A microcalorimetric study of the reaction catalysed by pyruvate kinase

The enthalpy change for phosphorylation of ADP^3? by PEP^3? catalysed by pyruvate kinase has been determined at 25°C using flow microcalorimetry. Measurements were made at pH 8 in three buffer systems TRIS, TEA and HEPES and also at pH 8.5 in TRIS buffer. The values of ΔH obtained, ?8.75 kJ mol^?1 in TRIS, ?7.3₉ kJ mol^? in TEA and ?6.1₉ kJ mol^?1 in HEPES surprisingly display a dependence on the buffer system used. The enthalpy change was combined with free energy data to calculate the entropy change for the catalysed reaction.