The Binding of Specific Ligands to Adenosine‐Triphosphate Phosphoribosyltransferase

Ligand binding by adenosine-triphosphate phosphoribosyltransferase was studied by different methods. 200000 daltons of enzyme bound approximately 3 molecules of histidine cooperatively with a Hill plot slope of 1.23 (half-maximal binding at 520 μM). AMP increased the affinity of the enzyme for histidine (half-maximal binding at 80 μM). In the presence of AMP the binding of histidine was strongly cooperative with a Hill plot slope of 2.3. The transferase binds a little more than 3 molecules of AMP per hexamer of enzyme with a dissociation constant of the transferase · AMP complex of approximately 25 μM. ATP was able to displace radioactive AMP from the enzyme only at a concentration ratio of 25 in favour of ATP. The transferase bound 3 molecules of ATP per 200000 daltons to an inhomogeneous population of sites, or by a mechanism of negative cooperativity. The binding of phosphoribosyladenosine triphosphate took place preferably at 1–2 sites per hexamer of enzyme, depending on several factors including the magnesium concentration.