Insight into the Radical Mechanism of Phycocyanobilin−Ferredoxin Oxidoreductase (PcyA) Revealed by X-ray Crystallography and Biochemical Measurements

The X-ray crystal structure of the substrate-free form of phycocyanobilin (PCB)−ferredoxin oxidoreductase (PcyA; EC 1.3.7.5) from the cyanobacterium Nostoc sp. PCC7120 has been solved at 2.5 Å resolution. A comparative analysis of this structure with those recently reported for substrate-bound and substrate-free forms of PcyA from the cyanobacterium Synechocystis sp. PCC6803 (Hagiwara et al. (2006) Proc. Natl. Acad. Sci. U.S.A. 103, 27−32; Hagiwara et al. (2006) FEBS Lett. 580, 3823−3828) provides a compelling picture of substrate-induced changes in the PcyA enzyme and the chemical basis of PcyA's catalytic activity. On the basis of these structures and the biochemical analysis of site-directed mutants of Nostoc PcyA, including mutants reported in recent studies (Tu et al. (2006) J. Biol. Chem. 281, 3127−3136) as well as mutants described in this study, a revised mechanism for the PcyA-mediated four-electron reduction of biliverdin IXα to 3E/3Z-phycocyanobilin via enzyme-bound bilin radical intermediates is proposed. The mechanistic insight of these studies, along with homology modeling, have provided new insight into the catalytic mechanisms of other members of the ferredoxin-dependent bilin reductase family that are widespread in oxygenic photosynthetic organisms.