Abstract
A crystal structure of the catalytic component, NB-protein, of light-independent protochlorophyllide (Pchlide) reductase from Rhodobacter capsulatus has been solved at 2.3 Å resolution. The structure contains two copies each of homologus BchN and BchB subunits, and a [4Fe-4S] cluster in the interface. A surprise of the structure is direct coordination of BchB-Asp36 to the cluster, instead of BchB-Cys95 anticipated to coordinate the cluster based on the sequence similarity. The structure in the presence or absence of Pchlide has revealed the displacement of C-terminal helix of BchB when accommodating Pchlide. Intriguingly, the orientation of bound Pchlide is mainly provided by hydrophobic interaction, keeping the reduction site of ring-D portion away from the [4Fe-4S] cluster. These structures would have important implications for the mechanism of stereospecific Pchlide reduction, and comparison with the evolutionary related nitrogenase, which catalyzes the reductive formation of ammonia from dinitrogen, illustrates a functional framework of nitrogenase-like enzymes.
