PROJECT SUMMARY/ABSTRACT This proposal aims to elucidate how the bacterial enzyme nitrogenase catalyzes the chemically difficult transformation of atmospheric dinitrogen into a bioavailable form, ammonia, and why/how it utilizes ATP hydrolysis to drive this reaction. Being the only enzyme responsible for reductive nitrogen fixation, nitrogenase sustains the agricultural/nutritional needs of ~40% of the human population. Aside from its global importance, nitrogenase is a unique model system with broad relevance to biological redox catalysis as well as ATP/GTP-dependent energy transduction processes, which are both central to proper cellular functioning and thus directly relevant to human health. Despite nearly five decades of extensive biochemical, biophysical and structural characterization, the two most important questions about nitrogenase mechanism have not answered in detail: a) Why and how ATP hydrolysis is ultimately utilized for the reduction of N2 or alternative substrates? b) What is the intimate mechanism of dinitrogen reduction on the nitrogenase active site cluster, FeMoco? The major experimental challenge in the investigations of nitrogenase arises from the fact that the catalytic activity of nitrogenase depends on continuous ATP turnover, which leads to a heterogeneous mixture of redox and nucleotide- bound states of nitrogenase that are difficult to distinguish from one another. To circumvent this challenge, we have initiated a research program in cryogenic electron microscopy (cryoEM) to structurally characterize dynamic states of nitrogenase at atomic resolution under enzymatic turnover conditions. Preliminary experiments have not only established the feasibility of this approach but also revealed unexpected structural features of nitrogenase which have fueled new mechanistic hypotheses. In the proposed project, we aim to build upon on our preliminary findings by a) mapping the ATP-driven conformational landscape of nitrogenase in unprecedented detail and b) elucidating FeMoco structural dynamics and FeMoco-small molecule interactions in atomic resolution, while also c) contributing to the development of cutting-edge cryoEM methodologies for the structural interrogation of highly complex/dynamic protein assemblies and metallocofactors.