Characterization of Slowly Interconvertible States of Phosphoribosyladenosine Triphosphate Synthetase Dependent on Temperature, Substrates, and Histidine

Phosphoribosyladenosine triphosphate (PR-ATP) synthetase from Salmonella typhimurium was found to undergo a slow temperature- and ligand-dependent activation. The kinetics of activation appeared first order with half-lives up to 17 min depending on the conditions. Activation by the substrate ATP diminished the extent of subsequent temperature activation. The extent of activation appeared to be independent of pH and buffer concentration over limited ranges and was reversible. Two high temperature (25–37°) states of PR-ATP synthetase having different inhibition responses toward histidine exist depending upon prior ligand action. Apparent patterns of histidine binding determined from kinetic data are similar to those determined from equilibrium data obtained by dialysis and protein fluorescence enhancement. The possible metabolic importance of the high temperature hysteretic behavior is discussed.