Characterization of Methyl- and Acetyl-Ni Intermediates in Acetyl CoA Synthase Formed during Anaerobic CO₂ and CO Fixation

Abstract

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.

Coupling CO2 Reduction and Acetyl‐CoA Formation: The Role of a CO Capturing Tunnel in Enzymatic Catalysis

“…However, in the methylated state the K d drops to 12 nM, probably due to the immediate insertion of the CO into the NiÀ CH 3 bond. [17] This is about 8-times lower than the K d of hemoglobin determined here (Figure S5). Still, even such a low affinity of the methylated A-cluster for CO is insufficient to explain the observed kinetics: due to the approximately 14-fold excess of hemoglobin over ACS in the assays (assuming a degree of maturation of 50 % for ACS), Hb-CO should still be able to compete with methylated ACS for CO binding.…”

Site specific redox properties in ligand differentiated di-nickel complexes inspired by the acetyl CoA synthase active site

“…Renewed interest in the ACS active site signifies the necessity for new model compounds in the development of catalysts that can valorize CO 2 into fuels. 34–36 Ongoing studies will leverage the knowledge obtained in this study towards bimetallic activation of small molecules.…”

Kopplung von CO2‐Reduktion und Acetyl‐CoA‐Bildung: Die Rolle eines CO‐fangenden Tunnels in der enzymatischen Katalyse

“…Im methylierten Zustand sinkt der K d jedoch auf 12 nM, was wahrscheinlich auf die sofortige Einbindung des CO in die Ni-CH 3 -Bindung zurückzuführen ist. [17] Dies ist etwa 8-mal niedriger als der hier ermittelte K d von Hämoglobin (Abbildung S5). Doch selbst eine so geringe Affinität des methylierten A-Clusters für CO reicht nicht aus, um die beobachtete Kinetik zu erklären: Aufgrund des etwa 14-fachen Überschusses von Hämoglobin gegenüber ACS in den Assays (unter der Annahme eines Besetzungsgrades von 50 % für ACS) sollte Hämoglobin immer noch mit methyliertem ACS um die CO-Bindung konkurrieren können.…”