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.
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.
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.
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