Carotenoid biosynthesis in the photosynthetic bacterium Rubrivivax gelatinosus leads to the formation of hydroxyspheroidene and spirilloxanthin as the products of a branched pathway. In this study we investigated the role of the desaturase encoded by crtD which catalyses the introduction of C-3,4 double bonds into acyclic carotenoids. The desaturase was expressed in Escherichia coli, and the activity and the substrate specificity of the enzyme were evaluated in vitro by application of structurally different carotenoids. The results indicate that the enzyme is a 3,4-desaturase that converts 1-hydroxy carotenoids. The 3,4-desaturation reaction can only occur with mono-1-hydroxy carotenoids at a ψ-end group or with 1,1ʹ-dihydroxy derivatives carrying a 3ʹ,4ʹ-double bond. In addition, 1-HO-ζ-carotene could also be converted by the desaturase. Enzyme kinetic studies showed a substrate preference of 1-HO-neurosporene over 1-HO-lycopene. Consequences from the biochemical data for the reaction sequence of hydroxyspheroidene and spirilloxanthin formation and the interconnection of both branches are discussed.

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