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The Fe protein of nitrogenase plays multiple roles in substrate reduction and metallocluster assembly. Best known for its function to transfer electrons to its catalytic partner during nitrogenase catalysis, the Fe protein is also a key player in the biosynthesis of the complex metalloclusters of nitrogenase. In addition, it can function as a reductase on its own and affect the ambient reduction of CO2 or CO to hydrocarbons. This review will provide an overview of the properties and functions of the Fe protein, highlighting the relevance of this unique FeS enzyme to areas related to the catalysis, biosynthesis, and applications of the fascinating nitrogenase system. 

ABSTRACT 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 ( Ma NifH), 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 Ma 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 2 on Ma NifH 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. IMPORTANCE 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 2 capture by a surface-exposed [Fe 4 S 4 ] cluster point to the possibility of developing novel strategies for CO 2 sequestration while providing the initial insights into the unique mechanism of FeS-based CO 2 activation. 

Abstract The Fe protein of nitrogenase plays multiple roles in substrate reduction and cluster maturation via its redox‐active [Fe₄S₄] cluster. Here we report the synthesis and characterization of a water‐soluble [Fe₄Se₄] cluster that is used to substitute the [Fe₄S₄] cluster of theAzotobacter vinelandiiFe protein (AvNifH). Biochemical, EPR and XAS/EXAFS analyses demonstrate the ability of the [Fe₄Se₄] cluster to adopt the super‐reduced, all‐ferrous state upon its incorporation intoAvNifH. Moreover, these studies reveal that the [Fe₄Se₄] cluster inAvNifH already assumes a partial all‐ferrous state ([Fe₄Se₄]⁰) in the presence of dithionite, where its [Fe₄S₄] counterpart inAvNifH exists solely in the reduced state ([Fe₄S₄]¹⁺). Such a discrepancy in the redox properties of theAvNifH‐associated [Fe₄Se₄] and [Fe₄S₄] clusters can be used to distinguish the differential redox requirements for the substrate reduction and cluster maturation of nitrogenase, pointing to the utility of chalcogen‐substituted FeS clusters in future mechanistic studies of nitrogenase catalysis and assembly.