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), and a methyl-group from the CH3-Co³⁺ site in the corrinoid iron-sulfur protein (CFeSP). These are the key steps in the Wood-Ljungdahl pathway of anaerobic CO and CO2 fixation. The active site of ACS is the A-cluster, which is an unusual nickel-iron-sulfur cluster. There is significant evidence for the catalytic intermediacy of a CO-bound paramagnetic Ni species, with an electronic configuration of [Fe4S4]²⁺-(Nip¹⁺–CO)-(Nid²⁺), where Nip and Nid represent the Ni centers in the A-cluster that are proximal and distal to the [Fe4S4]²⁺ cluster. This well-characterized Nip¹⁺–CO intermediate is often called NiFeC species. Photolysis of the Nip¹⁺–CO state generates a novel Nip¹⁺ species (Ared*) with a rhombic electron paramagnetic resonance spectrum (g-values of 2.56, 2.10, 2.01) and an extremely low (1 kJ/mol) barrier for recombination with CO. We suggest that the photolytically generated Ared* species is (or is similar to) the Nip¹⁺ species that binds CO (to form the Ni_p¹⁺–CO species) and the methyl group (to form Ni_p-CH₃) in the ACS catalytic mechanism. The results provide support for a binding site (an “alcove”) for CO near Ni_p, indicated by X-ray crystallographic studies of the Xe-incubated enzyme. We propose that, during catalysis, a resting Ni_p²⁺ state predominates over the active Ni_p¹⁺ species (A_red*) that is trapped by the coupling of a one-electron transfer step to the binding of CO, which pulls the equilibrium toward Ni_p¹⁺-CO formation.