Kinetic Studies Of Nitrogenase From Soya-bean Root-nodule Bacteroids
The apparent Michaelis constants [K′(N2) and K′(C2H2)] and the corresponding apparent maximum velocity values (V′) for soya-bean bacteroid nitrogenase increased concomitantly in response to increases in nitrogenase Fe–protein concentration and ATP concentration in cell-free assays and in response to O2 pressure in intact nodules and bacteroid suspensions. K′(C2H2) in cell-free assays was also affected by pH and by Na2S2O4 concentration. Nitrogenase Fe–protein behaved as a catalytic effector reacting at interacting sites on the nitrogenase Fe–Mo–protein. The results indicated that the Fe–Mo–protein probably bears the catalytic sites for N2 and C2H2 reduction. It is concluded that reduction of N2 or C2H2 by this nitrogenase involves a reaction mechanism with a sequence of unknown order. The sequence in which substrate, enzyme, effector, ATP and reductant react determines which of the various rate-constants are involved in the apparent Michaelis constant, whose true kinetic meaning was thus unresolved.