The Control of Pyruvate Kinases of Escherichia coli. II. Effectors and Regulatory Properties of the Enzyme Activated by Ribose 5-Phosphate

The pyruvate kinases of Escherichia coli activated by ribose 5-phosphate (RP) has been partially purified. The active form of the enzyme has a molecular weight of about 180 000 as judged by sucrose density gradient centrifugations and Sephadex G-150 chromatography. On dissociation in the absence of sulfhydryl reagents such as dithiothreitol, the enzyme is inactivated and it has a molecular weight of about 110 000. Various substrates and effectors of the enzyme, with the exception of phosphate, do not influence the association–dissociation equilibrium of the enzyme.The enzyme, unlike pyruvate kinases from many other sources, is not activated by potassium ions. Sulfate and phosphate ions are inhibitory to the enzyme. Phosphate seems to be an allosteric inhibitor and its effect is completely antagonized by activators. The enzyme is activated in an allosteric manner by two classes of compounds, nucleoside monophosphates and sugar phosphates of the hexose monophosphate pathway. Amongst the nucleotides, guanosine 5′-phosphate and adenosine 5′-phosphate are the most effective activators. Amongst the hexose monophosphate pathway intermediates, RP is the most powerful activator, with apparent activation constants as low as 1 μM. Sugar phosphates esterified at C-1 or both terminal positions are entirely ineffective in activation. The effectors act by changing the Michaelis constant for the substrates. Both of the substrates of the enzyme, adenosine diphosphate and phosphoenolpyruvate, yield cooperative rate–concentration plots in the presence of unsaturating concentrations of the fixed changing substrate. The initial velocity plots for both substrates become hyperbolic in the presence of saturating concentrations of RP.