Creatine Kinase Equilibration and ΔG over an Extended Range of Physiological Conditions: Implications for Cellular Energetics, Signaling, and Muscle Performance.

In this report, we establish a straightforward method for estimating the equilibrium constant for the creatine kinase reaction (CK K″) over wide but physiologically and experimentally relevant ranges of pH, Mg and temperature. Our empirical formula for CK K″ is based on experimental measurements. It can be used to estimate [ADP] when [ADP] is below the resolution of experimental measurements, a typical situation because [ADP] is on the order of micromolar concentrations in living cells and may be much lower in many in vitro experiments. Accurate prediction of [ADP] is essential for in vivo studies of cellular energetics and metabolism and for in vitro studies of ATP-dependent enzyme function under near-physiological conditions. With [ADP], we were able to obtain improved estimates of ΔG, necessitating the reinvestigation of previously reported ADP- and ΔG-dependent processes. Application to actomyosin force generation in muscle provides support for the hypothesis that, when [Pi] varies and pH is not altered, the maximum Ca-activated isometric force depends on ΔG in both living and permeabilized muscle preparations. Further analysis of the pH studies introduces a novel hypothesis around the role of submicromolar ADP in force generation.