CO Binding to the FeV Cofactor of CO-Reducing Vanadium Nitrogenase at Atomic Resolution.

Nitrogenases reduce N , the most abundant element in Earth's atmosphere that is otherwise resistant to chemical conversions due to its stable triple bond. Vanadium nitrogenase stands out in that it additionally processes carbon monoxide, a known inhibitor of the reduction of all substrates other than H . The reduction of CO leads to the formation of hydrocarbon products, holding the potential for biotechnological applications in analogy to the industrial Fischer-Tropsch process.

The Cofactors of Nitrogenases.

In biological nitrogen fixation, the enzyme nitrogenase mediates the reductive cleavage of the stable triple bond of gaseous N2at ambient conditions, driven by the hydrolysis of ATP, to yield bioavailable ammonium (NH4+). At the core of nitrogenase is a complex, ironsulfur based cofactor that in most variants of the enzyme contains an additional, apical heterometal (Mo or V), an organic homocitrate ligand coordinated to this heterometal, and a unique, interstitial carbide. Recent years have witnessed fundamental advances in our understanding of the atomic and electronic structure of the nitrogenase cofactor.

Establishing a Thermodynamic Landscape for the Active Site of Mo-Dependent Nitrogenase.

Nitrogenase enzymes are the only biological catalysts able to convert N to NH. Molybdenum-dependent nitrogenase consists of two proteins and three metallocofactors that sequentially shuttle eight electrons between three distinct metallocofactors during the turnover of one molecule of N. While the kinetics of isolated nitrogenase has been extensively studied, little is known about the thermodynamics of its cofactors under catalytically relevant conditions.

A bound reaction intermediate sheds light on the mechanism of nitrogenase.

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.