Carotenoid biosynthesis: Isolation and characterization of a bifunctional enzyme catalyzing the synthesis of phytoene

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Phytoene is the first C40 intermediate in the biogenesis of carotenoids. It is formed by two enzyme activities, catalyzing (i) the coupling of two molecules of geranylgeranyl diphosphate to yield prephytoene diphosphate and (ii) the conversion of prephytoene diphosphate into phytoene. We show now, with Capsicum chromoplast stroma, that the overall activity resides in a single protein, which has been purified to homogeneity by affinity chromatography. The monomeric structure and the molecular size (Mr 47,500) were demonstrated by NaDodSO4/PAGE and glycerol gradient centrifugation. Further characterization was achieved by using specific antibodies which allowed immunofractionation and immunoprecipitation of the enzymatic activity from chromoplast stroma. The two reactions followed conventional Michaelis-Menten kinetics, with Km values of 0.30 μM and 0.27 μM, respectively, for geranylgeranyl diphosphate and prephytoene diphosphate. The activity of the enzyme depends strictly upon the presence of Mn2+. This selectivity may be one of the factors regulating the competition with potentially rival enzymes converting geranylgeranyl diphosphate into other plastid terpenoids. The two enzymatic reactions were inhibited by inorganic pyrophosphate and by the arginine-specific reagent hydroxyphenylglyoxal. In no instance were the two reactions kinetically uncoupled. These properties strongly suggest that the same enzyme catalyzes the two consecutive reactions, and we propose to name it phytoene synthase.

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