Escherichia coli Phosphoenolpyruvate-Dependent Phosphotransferase System: Role of Divalent Metals in the Dimerization and Phosphorylation of Enzyme I

The function of divalent metal ions (Mg2+ and Mn2+) in the dimerization and phosphorylation of enzyme I has been studied. Only a dimeric form of the enzyme can be phosphorylated. Kinetic studies of phosphoryl-group exchange between phosphoenolpyruvate and pyruvate and measurements of initial enzyme I phosphorylation rates revealed that a divalent metal ion must be bound to the enzyme to render the dimer active. Mn2+ binding experiments by means of electron paramagnetic resonance showed binding of at least one Mn2+ per unphosphorylated dimer with a binding constant comparable to the activation constant found in the kinetic studies and a 10-fold tighter binding of only one Mn2+ per phosphorylated dimer. Gel filtration experiments provided evidence that divalent metals produce about a 10-fold stabilization of the dimers, in addition to their effect on the specific dimer activity. The stability of the dimer was also strongly dependent on salts such as LiCl, NaCl, KCl, and a series of tetraalkylammonium chlorides. The relative effects of these salts suggest that hydrophobic interactions possibly play a significant role in enzyme I dimerization.