Structural Analysis of a Nitrogenase Iron Protein from Methanosarcina acetivorans: Implications for CO(2) Capture by a Surface-Exposed [Fe(4)S(4)] Cluster

Nitrogenase iron (Fe) proteins reduce CO(2) to CO and/or hydrocarbons under ambient conditions. Here, we report a 2.4-Å crystal structure of the Fe protein from Methanosarcina acetivorans (MaNifH), which is generated in the presence of a reductant, dithionite, and an alternative CO(2) source, bicarbonate. Structural analysis of this methanogen Fe protein species suggests that CO(2) is possibly captured in an unactivated, linear conformation near the [Fe(4)S(4)] cluster of MaNifH 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(2) on MaNifH while suggesting a possible role of Arg in the initial coordination of CO(2) via hydrogen bonding and electrostatic interactions. These results provide a useful framework for further mechanistic investigations of CO(2) activation by a surface-exposed [Fe(4)S(4)] cluster, which may facilitate future development of FeS catalysts for ambient conversion of CO(2) into valuable chemical commodities.