: epoxy-, allenic, and acetylenic-carotenoids (e.g., fucoxanthin and diadinoxanthin) Euglenophyta: euterptielanone

Chloromonadophyta: diadinoxanthin, heteroxanthin, and vaucheriaxanthin Chryptophyta: acetilenic carotenoids (e.g., alloxanthin, monadoxanthin, crocoxanthin) Phaeophyta: fucoxanthin

In photosynthetic bacteria. High variability in the carotenoid pattern: carotenes, with aromatic or P rings (Chorobiaceae and Chloroflexaceae) and aldehydes; most bacterial carotenoids are involved in photosynthesis but carotenoid sulfates (eritoxanthin and caloxanthin) are not

In nonphotosynthetic bacteria. Uncommon carotenoids can be found (e.g., C30 carotenoids in Staphylococcus, C45 and C50 in flavobacteria, C^ carotenoid glycosides in mycobacteria) Accumulate carotenes, mono- and bi-cyclic carotenoids but without £ rings Canthaxanthin obtained from Cantharellus cinnabarinus is the most acknowledged carotenoid

In birds, yellow or red feather color is associated with carotenoids; fish (e.g., flesh of salmon and trout) and marine invertebrates (e.g., shrimps, crabs, lobsters) have astaxanthin and related carotenoids

Source: Adapted from Britton (1996)1 and Delgado-Vargas et al. (2000).2

in the membranes of photosynthetic bacteria in agreement with their lipophilic characteristic; their appearance in nonphotosynthetic bacteria is restricted and when they occur they have unique characteristics (Table 7.2).4 Fungi are nonphotosynthetic organisms and carotenoid appearance is capricious. The most common fungal car-otenoids are carotenes, mono- and bi-cyclic carotenoids but without £-rings. Can-thaxanthin is the most important fungal carotenoid and plectaniaxanthin has been reported in Ascomycetes.4

D. BIOSYNTHESIS: BIOCHEMISTRY AND MOLECULAR BIOLOGY

1. Biochemistry

Carotenoids are terpenoids, and they are produced by the isoprenoid pathway (Figure 7.4).29 A large number of structures are synthesized via this pathway. More than 29,000 in the plant kingdom have been identified with important metabolic roles: defensive toxins (sesquiterpene and diterpene phytoalexins), volatile defensive signals (monoterpenes and sesquiterpenes), photoprotectans (isoprene and caro-tenoids), pharmacological substances such as taxol (diterpene), monoterpene alkaloids (vincristine and camptothecin), and quinones that are involved in the redox processes. Interestingly, isopentenyl pyrophosphate (IPP) is a common precursor of this pathway (Figure 7.4A); thus sophisticated regulatory mechanisms must exist to ensure the organism functioning, whereas specific responses must be generated by developmental and environmental stimuli among other factors.10 Originally, it was believed that all isoprenoids were produced by using mevalonate (MVA) as precursor of IPP, but recently another pathway, which involves 1-deoxy-D-xylulose-5-phos-phate (DXP), was discovered (Figure 7.4A and B). This discovery is the most important of the last few years.11 On the other hand, the stages of the carotenoid biosynthesis have not been greatly modified (Figure 7.4B).

In animals and yeasts, the MVA pathway produces isoprenoids, and three enzymes are involved in the production of IPP, acetyl-CoA transferase, 3-hydroxy-3-methylglutaryl-coenzyme A (HMG-CoA) synthase, and HMG-CoA reductase (HMGR). HMGR is a highly regulated enzyme and uses NADPH as a cofactor. The production of IPP by the MVA pathway is shown in Figure 7.5A.2,12 The third reaction, MVA decarboxylation, requires ATP (adenosine triphosphate) and a divalent cation.10 On the other hand, MVA kinase activity was detected in plants but controversy arose regarding its subcellular localization. Later, the importance of compartmentalization in the biosynthesis of isoprenoids was corroborated. It has been shown by radioactive labeling (1-13C-glucose) that plastids (e.g., chloroplasts) of higher plants (e.g., Lemna gibba, Daucus carota, Hordeum vulgare) do not use the MVA pathway to produce carotenoids, phytyl, and plastoquinone. Pyruvate and glyceraldehyde-3-phosphate (from glycolysis) are used to produce the precursor of IPP (1-deoxy-D-xylulose-5-phosphate). Consequently, the existence of an alternative pathway for isoprenoid synthesis was demonstrated (Figure 7.5B). The DXP syn-thase (DXPS) enzyme has been cloned from different organisms and between them we have the plant DXPS obtained from pepper (C. annuum), plus Mentha piperita, Lycopersicon esculentum, and Arabidopsis thaliana. The DXP reductoisomerase (DXPR) was cloned from A. thaliana and M. piperita. The 4-diphosphocytidyl-2C-methyl-D-erythritol (DPME) synthase gene (ispD) was cloned from A. thaliana and the 2C-methyl-D-erythriol-2,4-cyclodiphosphate synthase gene (ispF) was cloned from M. piperita and tomato. In addition, it is supposed that the gene Lyt B, obtained from Adonis aestivalis, could be associated with the catalysis affecting the ratio IPP to dimethylallyldiphosphate (DMAPP).13

AcetoacetylCoA Pyruvate

Hydroximethyl 1 -Deoxy-D-xylulose-5-phosphate glutaril CoA

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