Mathematical modelling of ATP, K + and Na + interactions with ( [formula omitted] occurring under equilibrium conditions

Abstract

The controlling effect of ATP, K + and Na + on the rate of (Na + + K +)-ATPase inactivation by 7-chloro-4-nitrobenzo-2-oxa-1,3-diazole (NBD-Cl) is used for the mathematical modelling of the interaction of the effectors with the enzyme under equilibrium conditions. 1. 1. Of a series of conceivable interaction models, designed without conceptual restrictions to describe the effector control of inactivation kinetics, only one fits the experimental data described in a preceding paper. 2. 2. The model is characterized by the coexistence of two binding sites for ATP and the coexistence of two separate binding sites for K + and Na + on the enzyme-ATP complex. On the basis of this model, the effector parameters fitting the experimental data most closely are estimated by means of nonlinear least-squares fits. 3. 3. The apparent dissociation constants for ATP of the enzyme-ATP complex or of the enzyme-(ATP) 2 complex are computed to lie near 0.0024 mM and 0.34 mM, respectively, irrespective of whether K + and Na + were absent or K + and K + plus Na +, respectively, were present in the experiments. 4. 4. The origin of the high and the low affinity site for binding of ATP to the (Na + + K +)-ATPase molecule is traced back to the coexistence of two catalytic centres which, although primarily equivalent as to the reactivity of their thiol groups with NBD-Cl, are induced into anticooperative communication by ATP binding and thus show an induced geometric asymmetry.