Kinetics of acetylene and CN- reduction by the N 2-fixing system of Rhizobium Lupino

Abstract

1. 1. N 2-fixing extracts have been successfully isolated and party purified from bacteroids of Rhizobium lupini obtained from yellow lupin nodules. These extracts catalysed acetylene and CN- reduction, demonstrating the same substrate versatility as nitrogenase from other organisms. 2. 2. The enzyme saturation curves by ATP for both acetylene and CN- reduction were closely similar. These indicate that nitrogenase from this source has several sites for binding ATP. Intermediary plateaus suggest that cooperation of these sites involves both positive and negative changes in either binding or catalytic activity as the enzyme is saturated with ATP. 3. 3. Curves for inhibition of acetylene and CN- reduction by ADP also displayed mixed positive and negative cooperativity. It is proposed that ADP has an inverse effect to ATP on the rate of reductive catalysis by nitrogenase. 4. 4. The complex kinetics displayed are consistent with a hypothesis that on aspect of the role of ATP is to induce conformational changes in nitrogenase. Also, the kinetics provide a possible mechanism for nitrogenase regulation by the available energy supply. 5. 5. Tests with a variety of nitrogenous products of fixation and other compounds failed to provide convincing evidence for feed-back inhibition of the nitrogenase in Rhizobium lupini.