Changes in the EPR signal of dinitrogenase from Azotobacter vinelandii during the lag period before hydrogen evolution begins.

During the lag period before H2 is evolved by the nitrogenase system, the EPR signal of dinitrogenase decreases steadily, indicating transfer of electrons into dinitrogenase. The rate constant for the decrease in amplitude of the EPR signal, the steady state rate of H2 evolution from nitrogenase, and the length of the lag period have been measured. The data suggest that H2 is evolved only after dinitrogenase has been reduced by 2 electrons/molybdenum. The electrons that have been transfered into dinitrogenase during the lag period are not evolved as H2 upon denaturation of dinitrogenase. The existence of a lag indicates that the two nitrogenase proteins dissociate after every electron transfer. The lag occurs and the nitrogenase proteins dissociate under a variety of conditions of pH and temperature.