Fructose-1,6-bisphosphate-activated pyruvate kinase from Escherichia coli. Nature of bonds involved in the allosteric mechanism

The allosteric properties of the fructose-1,6-bis-phosphate-activated pyruvate kinase from Escherichia coli were examined in the presence of a number of fructose bisphosphate analogues, as well as of increased ionic strength (NaCl) and of the hydrogen-bond-breaking agent, formamide. Fructose 2,6-bisphosphate, ribulose 1,5-bisphosphate and 5-phosphorylribose 1-pyrophosphate gave allosteric activation (additive to that of fructose 1,6-bisphosphate). Formamide always decreased Vmax, but left unchanged the Km for phosphoenolpyruvate, while it decreased the concentration of fructose bisphosphate required to give half-maximal activity (K0.5). NaCl increased the K0.5 for both phosphoenolpyruvate and fructose bisphosphate, leaving Vmax unchanged. These results are consistent with ionic binding of fructose bisphosphate through phosphates and with a critical role of hydrogen bonds in stabilizing both the inactive and the active enzyme conformers.