Reduction of N by nitrogenases occurs at an organometallic iron cofactor that commonly also contains either molybdenum or vanadium. The well-characterized resting state of the cofactor does not bind substrate, so its mode of action remains enigmatic. Carbon monoxide was recently found to replace a bridging sulfide, but the mechanistic relevance was unclear. Here we report the structural analysis of vanadium nitrogenase with a bound intermediate, interpreted as a μ-bridging, protonated nitrogen that implies the site and mode of substrate binding to the cofactor. Binding results in a flip of amino acid glutamine 176, which hydrogen-bonds the ligand and creates a holding position for the displaced sulfide. The intermediate likely represents state E or E of the Thorneley-Lowe model and provides clues to the remainder of the catalytic cycle.
Nitrogenases are
responsible for biological nitrogen fixation,
a crucial step in the biogeochemical nitrogen cycle. These enzymes
utilize a two-component protein system and a series of iron–sulfur
clusters to perform this reaction, culminating at the FeMco active
site (M = Mo, V, Fe), which is capable of binding and reducing N2 to 2NH3. In this review, we summarize how different
spectroscopic approaches have shed light on various aspects of these
enzymes, including their structure, mechanism, alternative reactivity,
and maturation. Synthetic model chemistry and theory have also played
significant roles in developing our present understanding of these
systems and are discussed in the context of their contributions to
interpreting the nature of nitrogenases. Despite years of significant
progress, there is still much to be learned from these enzymes through
spectroscopic means, and we highlight where further spectroscopic
investigations are needed.
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