C30 Carotenoids: Biosynthesis, Chemical and Biological Aspects

Some of non-photosynthetic bacteria that belong to the phyla Firmicutes and Proteobacteria can produce acyclic carotenoids with a basic 30 carbon skeleton (C30 carotenoids). It has been shown that the final products of such C30-carotenoids often have a complicated structure, which adds D-glucose with/without an acylated aliphatic acid to the one terminus region. Here the principle is reported that the chemical structures of their aglycones converge on three types, i.e., 4,4′-diaponeurosporen-4-oic acid as staphyloxanthin-type, present in Staphylococcus aureus, 4,4′-diapolycopene-4,4′-dioic acid as dioic acid-type, observed in Methylobacterium, 5-hydroxy-5,6-dihydro-4,4′-diapolycopen-4′-oic acid (methyl ester) as high modification-type, present in Planococcus. This chapter reveals the biosynthetic pathway from farnesyl diphosphate (FPP) to the final C30-carotenoid metabolites by way of the three aglycones in a comprehensive manner. For example, biosynthesis from FPP to staphyloxanthin-type and to dioic acid-type aglycones needs the action of 4 gene products individually, which contain 3 enzymes common to each other, CrtM, CrtP/CrtNb and AldH/CrtNc, and CrtN (CrtNa) with different reaction products, 4,4′-diaponeurosporene and 4,4′-diapolycopene, respectively. In the case of the biosynthesis of high modification-type aglycone, two more gene products are required, i.e., 4,4′-diaponeurosporene 5,6-hydratase, named CruF, and 5-hydroxy-5,6-dihydro-4,4′-diaponeurosporene 7,8-desaturase, designated CrtNb, but properly better designated as CrtNd. Biological functions of such C30-carotenoids are lastly discussed.