pThe equilibrium constant of the phosphoglyceromutase reaction was determined over a range of pH (5.4–7.9), in solutions of different ionic strength (0.06–0.3) and in the presence of Mgsup2+/sup, at 30°C and at 20°C. The values obtained (8.65–11.65) differ substantially from previously published values. The third acid dissociation constants were redetermined for 2- and 3-phosphoglycerate, and in contrast with previous reports the piK/i values (7.03 and 6.97 respectively at zero ionic strength) were closely similar. The Mgsup2+/sup-binding constants were measured spectrophotometrically and the values, 286mmsup-1/sup and 255mmsup-1/sup for 2- and 3-phosphoglycerate at pH7 and ionic strength 0.02, were also very similar. From the relative lack of effect of temperature, pH and ionic strength it is concluded that the equilibrium constant differs from unity largely because of entropic factors. At low ionic strength, in the neutral region, the pH-dependence can be attributed to the small difference in the acid dissociation constants, but the difference in dissociation constants does not explain the pH-dependence in the acid region or at high ionic strength. Within physiological ranges of pH, Mgsup2+/sup concentration and ionic strength there will be little variation in equilibrium constant./p
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