Self-association and allosteric properties of glutamine-dependent carbamyl phosphate synthetase. Reversible dissociation to monomeric species.

Glutamine-dependent carbamyl phosphate synthetase from Escherichia coli can exist in two different conformations which exhibit, respectively, monomer sedimentation coefficients (.&J of 7.3 S and 8.7 S. Both conformations undergo rapid reversible self-association upon addition of potassium ions or ornithine. Formation of the 8.7 S conformation is promoted by the addition of phosphate ions. Low concentrations of urea (0.2 to 2.0 M) or guanidine hydrochloride (0.05 to 0.5 M), both of which inhibit the enzymatic activity, favor dissociation in potassium phosphate buffer, which is reversible. Ammonia has been found to be a potent allosteric activator of the enzyme. The presence of ammonia or ornithine (also an allosteric activator) promotes oligomer formation; the maximum sedimentation coefficient observed (in potassium phosphate buffer containing ornithine) is 14.8 S. Sedimentation equilibrium studies indicate the existence of tetramer or higher species in potassium phosphate buffer containing ornithine. Models are presented which explain the apparent relationship between allosteric regulation of the enzyme and its state of association.