On the enthalpy of binding of ADP to heavy meromyosin

The binding of ADP to heavy meromyosin has been studied by microcalorimetry. Minute amounts of myokinase interfere with binding measurements, but by selection of appropriate conditions, we can estimate that the value of the apparent ΔH_binding lies between −1.0 and −3.0 kcal per mole of ADP bound (0.3 M KCl, 2 mM MgCl₂, 20 mM Tris, pH 8.00, 20°C). Values of ΔH_binding reported to date are an order of magnitude larger, and we suggest that these values are artifactual results due to myokinase contamination.