title: A 70-amino acid zinc-binding polypeptide fragment from the regulatory chain of aspartate transcarbamoylase causes marked changes in the kinetic mechanism of the catalytic trimer.
abstract: Interaction between a 70-amino acid and zinc-binding polypeptide from the regulatory chain and the catalytic (C) trimer of aspartate transcarbamoylase (ATCase) leads to dramatic changes in enzyme activity and affinity for active site ligands. The hypothesis that the complex between a C trimer and 3 polypeptide fragments (zinc domain) is an analog of R state ATCase has been examined by steady-state kinetics, heavy-atom isotope effects, and isotope trapping experiments. Inhibition by the bisubstrate ligand, N-(phosphonacetyl)-L-aspartate (PALA), or the substrate analog, succinate, at varying concentrations of substrates, aspartate, or carbamoyl phosphate indicated a compulsory ordered kinetic mechanism with carbamoyl phosphate binding prior to aspartate. In contrast, inhibition studies on C trimer were consistent with a preferred order mechanism. Similarly, 13C kinetic isotope effects in carbamoyl phosphate at infinite aspartate indicated a partially random kinetic mechanism for C trimer, whereas results for the complex of C trimer and zinc domain were consistent with a compulsory ordered mechanism of substrate binding. The dependence of isotope effect on aspartate concentration observed for the Zn domain-C trimer complex was similar to that obtained earlier for intact ATCase. Isotope trapping experiments showed that the compulsory ordered mechanism for the complex was attributable to increased "stickiness" of carbamoyl phosphate to the Zn domain-C trimer complex as compared to C trimer alone. The rate of dissociation of carbamoyl phosphate from the Zn domain-C trimer complex was about 10(-2) that from C trimer.(ABSTRACT TRUNCATED AT 250 WORDS)
authors.affiliation: Department of Molecular and Cell Biology, University of California, Berkeley 94720.
authors.forename: B B
authors.initials: BB
authors.lastname: Zhou
authors.forename: G L
authors.initials: GL
authors.lastname: Waldrop
authors.forename: L
authors.initials: L
authors.lastname: Lum
authors.forename: H K
authors.initials: HK
authors.lastname: Schachman
mesh_terms: D001221:Aspartate Carbamoyltransferase / Q000037:antagonists & inhibitors / Q000737:chemistry* / Q000378:metabolism*; D001224:Aspartic Acid / Q000031:analogs & derivatives / Q000378:metabolism / Q000494:pharmacology; D001665:Binding Sites; D002221:Carbamyl Phosphate / Q000378:metabolism; D002247:Carbon Isotopes; D002384:Catalysis; D004926:Escherichia coli / Q000201:enzymology; D006863:Hydrogen-Ion Concentration; D007700:Kinetics; D046911:Macromolecular Substances; D010446:Peptide Fragments / Q000737:chemistry* / Q000378:metabolism*; D010746:Phosphonoacetic Acid / Q000031:analogs & derivatives / Q000494:pharmacology; D013329:Structure-Activity Relationship; D013386:Succinates / Q000494:pharmacology; D019802:Succinic Acid; D015032:Zinc / Q000378:metabolism*
chemical_list: D002247:Carbon Isotopes; D046911:Macromolecular Substances; D010446:Peptide Fragments; D013386:Succinates; D001224:Aspartic Acid; D002221:Carbamyl Phosphate; C013195:sparfosic acid; D019802:Succinic Acid; D001221:Aspartate Carbamoyltransferase; D015032:Zinc; D010746:Phosphonoacetic Acid