Kinetic evidence for interacting active sites in the Neurospora crassa plasma membrane ATPase.

The rate of MgATP hydrolysis (v) by the Neurospora plasma membrane ATPase shows a sigmoid relationship to substrate concentration ( [S] ), which is precisely fit by the equation: v = (Vmax X [S]2)/(Km + [S]2). This equation describes an enzyme with two substrate-binding sites, both of which must be filled for hydrolysis to occur. At concentrations above 1 mM, both free Mg2+ and free ATP behave as competitive inhibitors of the ATPase. Free ATP, although not hydrolyzed, can also significantly stimulate the rate of activity at low substrate concentrations, thus reducing the sigmoidicity in the v versus [S] curve. Vanadate also stimulates the ATPase if the MgATP concentration is below the Km. (Vanadate is a potent inhibitor of activity at saturating MgATP concentrations.) The effect of vanadate is to eliminate the sigmoidicity in the v versus [S] plot. The kinetic behavior of the ATPase suggests that binding to one active site by MgATP, free ATP, or vanadate permits hydrolysis of MgATP at a second site.