Structural Analysis of a Nitrogenase Iron Protein from Methanosarcina acetivorans: Implications for CO Capture by a Surface-Exposed [FeS] Cluster.

Nitrogenase iron (Fe) proteins reduce CO to CO and/or hydrocarbons under ambient conditions. Here, we report a 2.4-Å crystal structure of the Fe protein from (NifH), which is generated in the presence of a reductant, dithionite, and an alternative CO source, bicarbonate. Structural analysis of this methanogen Fe protein species suggests that CO is possibly captured in an unactivated, linear conformation near the [FeS] cluster of NifH by a conserved arginine (Arg) pair in a concerted and, possibly, asymmetric manner. Density functional theory calculations and mutational analyses provide further support for the capture of CO on NifH while suggesting a possible role of Arg in the initial coordination of CO via hydrogen bonding and electrostatic interactions. These results provide a useful framework for further mechanistic investigations of CO activation by a surface-exposed [FeS] cluster, which may facilitate future development of FeS catalysts for ambient conversion of CO into valuable chemical commodities. This work reports the crystal structure of a previously uncharacterized Fe protein from a methanogenic organism, which provides important insights into the structural properties of the less-characterized, yet highly interesting archaeal nitrogenase enzymes. Moreover, the structure-derived implications for CO capture by a surface-exposed [FeS] cluster point to the possibility of developing novel strategies for CO sequestration while providing the initial insights into the unique mechanism of FeS-based CO activation.