Abstract
In this report, we establish a straightforward method for estimating the equilibrium constant for the creatine kinase reaction (CK K(eq)″) over wide but physiologically and experimentally relevant ranges of pH, Mg(2+) and temperature. Our empirical formula for CK K(eq)″ 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(ATP), necessitating the reinvestigation of previously reported ADP- and ΔG(ATP)-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(2+)-activated isometric force depends on ΔG(ATP) 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.