The regulation of yeast pyruvate carboxylase by acetyl-coenzyme A and l-aspartate

In contrast with pyruvate carboxylases from animal tissues, baker's yeast pyruvate carboxylase is activated by acetyl-CoA following Michaelis-Menten kinetics and the apparent Hill coefficient n is equal to one. However, in the presence of l-aspartate (an inhibitor of the enzyme reaction), plots of the reaction velocity vs. activator concentration become sigmoid and the apparent n for acetyl-CoA increases to nearly 2. l-aspartate (5 and 10 m m) increases 4 and 10 times, respectively, the concentration of acetyl-CoA necessary for half-maximal activation [(A) 0.5 value]. Similar effects have been also demonstrated for the enzyme-catalysed decarboxylation of oxaloacetate. Furthermore, in the carbon dioxide fixation reaction, acetyl-CoA counteracts the inhibitory effect of l-aspartate, as shown by the significant increase in the 50% inhibitory concentration for l-aspartate [(I) 0.5 value]. Accordingly, the apparent n value for l-aspartate increases significantly in the presence of activator. The evidence presented shows that, in spite of circumstancial differences, pyruvate carboxylase from baker's yeast may be of allo-steric nature, like the chicken liver and sheep kidney pyruvate carboxylases.