Hydrogen Inhibition of Nitrogen Reduction by Nitrogenase in Isolated Soybean Nodule Bacteroids

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Abstract
Dihydrogen, a by-product of biological nitrogen fixation, is a competitive inhibitor of N2 reduction by nitrogenase. To evaluate the significance of H2 inhibition in vivo, we have measured the apparent inhibition constant for H2 inhibition of N2 reduction in Bradyrhizobium japonicum bacteroids isolated from soybean nodules. The rate of N2 reduction was measured as ammonia production by bacteroids incubated in a buffer containing 200 micromolar leghemoglobin and 10 millomolar succinate under 0.02 atmosphere O2 and various concentrations of N2 and H2. The apparent inhibition constant for H2 under these conditions was determined to be approximately 0.03 atomsphere. This relatively low value strengthens the proposal than H2 inhibition of N2 reduction may be a significant factor in lowering the efficiency of nitrogen fixation in legume nodules.