In vitro activity of C-20 methyltransferase, BchU, in biosynthetic pathway of bacteriochlorophyll c

Abstract

The enzymatic activity of a methyltransferase, BchU, which catalyzes methylation at the C-20 position of a chlorin moiety, was investigated in vitro. The bchU gene derived from a photosynthetic green sulfur bacterium, Chlorobium tepidum, was over-expressed in Escherichia coli as a His-tagged protein (His₆-BchU), and the His₆-BchU was purified. The His₆-BchU catalyzed the methylation of zinc bacteriopheophorbide d at the 20 position in the presence of S-adenosylmethionine to give zinc bacteriopheophorbide c. However, metal-free bacteriopheophorbide d could not be methylated by the BchU, indicating that the central metal in the chlorin ring should be required for the recognition in the BchU catalyzed methylation. The zinc chlorin possessing a 1-hydroxyethyl group at the 3-position was methylated more efficiently than the other substrates used in this study. We would like to discuss the role of BchU in the bacteriochlorophyll c biosynthetic pathway.