Carotene isomerase

Isomerisation of 15-c/5-phytoene to the all-/ra x configuration must occur during the desaturation steps, since most desaturated carotenes are in the all-trans form. The CRTI type desaturases appear to be able to carry out this isomerisation themselves (Fraser et al, 1992 Bartley etal, 1999), but mutants of PDS/ZDS-type organisms accumulate cis isomers of unsaturated carotenes, suggesting the presence of a separate isomerase (Clough and Pattenden, 1983 Ernst and Sandmann, 1988). Three recent publications have reported the cloning of a carotene isomerase (CrtlSO) from tomato (Isaacson et al, 2002), Arabidopsis (Park et al, 2002) and Synechocystis 6803 (Breitenbach... 

Breitenbach, J., Vioque, A., and Sandmann, G., Gene sll0033 from Synechocystis 6803 encodes a carotene isomerase involved in the biosynthesis of aU-E lycopene,... 

Fig. 5.3 Carotenoid biosynthesis in maize endosperm. Compounds IPP, isopentenyl pyrophosphate FPP, famesyl pyrophosphate GGPP, geranylgeranyl pyrophosphate DMAPP, dimethallyl pyrophosphate. Carotenoid biosynthetic pathway enzymes PSY, phytoene synthase PDS, phytoene desaturase ZDS, zetacarotene desaturase ISO, carotene isomerase LCY-B, lycopene beta cyclase LCY-E, lycopene epsilon cyclase HYD-B, beta-carotene hydroxylase HYD-E, alpha-carotene hydroxylase Isonrenoid biosynthetic pathway enzymes IPPI (IPP isomerase) GGPPS (GGPP synthase). Structures are not representative of the geometrical isomer substrates (e.g. Z-phytoene is a bent structure).

Cyclization of lycopene proceeds only after an d -trans lycopene is formed by the action of carotene isomerase (CRTISO) in nongreen tissue. In the photosynthetic tissues, this conversion is catalyzed by light and chlorophyll (acting as a sensitizer). Oxygenation of the cyclic carotenoids yields xanthophylls. Introduction of hydroxyl groups at positions 3 and 3 in p-carotene produces zeaxanthin. Zeaxan-thin epoxidase (ZEP) and violaxanthin de-epoxidase (VDE) act in tandem to regulate the formation of violaxanthin. Violaxanthin is next converted to 9-cis-neoxanthin, ABA precursor, by neoxanthin synthase. Lutein is mainly present in photosynthetic tissues, biosynthesized from a-carotene via catalysis by p- and E-hydroxylases. 

The analysis of the tangerine (t) mutation with orange fruit phenotype due to the accumulation of prolycopene enabled the identification of a carotene isomerase, CRTISO, which is responsible for the conversion of prolycopene to aVi-trans-lycopene [32]. The accumulation of all-irani-lycopene as the primary carotenoid in ripe tomato fruits enables the development of red color starting from the colorless precursor phytoene. 

Figure 3. Carotenoid biosynthesis in plants. Enzyme abbreviation El - phytoene synthase E2 -phytoene desaturase E3 - -carotene isomerase E4 - -carotene desaturase E5 - lycopene e-cyclase E6 - lycopene p-cyclase E7 -s-carotene hydroxylase E8 - P-ring hydroxylase E9 -p-carotene hydroxylase E10 -s-ringhydroxylase Ell - p-caroteneketolase.

ISAACSON T, RONEN G, ZAMIR D and HIRSCHBERG J (2002) Cloning of tangerine from tomato reveals a carotenoid isomerase essential for production of 3-carotene and xanthophyUs in plants . Plant Cell, 14, 333-42. 

Figure 73. The carotenoid biosynthetic pathway. Enzymes are named according to the designation of their genes Ccs, capsanthin-capsorubin synthase CrtL-b, lycopene-b-cyclase CrtL-e, lycopene-e-cyclase CrtR-b, b-ring hydroxylase, CrtR-e, e-ring hydroxylase DMADP, dimethylallyl diphosphate GGDP, geranylgeranyl diphosphate Ggps, geranylgeranyl-diphosphate synthase IDP, isopentenyl diphosphate Ipi, IDP isomerase Pds, phytoene desaturase Psy, phytoene synthase Vde, violaxanthin de-epoxidase Zds, z-carotene desaturase Zep, zeaxanthin epoxidase. (From van den Berg and others 2000.)...

Fig. 11.3 Simplified diagram of isoprenoid metabolic pathway relative to the conversions among carotenoids that have been genetically manipulated. Enzymes catalysing some key reactions are indicated by the following abbreviations IPI, isopentenyl pyrophosphate isomerase PSY, phytoene synthase PDS, phytoene desaturase (crti, gene from Erwinia uredovora) ZDS, -carotene desaturase LCYB, lycopene / -cyclase CRTR-b, /3-carotene 3,3 -hydroxylase CCS, capsanthin-capsombin synthase CRTO, /3-carotene ketolase.

ISAACSON, T., RONEN, G., ZAMIR, D., HIRSCHBERG, J., Cloning of tangerine from tomato reveals a earotenoid isomerase essential for the production of i— carotene and xanthophylls in plants.. Plant Cell 2002,14, 333-342. 

Table 2. Shows the current selection of -carotene and other desaturase and isomerase genes from plants, algae, fungi, cyanobacteria and bacteria. Clones are classified as genomic or cDNA, and some ESTs. GenBank accession numbers and EST ids are indicated.

Fig. 56.2 Carotenoid biosynthesis pathway in plants. Enzymes (with abbreviations) indicated are isopentenyl pyrophosphate isomerase (IPI), geranylgeranyl pyrophosphate synthase (GGPS), phytoene synthase (PSY), phytoene desaturase (PDS), zeta-carotene desaturase (ZDS), carotenoid isomerase (CRTISO), lycopene beta-cyclase (LCYB), lycopene epsilon-cyclase (LCYE), beta-ring carotene hydroxylase (CHXB), epsilon-ring carotene hydroxylase (CHXE), zeaxanthin epoxidase (ZEP), violaxanthin de-epoxidase (VDE), capsorubin-capsanthin synthase (CCS), neoxanthin synthase (NXS), 9-cis epoxycarotenoid dioxygenase (NCED), and carotenoid cleavage dioxygenase (CCD). (Source [101], drawn using KeGG pathway)...

---