The Many Roles of the Nitrogenase Iron Protein

Citation

Wenke, Belinda B. (2019) The Many Roles of the Nitrogenase Iron Protein. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/8RB1-HC30. https://resolver.caltech.edu/CaltechTHESIS:04222019-175445561

Abstract

Nitrogenase is the only known enzyme capable of reducing atmospheric nitrogen (N₂) into ammonia (NH₃) for incorporation into cellular material. N₂ reduction by nitrogenase is accomplished by sequential electron transfer between two component proteins: the substrate reductase (the MoFe-protein), and a specialized low-potential electron donor (the Fe-protein). The MoFe-protein contains the active site for nitrogen reduction, the FeMocofactor (FeMo-co). During nitrogen reduction, each Fe-protein dimer docks onto the MoFe-protein, transferring electrons to an intermediate cluster (P-cluster), and ultimately to the FeMo-co.

Strikingly, the Fe-protein has another critical role in nitrogen fixation. The Fe-protein is required for the biosynthesis of the two unique metalloclusters of the MoFe-protein: the Pcluster [8Fe:7S] and the active site FeMo-co ([Mo:7Fe:9S:C]-R-homocitrate) cluster. During FeMo-co-cluster maturation, the Fe-protein forms a complex with NifEN, a scaffolding protein homologous to the MoFe-protein, catalyzing the final step in the FeMoco biosynthesis. Studies indicate that the Fe-protein catalyzes insertion of molybdenum and R-homocitrate into an all-iron FeMo-co precursor in a reductant and nucleotide dependent manner. The remaining questions about the cellular functions of the Fe-protein include how the Fe-protein interacts with other maturation proteins in distinct (or similar) ways compared to the MoFe-protein, and how the Fe-protein contributes to the activation and insertion of molybdenum into the FeMo-co.

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