Relation between the thermostability of the tetrameric molecule of lactate dehydrogenase from swine muscles and the degree of occupancy of its active sites with ligands

Using differential scanning microcalorimetry and measurements of protein fluorescence, the thermal denaturation of lactate dehydrogenase (LDH) from porcine muscle (in the apo-form as well as in the form of the enzyme-pyruvate, enzyme-NAD+ and enzyme-NAD-pyruvate-adduct complexes) was studied. Pyruvate binding did not affect the thermal stability of LDH. NAD+ exerted a stabilizing effect on the enzyme, the value of which was proportional to the number of ligand molecules bound per LDH tetramer. The formation of the abortive LDH-NAD-pyruvate complex in one, two or three active centers of the enzyme tetramer did not influence the values of calorimetric parameters of thermal denaturation in comparison with those for the apoenzyme. The occupancy of all four active centers of LDH by the adduct resulted in a sharp increase of the enzyme thermal stability and tightness of the LDH adduct complex as compared with complexes formed upon partial saturation. The experimental results are suggestive of the existence of a concerted conformational transition of the LDH tetramer induced by the formation of the LDH-NAD-pyruvate complex in the last active center of the tetramer.