Abstact

Many structural studies have been reported for ferredoxin:NADP+ reductase family members, but an experimental validation of the catalytic hydride and proton transfer steps through a direct detection of the involved hydrogen atoms has not been achieved so far. Here, we determined high-resolution X-ray and neutron crystal structures of NADH-cytochrome b5 reductase, which acts as an electron supplier for various metabolic processes and mediates hydride and proton transfer reactions via its FAD and NADH cofactors. The X-ray structures identify the FADH--NAD+ and FAD-NADH complexes based on the electron densities of the hydrogen atoms bound to the cofactors. The neutron structures determined at different pD-values show a difference in the protonation state of the histidine residue in the hydrogen-bond network from FAD to the protein surface. The observation of the hydrogen atoms reveals the structural basis for the hydride and proton transfer reactions catalyzed by NADH-cytochrome b5 reductase.