Characterization of Methyl- and Acetyl-Ni Intermediates in Acetyl CoA Synthase Formed during Anaerobic CO₂ 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 (Nip) within the A cluster of ACS undergoes major geometric and redox changes as it transits the planar Nip, tetrahedral Nip–CO and planar Nip–Me and Nip–Ac intermediates. We propose that the Nip 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.