Nitrogenase of Klebsiella pneumoniae: An MgATP Hydrolysing Energy Transduction System with Similarities to Actomyosin and p21ras [and Discussion]

The mechanism of ATP hydrolysis by nitrogenase shows some similarity to that proposed for actomyosin and for GTP hydrolysis by p21ras. All three systems involve the formation of an active complex from two component proteins, nucleotide-induced changes in protein conformation, energy transduction that in the case of nitrogenase involves a decrease in redox potential of metal centres, and a slow dissociation of the protein complex. Metal ion activation (Mg$^{2+}$ or Ca$^{2+}$) and in-line displacement of ADP by H$_{2}$O without enzyme phosphorylation are also common features. At 5 degrees C, stopped-flow calorimetry shows that the kinetic and thermodynamic parameters for endothermic, reversible on-enzyme cleavage of MgATP by nitrogenase and myosin subfragment 1 are remarkably similar. [$^{18}$O$_{4}$]P$_{\text{i}}$-water exchange studies also show that ATP cleavage on nitrogenase and myosin are reversible.