Activation of yeast pyruvate carboxylase: interactions between acyl coenzyme A compounds, aspartate, and substrates of the reaction.

Chicken liver pyruvate carboxylase has an absolute requirement for short-chain acyl coenzyme A (CoA), whereas the same enzyme from yeast has less stringent requirements. The yeast enzyme has now been studied in an effort to elucidate the mechanism by which acyl-CoA stimulates pyruvate carboxylase activity. Yeast pyruvate carboxylase has an apparent basal level of activity above which CoA and acyl-CoAs of 2-20 carbons activate; the concentration of acyl-CoA required for half-maximum activation (K0.5) decreases as the chain length of the acyl moiety increases to 16 carbons. Activation of yeast pyruvate carboxylase by acyl-CoA is brought about in part by increasing the affinity of pyruvate carboxylase for two substrates, bicarbonate and pyruvate. The affinity of pyruvate carboxylase for bicarbonate is also increased by potassium ions. The observation of only low levels of activity in the absence of acyl-CoA or potassium ion leads to the conclusion that the basal activity so frequently referred to is probably due to the presence of activating monovalent cations. Pyruvate carboxylase from yeast probably has an absolute requirement for monovalent cations or acyl-CoA with a combination of the two being required for optimum conditions for maximal activity. Stimulation by acyl-CoA and inhibition by aspartate are mutually antagonistic with each affecting the activation or inhibition constant and the degree of cooperativity brought about by the other. The enzyme from liver is unaffected by aspartate.