The enthalpy of interaction of malonaldehyde with amino acids and myosin: effect of pH

Abstract

The enthalpies of interaction between malonaldehyde with myosin and heavy meromyosin (HMM) in aqueous solution over a range of pH (3.6–9.0) at 25° C have been measured as a function of malonaldehyde concentration by microcalorimetry. The enthalpies of interaction are strongly pH dependent and for myosin change sign, becoming exothermic in acid solution at pH 3.6. To aid interpretation of the data the enthalpies of interaction of malonaldehyde with the amino acids arginine, histidine, lysine and methionine have also been measured. It was found that the side chain guanidino and imidazolyl groups of arginine and histidine relative to the other amino acid side chains react very exothermically with malonaldehyde in acid solution (pH 3.6). However, the enthalpies of reaction with the amino acid side chains in the myosins cannot account for the observed enthalpies of interaction with malonaldehyde but the interaction initiates denaturation resulting in absorption of heat. As for thermal denaturation, malonaldehyde-induced denaturation is strongly pH dependent in acid solution.