The substrate curve of the Na,K-ATPase

ROSSI, R. C.; GARRAHAN, P. J.

The Sodium Pump

The Company of Biologists Ltd

Año: 1985; p. 443 - 455

Most, if not all, of the equations for those enzymic mechanisms that show biphasic responses to the concentration of the substrate can be written as quotients of polynomials of degree two and are therefore mathematically equivalent. For this reason, rate equations of very different models will fit, with the same precision, the experimental data of Na,K-ATPase activity versus ATP concentration, making it impossible to use curve fitting to discriminate between the different models.The Na-ATPase activity of the Na,K-ATPase shows a simple hyperbolic response to the concentration of ATP with Km similar to Ks, K(Mg)<<Kdiss, lack of interactions between the apparent affinities for ATP and Mg, and ping-pong kinetics between activation by ATP and low-affinity activation by Na+. These properties would be explained by the currently accepted reaction schemes if ATP were far from equilibrium and some plausible numerical relations held among the values of the rate constants for the elementary steps.