K-055. Increased Expression of Hemoprotein BchZ of Chlorophyllide a Reductase upon Semi-aerobiosis Results in the Enhancement of Bacteriochlorophyll Synthesis of Rhodobacter sphaeroides
Chlorophyllide a reductase of Rhodobacter sphaeroides, which is composed of BchX, BchY, and BchZ, reduces ring B of chlorophyllide a under anaerobic conditions. Although the enzyme is labile in the presence of O2, it generates superoxide at low O2. BchX, a flavo-iron sulfur protein, acts as an ATP-dependent NADH:FMN oxidoreductase, whereas BchY is an iron-sulfur protein that can be co-purified with BchZ containing b-type heme. Superoxide radical is generated from heme of BchZ. The expression of chlorophyllide a reductase is highly induced upon lowering O2 tension. Especially, the induction of BchZ synthesis therein accompanies the titration of heme in cells. The resulting effect is to lower heme availability, which results in further activation of the expression of 5-aminolevulinate synthase in addition to its O2-dependent expression. Thus, the induction of BchZ expression upon semi-aerobiosis results in the enhanced metabolic flow to protoporphyrin IX, which could be readily used for bacteriochlorophyll synthesis of R. sphaeroides.