In vitro C13-dealkoxycarbonylations of zinc chlorophyll a derivatives including C13-substitutes by a BciC enzyme.

Chlorosomes in the green photosynthetic bacteria are the largest and most efficient light-harvesting antenna systems of all phototrophs. The core part of chlorosomes consists of bacteriochlorophyll c, d, e, or f molecules. In their biosynthetic pathway, a BciC enzyme catalyzes the removal of the C13-methoxycarbonyl group of chlorophyllide a. In this study, in vitro C13-dealkoxycarbonylations of zinc chlorophyll a derivatives bearing a methyl-, ethyl- or propyl-esterifying group and its methyl ester analogs with additional alkyl and hydroxy groups at the C13-position were examined using the BciC enzyme. The BciC-catalyzed reaction activity for the C13-methoxycarbonylated substrate was comparable to that for the ethoxycarbonylated compound; however, depropoxycarbonylation did not proceed. The BciC enzymatic demethoxycarbonylation of zinc methyl C13-alkylated pheophorbides a was gradually suppressed with the elongation of the alkyl chain and finally became inactive for the propyl substrate. The reaction of the C13-hydroxylated substrate (allomer) was accelerated compared to that of the C13-methyl analog possessing a similar steric size, and gave the corresponding C13-oxo product via further air-oxidation. All of the abovementioned enzymatic reactions occurred for one of the C13-epimers with the same configuration as in chlorophyllide a. The above substrate specificities and product distributions indicated the stereochemistry and size of the BciC enzymatic active site (pocket).