Hysteretic behaviour of citrate synthase. Symmetry in the kinetics of the synthase partial reactions.

The description of hysteretic behaviour of citrate synthase is completed with the demonstration of a burst period in the citryl-CoA lyase reaction. The kinetics of this partial reaction show symmetry to those of the citryl-CoA hydrolase reaction. The amplitudes of the burst periods of each partial reaction are proportional to synthase activity. Using the synthase species proteolytically nicked by endoproteinase Lys-C, a standard was elaborated to determine the actual ratio of hydrolase over lyase reactions which was found to be 0.72:0.28. The ratio found with native synthase averaged 0.8:0.2. These and other results indicate that less oxaloacetate is liberated from the synthase than is actually generated in the lyase reaction of citryl-CoA. The temperature dependence of hysteretic behaviour of both partial reactions is consistent with the participation of citryl-CoA-derived physiological substrates in the generation of this behaviour. More hydrolytic products were formed at low than at high temperature. As shown with the proteolytically nicked synthase species indicated above, this effect is related to different temperature coefficients of the partial reactions. The apparent activation energies of the citryl-CoA hydrolase and lyase reactions, 26.7 kJ X mol-1 and 44.6 kJ X mol-1, respectively, were determined. The action of established synthase inhibitors on the expression of hysteretic behaviour is described and discussed.