Interaction of Macroions and Dioxane with the Allosteric Phosphoenolpyruvate Carboxylase

Phosphoenolpyruvate carboxylase from Salmonella typhimurium is activated by basic compounds such as polylysine, protamine, histone, and spermidine. Ionic strength and pH of the assay medium are contributory factors for the activation of the enzyme by polylysine. Gradient centrifugation experiments show that polylysine binds with the enzyme. The polylysine-enzyme aggregate remains susceptible to the modulation of its activity by the allosteric effectors, aspartate (inhibitor) and fructose 1,6-diphosphate (activator). Various nonpolar solvents such as dioxane, 2-propanol, ethanol, dimethyl sulfoxide, and propylene glycol also activate the enzyme reversibly. In the presence of 10% dioxane but not the other solvents the enzyme gets desensitized to the effect of aspartate and fructose 1,6-di-P. This desensitization is a reversible process and does not seem to be due to a dissociation of the enzyme polymer into subunits. It is concluded that the enzyme possibly has accessible hydrophobic regions coupled with exposed areas with a preponderance of negatively charged groups. Long range electrostatic interactions or weakening of the hydrophobic bonds of the enzyme in its native configuration both lead to the activation of the enzyme.