Characterization of Methyl- and Acetyl-Ni Intermediates in Acetyl CoA Synthase Formed during Anaerobic CO 2 and CO Fixation.

The Wood-Ljungdahl Pathway is a unique biological mechanism of carbon dioxide and carbon monoxide fixation proposed to operate through nickel-based organometallic intermediates. The most unusual steps in this metabolic cycle involve a complex of two distinct nickel-iron-sulfur proteins: CO dehydrogenase and acetyl-CoA synthase (CODH/ACS). Here, we describe the nickel-methyl and nickel-acetyl intermediates in ACS completing the characterization of all its proposed organometallic intermediates. A single nickel site (Ni p ) within the A cluster of ACS undergoes major geometric and redox changes as it transits the planar Ni p , tetrahedral Ni p -CO and planar Ni p -Me and Ni p -Ac intermediates. We propose that the Ni p intermediates equilibrate among different redox states, driven by an electrochemical-chemical (EC) coupling process, and that geometric changes in the A-cluster linked to large protein conformational changes control entry of CO and the methyl group.