In Vitro Assays of BciC Showing C132-Demethoxycarbonylase Activity Requisite for Biosynthesis of Chlorosomal Chlorophyll Pigments

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A BciC enzyme is related to the removal of the C132-methoxycarbonyl group in biosynthesis of bacteriochlorophylls (BChls) c, d and e functioning in green sulfur bacteria, filamentous anoxygenic phototrophs and phototrophic acidobacteria. These photosynthetic bacteria have the largest and the most efficient light-harvesting antenna systems, called chlorosomes, containing unique self-aggregates of BChl c, d or e pigments, that lack the C132-methoxycarbonyl group which disturbs chlorosomal self-aggregation. In this study, we characterized the BciC derived from the green sulfur bacterium Chlorobaculum tepidum, and examined the in vitro enzymatic activities of its recombinant protein. The BciC-catalyzing reactions of various substrates showed that the enzyme recognized chlorophyllide (Chlide) a and 3,8-divinyl(DV)-Chlide a as chlorin substrates to give 3-vinyl-bacteriochlorophyllide (3V-BChlide) d and DV-BChlide d, respectively. Since the BciC afforded a higher activity with Chlide a than that with DV-Chlide a and no activity with (DV-)protoChlides a (porphyrin substrates) and 3V-BChlide a (a bacteriochlorin substrate), this enzyme was effective for diverting the chlorosomal pigment biosynthetic pathway at the stage of Chlide a away from syntheses of other pigments such as BChl a and Chl a. The addition of methanol to the reaction mixture did not prevent the BciC activity, and we identified this enzyme as Chlide a demethoxycarbonylase, not methylesterase.

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