Reactivity of C-20 methyltransferase, BchU, derived from green sulfur bacterium, Chlorobium tepidum, involved in biosynthesis of bacteriochlorophyll c

Abstract

In vitro activity of a methyltransferase, BchU, which catalyzes methylation at the C-20 position of a chlorin moiety in biosynthetic pathway of bacteriochlorophyll c, was investigated. 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 enzymatic activities of this His₆-BchU were measured using a variety of synthetic pigments as substrates and S-adenosylmethionine as a donor of methyl group. These results suggest that in the biosynthetic pathway, native BchU catalyzes the methylation of bacteriochlorophyllide d homologs possessing a 1-hydroxyethyl group at the C-3 position and functions after the hydration of the C-3 vinyl group by BchF and V. This finding was supported by analysis of the crystal structure of BchU. We will discuss the role of BchU in the bacteriochlorophyll c biosynthetic pathway.