1 research outputs found
Insights into the Proton Transfer Mechanism of a Bilin Reductase PcyA Following Neutron Crystallography
Phycocyanobilin, a light-harvesting
and photoreceptor pigment in
higher plants, algae, and cyanobacteria, is synthesized from biliverdin
IXα (BV) by phycocyanobilin:ferredoxin oxidoreductase (PcyA)
via two steps of two-proton-coupled two-electron reduction. We determined
the neutron structure of PcyA from cyanobacteria complexed with BV,
revealing the exact location of the hydrogen atoms involved in catalysis.
Notably, approximately half of the BV bound to PcyA was BVH<sup>+</sup>, a state in which all four pyrrole nitrogen atoms were protonated.
The protonation states of BV complemented the protonation of adjacent
Asp105. The “axial” water molecule that interacts with
the neutral pyrrole nitrogen of the A-ring was identified. His88 Nδ
was protonated to form a hydrogen bond with the lactam O atom of the
BV A-ring. His88 and His74 were linked by hydrogen bonds via H<sub>3</sub>O<sup>+</sup>. These results imply that Asp105, His88, and
the axial water molecule contribute to proton transfer during PcyA
catalysis