A protein-based model of carbon monoxide dehydrogenase exhibits tunable covalency across cluster oxidation and ligand-bound states

Abstract

The nickel-containing carbon monoxide dehydrogenase (CODH) uses a unique heterometallic [NiFe₄S₄] cluster active site, called the C-cluster, to catalyze the reversible reduction of carbon dioxide (CO₂) to carbon monoxide (CO) at low overpotential and with perfect selectivity. Only the properly assembled nickel-bound form is capable of this reactivity, though how the structure of the cluster promotes such selectivity remains poorly understood. We have developed a model of the C-cluster by constructing a [NiFe₃S₄] cluster in the iron-sulfur cluster binding site of the Pyrococcus furiosus ferredoxin protein (NiFd) that replicates the thiolate ligation and aqueous environment of the native system. In this work, we interrogate the roles of each individual metal site and the whole-cluster covalency across two oxidation states that mirror the C-cluster in the C_ox and C_red1 states. We have also studied the system bound to a CODH substrate (CO) and C-cluster inhibitor (CN^-). A comprehensive suite of spectroscopic techniques, including pulsed electron paramagnetic resonance (EPR), variable-temperature, variable-field Mössbauer, and high-energy resolution fluorescence-detected X-ray absorption (HERFD-XAS) spectroscopy, have been used in conjunction with quantum mechanics/molecular mechanics (QM/MM) and broken symmetry density functional theory (BS-DFT) calculations to elucidate the electronic properties of these heterometallic clusters. This work reveals that the supporting iron sulfide subcluster and thiolate ligands play a critical role in buffering charge density as the cluster traverses multiple states. An unusually weak exchange interaction between the Ni site and the iron atoms is examined in the CO-bound form, suggesting that substrate binding electronically isolates the nickel site, giving a low-spin ground state that drives localized chemistry to occur at the nickel center. These results have implications for understanding how reactivity is controlled in native CODH to promote CO oxidation and CO₂ reduction rather than deleterious hydrogen evolution.