ビリン還元酵素PcyAと基質ビリベルジン複合体の中性子結晶構造解析で見えてきたもの

Phycocyanobilin:ferredoxin oxidoreductase (PcyA) catalyzes the conversion of　biliverdin (BV) to phycocyanobilin, a light harvesting pigment. To visualize the hydrogens (and protons) in　this unique enzyme, we conducted neutron crystallography of the PcyA-BV complex. In　the enzyme, BV bound to PcyA was in equilibrium between the neutral (BV) and the　protonated (BVH+) forms. Both of the structures including the hydrogen atoms could be　determined. The protonation of the nearby essential residue Asp105 was complemented　with the BV protonation states. His88, an essential residue, was protonated to form a　hydrogen bond to a lactam oxygen of the BV A-ring. His88 is also hydrogen-bonded to　His74 via a hydronium ion. In this article, we also describe the story to grow the large　crystals of the PcyA-BV complex and its neutron diffraction experiment