Myokinase Equilibrium

Apparent equilibrium constants of many biochemical reactions are influenced by metal ions. Quantitative prediction of this effect, which is due to complex formation between metal ion and equilibrating species, requires knowledge of the relevant metal-ligand stability constants. The inconsistency of the published values for some such constants is notorious, and methods for their determination often cannot be applied under conditions close to those in the living cell. This paper proposes an enzymatic method for the determination of metal-ligand stability constants at constant pH, based on observations of apparent equilibria over a range of metal ion concentrations. A nonlinear least squares data fitting program is developed which extracts from these observations the set of stability constants, with standard errors, which best predicts the observations. The method is applied to the determination of stability constants for MgATP, MgADP, and MgAMP in 0.46 m tetrabutylammonium chloride at pH 7.25, using the myokinase equilibrium. Subject to a few constraints, extension of this data fitting procedure to other equilibria is straightforward.