Multiple Aggregation States of Phosphoribosyladenosine Triphosphate Synthetase

The association states of phosphoribosyladenosine triphosphate synthetase from Salmonella typhimurium were studied using ultracentrifugation, gel filtration, and fluorescence spectroscopic techniques. The enzyme exists predominantly as a hexamer at 25 and 37° under mild solvent conditions. At 4–7.5° it aggregates to species smaller and larger than a hexamer depending on protein concentration. High ionic strength cesium chloride at 25° leads to a species larger than the hexamer. Low ionic strength, pH 10, or aging dissociate the enzyme to a dimer. The combination of low ionic strength and pH 10 can dissociate the enzyme further to a monomer. Either of the substrates or histidine stabilizes the hexamer form of the enzyme, but sodium ion is necessary for effective stabilization by histidine. Active enzyme was shown to be a hexamer under assay conditions, even when incubated under conditions leading to indefinite aggregation prior to ultracentrifugation in assay media.