Kinetic interactions of glycine with substrates and effectors of phosphenolpyruvate carboxylase from maize leaves

Abstract

Glycine enhanced the sensitivity of maize phosphenolpyruvate carboxylase to the activator glucose 6-phosphate and reduced the sensitivity of the enzyme to the inhibitors malate and aspartate. The effects of glycine on the kinetic constants for these other effectors were greater than its effect on the Km for substrate, raising the Ki(malate) 11-fold and reducing Ka(glucose6-P) 7-fold, while reducing the Km(PEP) by 3-fold. Kinetically saturating levels of glycine and glucose 6-phosphate acted synergistically to raise Ki(malate) higher than that observed with either activator alone. Glycine and glucose 6-phosphate also synergistically reduced aspartate inhibition. Dual inhibitor analysis indicated that aspartate and malate bind in a mutually exclusive manner, and thus probably compete for the same inhibitor site. In contrast, the synergism between glycine and glucose 6-phosphate indicate that these activators bind at separate sites. Glycine also reduced the Km(Mg) by 3-fold but had no significant effect on the Km of bicarbonate.