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 Reactiont

摘要

Acetyl-CoA synthase (ACS) catalyzes the synthesis of acetyl-CoA from CO, coenzyme A (CoA), and a methyl group from the CH_3-Co~(3+) site in the corrinoid iron-sulfur protein (CFeSP). These are the key steps in the Wood-Ljungdahl pathway of anaerobic CO and CO_2 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 [Fe_4S_4]~(2+)-( Ni_p~+- CO)-(Nid~(2+)), where Nip and Nid represent the Ni centers in the A-cluster that are proximal and distal to the [Fe_4S_4]~(2+) cluster, respectively. This well-characterized Ni_p~ +-CO intermediate is often called the NiFeC species. Photolysis of the Ni_p~ +-CO state generates a novel Ni_p~ + species (A_(rad)~*) with a rhombic electron paramagnetic resonance spectrum (g values of 2.56, 2.10, and 2.01) and an extremely low (1 kJ/mol) barrier for recombination with CO. We suggest that the photolytically generated A_(rad)~* species is (or is similar to) the Ni_p~ + species that binds CO (to form the Ni_p~ +-CO species) and the methyl group (to form Ni_p-CH_3) 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_p~(2+) state predominates over the active Ni_p~ + species (A_(rad)~*) that is trapped by the coupling of a one-electron transfer step to the binding of CO, which pulls the equilibrium toward Ni_p~ +-CO formation.