Reaction mechanism for stereo-specific reduction of porphyrin D-ring based on the crystallographic structure of NB-protein of protochlorophyllide oxidoreductase from Rhodobacter capsulatus

Abstract

The chlorin ring structure is formed by the reduction of the porphyrin D-ring by a nitrogenase-like enzyme, dark-operative protochlorophyllide (Pchlide) oxidoreductase (DPOR) in the biosynthetic pathway of bacteriochlorophyll. In the previous work, we proposed a reaction mechanism for the stereo-specific reduction of Pchlide D-ring (C17-C18 double bond) based on the X-ray crystallographic structure of the DPOR catalytic component NB-protein (BchN-BchB heterotetramer) from Rhodobacter capsulatus. In this study, we isolated and characterized a series of NB-protein mutants in which some amino acid residues expected to be involved in the D-ring reduction were altered. In addition, we also examined the effect of a substrate analogue chlorophyll c, which has an acrylate at the C17 position instead of the propionate, on DPOR reaction. The results were consistent with the involvement of BchB-Asp274 and the propionate at C17 of Pchlide as the proton donors for C17 and C18 carbons, respectively.