AFFINITY CLEAVAGE OF CARBAMOYL-PHOSPHATE SYNTHETASE-I LOCALIZES REGIONS OF THE ENZYME INTERACTING WITH THE MOLECULE OF ATP THAT PHOSPHORYLATES CARBAMATE

Two ATP molecules are used in the reaction catalyzed by carbamoyl-phosphate synthetase I. One molecule (ATP(A)) phosphorylates HCO3- and the other (ATP(B)) phosphorylates carbamate. Carbamoylphosphate synthetase I is a 10-kDa polypeptide consisting of a 40-kDa N-terminal moiety and a 120-kDa C-terminal moiety, the latter being composed of two similar halves of molecular mass 60 kDa. We showed [Alonso, E., Cervera, J., Garcia-Espana, A., Bendala, E. and Rubio, V. (1992) J. Biol. Chem. 267, 4524-4532] that Fe . ATP bound at the site for ATP(B) catalyzes the oxidative inactivation of carbamoyl-phosphate synthetase I in a model oxidative system consisting of Fe3+, ascorbate, and O-2, and we detected ATP-promoted oxidative cleavage of the enzyme. We now provide further evidence indicating that this cleavage is catalyzed by bound Fe . ATP(B), and we demonstrate that the enzyme is cleaved at seven points, which we identify as residues 1002, 1064, 1083, 1128, 1200, 1242, and 1270. All these cleavage points are confined within and distributed throughout the more N-terminal 40-kDa region of the C-terminus of the 120-kDa moiety. Thus, this 40-kDa region contains the ATP, site, is folded as a globular domain with the polypeptide recurring several times towards the nucleotide, and appears to be a modular unit equivalent to carbamate kinase, with full responsibility for ATP(B) binding and carbamate phosphorylation. The present results and our previous demonstration [Rodriguez Aparicio, L., Guadalajara, A. M. & Rubio, V. (1989) Biochemistry 28, 3070-3074] of the binding of N-acetyl-L-glutamate in the C-terminal 20-kDa region, strongly support the idea that each homologous half of the 120-kDa moiety of carbamoyl-phosphate synthetase I is composed of a 40-kDa ATP-binding domain and a 20-kDa domain that, in the carboxyl half, is the regulatory domain.