UC San Diego

Nitrogenase is a microbial enzyme critical for nitrogen fixation, catalyzing the reduction of dinitrogen (N₂) to ammonia (NH₃). This process is vital for the biosynthesis of nucleic acids and amino acids, contributing to half of global ammonia production. Despite extensive research, the mechanisms of substrate reduction, protein interaction, and ATP-dependent energy transduction remain unclear. Recently, cryogenic electron microscopy (cryoEM) has provided significant structural information on nitrogenase, although its oxygen-sensitive nature poses substantial challenges for this method.This study employs cryoEM to investigate the protein interactions within homologous and heterologous nitrogenase complexes. An optimized sample vitrification method was developed specifically for studying nitrogenase by cryoEM. Using this approach, we structurally characterized one homologous and two heterologous nitrogenase complexes. The electron microscopy (EM) density data, coupled with biochemical assays, revealed that while homologous and heterologous nitrogenase complexes share similar conformations in the activated complex state, they exhibit varied catalytic efficiencies and energy consumption rates. This study lays the groundwork for future cryoEM investigations of nitrogenase and provides important insights into its protein interactions, potentially guiding efforts to enhance the efficiency of biological nitrogen fixation.