Stereoselective C3-substituent modification and substrate channeling by oxidoreductase BchC in bacteriochlorophyll a biosynthesis

We report the in vitro activity of recombinant BchC oxidoreductase involved in bacteriochlorophyll a biosynthesis. BchC of Rhodobacter capsulatus preferentially oxidizes 3(1)R-3-(1-hydroxyethyl)-chlorophyllide a and 3(1)R-3-(1-hydroxyethyl)-bacteriochlorophyllide a in the presence of NAD(+) to 3-acetyl-chlorophyllide a and bacteriochlorophyllide a, respectively, leaving the unreacted 3(1)S-epimers. In the reverse reaction, BchC with NADH predominately produces 3(1)R-epimeric alcohols from the 3-acetyl-(bacterio)chlorins. BchC of Chlorobaculum tepidum demonstrates the same 3(1)R-selectivity, suggesting that utilization of 3(1)R-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.