Sedimentation behaviour of phosphoribosyladenosine triphosphate synthetase effects of substrates and modifiers

Abstract We have studied the sedimentation properties of purified phosphoribosyladenosine triphosphate:pyrophosphate phosphoribosyltransferase (phosphoribosyladenosine triphosphate synthetase) from Escherichia coli as influenced by the ligands phosphoribosyladenosine triphosphate, AMP and histidine. 1. 1. In dilute imidazole buffer the enzyme had an s 20, w of 12.6 S, while in higher ionic strength buffer with 0.4 mM histidine the s 20, w was 8.9 S. 2. 2. An 8.9-S species was stabilized by the product of the synthetase reaction, phosphoribosyladenosine triphosphate, and AMP had a similar effect. High concentrations of ATP also stabilized the 8.9-S species. 3. 3. The area under the 8.9-S peak gives a rough estimate of the ligand effect. When histidine was acting alone or in the presence of low concentrations of ATP, cooperative behaviour by histidine was apparent. When AMP was added, the enzyme responded to histidine at lower ligand concentrations and apparently with less cooperativity. 4. 4. A hypothetical model is discussed in the light of earlier and new results.