Structural and Mechanistic Insights into CO2 Activation by Nitrogenase Iron Protein

Abstract

The Fe protein of nitrogenase catalyzes the ambient reduction of CO2 when its cluster is present in the all-ferrous, [Fe4S4](0) oxidation state. Here, we report a combined structural and theoretical study that probes the unique reactivity of the all-ferrous Fe protein toward CO2. Structural comparisons of the Azotobacter vinelandii Fe protein in the [Fe4S4](0) and [Fe4S4](+) states point to a possible asymmetric functionality of a highly conserved Arg pair in CO2 binding and reduction. Density functional theory (DFT) calculations provide further support for the asymmetric coordination of O by the "proximal" Arg and binding of C to a unique Fe atom of the all-ferrous cluster, followed by donation of protons by the proximate guanidinium group of Arg that eventually results in the scission of a C-O bond. These results provide important mechanistic and structural insights into CO2 activation by a surface-exposed, scaffold-held [Fe4S4] cluster.