Organometallic intermediates are implicated in many nickel metalloenzymes, including carbon monoxide dehydrogenase (CODH), acetyl coenzyme A synthase (ACS), and methyl coenzyme M reductase (MCR). However, these bioorganometallic species have largely evaded characterization, thwarting efforts to understand the catalytic mechanisms of these important yet complex multimetallic enzymes. To circumvent the challenges associated with studying the native enzymes and establish spectroscopic and reactivity metrics, we have developed models of nickel metalloenzymes based on robust protein scaffolds. A mutant of nickel-substituted azurin has been established as a model of ACS, capable of binding both of the native substrates, CO and a methyl group. Using pulsed EPR spectroscopy in conjunction with computational analyses, we have characterized the electronic and geometric structures of the S = ½ Ni-CO and Ni-CH3 Az species, which mimic intermediates proposed in ACS. While the Ni-CO state shows conventional metal-ligand interactions and a classical ligand field, structural analysis of the Ni-CH3 species suggests a near-planar methyl ligand that can be best described as cationic. Consistent with this conclusion, the frontier molecular orbitals of the Ni-CH3 species indicate a ligand-centered LUMO, with a d9 population on the metal center. Collectively, these data support the presence of an inverted ligand field configuration for the Ni-CH3 Az species, providing the first evidence for an inverted ligand field within a biological system. The functional relevance of the electronic structures of both the Ni-CO and Ni-CH3 species will be discussed in the context of native ACS, whereby an inverted ligand field is proposed as a mechanism to gate reactivity in thiolate-containing metalloenzymes.

Uncovering ligand field inversion in a biochemical model of acetyl coenzyme A synthase using pulsed EPR spectroscopy

ACS Spring Meeting 2021

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.