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Bacteriochlorophyll c molecules self‐aggregate to form large oligomers in the core part of chlorosomes, which are the main light‐harvesting antenna systems of green photosynthetic bacteria. In the biosynthetic pathway of bacteriochlorophyll c, a BciC enzyme catalyzes the removal of the C132‐methoxycarbonyl group of chlorophyllide a, which possesses a free propionate residue at the C17‐position and a magnesium ion as the central metal. The in vitro C132‐demethoxycarbonylations of chlorophyll a derivatives with various alkyl propionate residues and central metals were examined by using the BciC enzyme derived from one green sulfur bacteria species, Chlorobaculum tepidum. The BciC enzymatic reactions of zinc pheophorbide a alkyl esters were gradually suppressed with an increase of the alkyl chain length in the C17‐propionate residue (from methyl to pentyl esters) and finally the hexyl ester became inactive for the BciC reaction. Although not only the zinc but also nickel and copper complexes were demethoxycarbonylated by the BciC enzyme, the reactions were largely dependent on the coordination ability of the central metals: Zn>Ni>Cu. The above substrate specificity indicates that the BciC enzyme would not bind directly to the carboxy group of chlorophyllide a, but would bind to its central magnesium to form the stereospecific complex of BciC with chlorophyllide a, giving pyrochlorophyllide a, which lacks the (132R)‐methoxycarbonyl group.

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Chlorosomes in 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, or e molecules. In their biosynthetic pathway, a BciC enzyme catalyzes the removal of the C132-methoxycarbonyl group of chlorophyllide a. In this study, the in vitro enzymatic reactions of chlorophyllide a analogues, C132-methylene- and ethylene-inserted zinc complexes, were examined using a BciC protein from Chlorobaculum tepidum. As the products, their hydrolyzed free carboxylic acids were observed without the corresponding demethoxycarbonylated compounds. The results showed that the in vivo demethoxycarbonylation of chlorophyllide a by an action of the BciC enzyme would occur via two steps: (1) an enzymatic hydrolysis of a methyl ester at the C132-position, followed by (2) a spontaneous (nonenzymatic) decarboxylation in the resulting carboxylic acid.

We report the in vitro activity of recombinant BchC oxidoreductase involved in bacteriochlorophyll a biosynthesis. BchC of Rhodobacter capsulatus preferentially oxidizes 31R‐3‐(1‐hydroxyethyl)‐chlorophyllide a and 31R‐3‐(1‐hydroxyethyl)‐bacteriochlorophyllide a in the presence of NAD+ to 3‐acetyl‐chlorophyllide a and bacteriochlorophyllide a, respectively, leaving the unreacted 31S‐epimers. In the reverse reaction, BchC with NADH predominately produces 31R‐epimeric alcohols from the 3‐acetyl‐(bacterio)chlorins. BchC of Chlorobaculum tepidum demonstrates the same 31R‐selectivity, suggesting that utilization of 31R‐epimers in BchC‐catalyzed reductions may be conserved across different phyla of photosynthetic bacteria. Additionally, the presence of BchC accelerates the 3‐vinyl hydration by BchF hydratase of Chlorobaculum tepidum during conversion of chlorophyllide a to 3‐acetyl‐chlorophyllide a through 3‐(1‐hydroxyethyl)‐chlorophyllide a, indicating that these enzymes work cooperatively to promote efficient bacteriochlorophyll a biosynthesis.

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Green photosynthetic bacteria, one of the phototrophs, have the largest and most efficient light-harvesting antenna systems, called chlorosomes. The core part of chlorosomes consists of unique bacteriochlorophyll c/d/e molecules. In the biosynthetic pathway of these molecules, a BciC enzyme catalyzes the removal of the C13 2 -methoxycarbonyl group of chlorophyllide a. Two sequential reactions have been proposed for the BciC enzymatic demethoxycarbonylation: the BciC enzyme would catalyze the hydrolysis of the C13 2 -methoxycarbonyl group, and the resulting carboxylic acid would be rapidly decarboxylated to generate pyrochlorophyllide a. In this study, we computationally predicted the three-dimensional structure of the BciC protein. Its active site was proposed based on structural analysis using docking simulation. In vitro enzymatic reaction assays of mutated BciC supported the prediction. The BciC enzymatic hydrolysis would be an aspartic/glutamic acid hydrolase, which involves the amino residues E85 and D180. Furthermore, Y58 and H126 might depend on stabilization and/or recognition with the substrate. Most importantly, H137 would protonate 13-C�O or deprotonate C13 2 -COOH in the hydrolyzed product to promote decarboxylation. In conclusion, the BciC enzyme has the dual functions of hydrolysis and decarboxylation.