Subunit association of enzyme I of the Salmonella typhimurium phosphoenolpyruvate: glycose phosphotransferase system. Temperature dependence and thermodynamic properties.

The bacterial phosphoenolpyruvate:glycose phosphotransferase system plays an essential role in diverse physiological phenomena. To perform these functions, the system is stringently regulated, although the underlying molecular regulatory mechanisms have not been established. A potential target for this type of regulation is the first protein in the phosphotransfer sequence, Enzyme I, which catalyzes the following reaction: P-enolpyruvate + Enzyme I Mg2+ in equilibrium phospho-I + pyruvate. We reported previously that Enzyme I from Salmonella typhimurium consists of identical subunits which associate in a temperature-dependent manner; the mode of association was found to be either monomer-dimer or isodesmic. The association reaction has now been investigated by analytical gel chromatography at 8, 11, and 23 degrees C. At each temperature, the mode of association was strictly monomer-dimer. The apparent association equilibrium constant, K'a, increased dramatically with temperature, with an enthalpy of 54.8 +/- 6.3 kcal/mol. At 23 degrees C, K'a decreased slightly when the enzyme solution contained either Mg2+ or phosphoenolpyruvate. However, when both ligands were present, i.e. under conditions where Enzyme I is phosphorylated, K'a decreased significantly (25-fold at 11 degrees C and 50-fold at 23 degrees C). These results are in accord with a model for the action of Enzyme I which involves a cycle of association and dissociation. This model has potentially important implications for regulating Enzyme I and the bacterial phosphoenolpyruvate:glycose phosphotransferase system.