Infrared and EPR Spectroscopic Characterization of a Ni(I) Species Formed by Photolysis of a Catalytically Competent Ni(I)-CO Intermediate in the Acetyl-CoA Synthase Reaction

摘要

Acetyl-CoA synthase (ACS) catalyzes the synthesis of acetyl-CoA from CO, coenzyme A (CoA),nand a methyl group from the CH3-Co3þ site in the corrinoid iron-sulfur protein (CFeSP). These are the keynsteps in the Wood-Ljungdahl pathway of anaerobic CO and CO2 fixation. The active site of ACS is thenA-cluster, which is an unusual nickel-iron-sulfur cluster. There is significant evidence for the catalyticnintermediacy of a CO-bound paramagnetic Ni species, with an electronic configuration of [Fe4S4]n2þ-(Nipnþ-nCO)-(Nidn2þ), where Nip and Nid represent the Ni centers in the A-cluster that are proximal and distal to then[Fe4S4]n2þ cluster, respectively. This well-characterized Nipnþ-CO intermediate is often called the NiFeC species.nPhotolysis of the Nipnþ-CO state generates a novel Nipnþ species (Ared*) with a rhombic electron paramagneticnresonance spectrum(g values of 2.56, 2.10, and 2.01) and an extremely low (1 kJ/mol) barrier for recombinationnwith CO. We suggest that the photolytically generated Ared* species is (or is similar to) the Nipnþ species thatnbinds CO (to form the Nipnþ-CO species) and the methyl group (to form Nip-CH3) in the ACS catalyticnmechanism. The results provide support for a binding site (an “alcove”) for CO near Nip, indicated by X-rayncrystallographic studies of the Xe-incubated enzyme. We propose that, during catalysis, a resting Nipn2þ statenpredominates over the active Nipnþ species (Ared*) that is trapped by the coupling of a one-electron transfer stepnto the binding of CO, which pulls the equilibrium toward Nipnþ-CO formation.