An unexpectedly branched biosynthetic pathway for bacteriochlorophyll b capable of absorbing near-infrared light

Chlorophyllous pigments are essential for photosynthesis. Bacteriochlorophyll (BChl) b has the characteristic C8-ethylidene group and therefore is the sole naturally occurring pigment having an absorption maximum at near-infrared light wavelength. Here we report that chlorophyllide a oxidoreductase (COR), a nitrogenase-like enzyme, showed distinct substrate recognition and catalytic reaction between BChl a- and b -producing proteobacteria. COR from BChl b -producing Blastochloris viridis synthesized the C8-ethylidene group from 8-vinyl-chlorophyllide a . In contrast, despite the highly conserved primary structures, COR from BChl a -producing Rhodobacter capsulatus catalyzes the C8-vinyl reduction as well as the previously known reaction of the C7 = C8 double bond reduction on 8-vinyl-chlorophyllide a . The present data indicate that the plasticity of the nitrogenase-like enzyme caused the branched pathways of BChls a and b biosynthesis, ultimately leading to ecologically different niches of BChl a - and b -based photosynthesis differentiated by more than 150 nm wavelength.