Pathway okenone

The Rhodospirillaceae and Chromatiaceae have the spirilloxanthin or the okenone pathway depending on the genus or species. All of the Ectothiorhodospiraceae have the spirilloxanthin pathway. The isorenieratene, the y-and -carotene, and the diapocarotene pathways are found specifically in the Chlorobiaceae, Chloroflexaceae, and Heliobacteriaceae, respectively. Aerobic photosynthetic bacteria mostly have the spirilloxanthin pathway, further most of these species have unusual carotenoids including non-photosynthetic carotenoids, such as carotenoid sulfates and carotenoic acids, which have no photosynthetic functions. 

Okenone pathway (okenone, and. g.-keto carotenoid pathways). 

The Chromatiaceae have either the spirilloxanthin pathway (normal spirilloxanthin, unusual spirilloxanthin and carotenal pathways) or okenone of the okenone pathway except for one species, Tea. halophila, which has both pathways (Tables 1 and4). Even in the same genus, some species have the spirilloxanthin pathway and others have the okenone pathway. Therefore, the carotenogenesis pathways are not well related to the bacteria s classification in the Chromatiaceae. 

Fig. 7. The predicted pathway for the biosynthesis of okenone and f .g.-keto carotenoids.

A small amount of lycopene is found in Rpi. globiformis (Table 3 S. Takaichi, unpublished) and in the diphenylamine-inhibited cultures of Chr. okenii (Schmidt et al, 1963). However in diphenylamine-inhibited cultures of Rpi. globiformis, lycopene was not reported (Schmidt and Liaaen-Jensen, 1973). Although small amounts of the intermediates of phytoene desaturase are usually found in photosynthetic bacteria, if the activity of the C-1,2 hydratase is high, lycopene as the substrate may not be found. Therefore, the carotenogenesis pathways of okenone and / .g.-keto carotenoids could possibly start from lycopene rather than from neurosporene (Fig. 7). Nothing is known about how the keto group is introduced under the anaerobic conditions. 

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