Catalytic cycle of cytochrome-c(3) hydrogenase, a [NiFe]-enzyme, deduced from the structures of the enzyme and the enzyme mimic

Hydrogenases catalyze uptake and production of H-2. Heterolytic cleavage of H-2 bound on [NiFe]-hydrogenase (E) produces two unequal H species to form E:HaHb, where Ha and H-b behave differently. The structures of various states of the enzyme established by crystallography and spectroscopy were used to construct a catalytic cycle of the enzyme. The Ni Fe center of the active enzyme has the Ni Fe bridging site vacant. The enzyme is suggested to bind H-2 either at Ni or Fe atom. In E:HaHb, Ha is considered to be a protein-bound hydron (proton or deuteron) at the entrance to the hydrophobic gas tunnel. The structure of a synthetic hydrogenase-mimic suggests Hb to be the 6th ligand to Fe. Two successive one-electron processes from E:HaHb complete the catalytic cycle of H-2 uptake. The reverse of the cycle operates in the H-2 production. The proposed catalytic cycle is consistent with the kinetic, crystallographic and spectroscopic studies. Copyright (C) 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.