Enzymatic activity of C-20 methyltransferase BchU evaluating its role in bacteriochlorophyll <i>c</i> biosynthetic pathway

Bacteriochlorophylls (BChls) c and e are responsible for the main part of the light-harvesting process in chlorosome antenna systems of green sulfur bacteria, and contain a methyl group at the peripheral C-20 position of their core chlorin rings. This study performed in vitro and in vivo analysis of the C-20 methyltransferase BchU derived from the green sulfur bacterium Chlorobaculum tepidum, which synthesizes BChl c, to clarify the role of this enzyme in the biosynthetic pathway. Although the reaction step of BchU in the biosynthesis could not be determined by genetic analysis, enzymatic assays using various substrates showed that BchU reacts primarily with substrates after hydration of BchF and BchV at the C-3 position. The results in this study allow the proposition of a biosynthetic pathway for BChl c and e involving this enzyme.