title: Ligand-mediated conformational changes in wheat-germ aspartate transcarbamoylase indicated by proteolytic susceptibility.
abstract: Ligand-mediated effects on the inactivation of pure wheat-germ aspartate transcarbamoylase by trypsin were examined. Inactivation was apparently first-order in all cases, and the effects of ligand concentration on the pseudo-first-order rate constant, k, were studied. Increase in k (labilization) was effected by carbamoyl phosphate, phosphate and the putative transition-state analogue, N-phosphonoacetyl-L-aspartate. Decrease in k (protection) was effected by the end-product inhibitor, UMP, and by the ligand pairs aspartate/phosphate and succinate/carbamoyl phosphate, but not by aspartate or succinate alone up to 10 mM. Except for protection by the latter ligand pairs, all other ligand-mediated effects were also observed on inactivation of the enzyme by Pronase and chymotrypsin. Ligand-mediated effects on the fragmentation of the polypeptide chain by trypsin were examined electrophoretically. Slight labilization of the chain was observed in the presence of carbamoyl phosphate, phosphate and N-phosphonoacetyl-L-aspartate. An extensive protection by UMP was observed, which apparently included all trypsin-sensitive peptide bonds. No significant effect by the ligand pair succinate/carbamoyl phosphate was noted. It is concluded from these observations that UMP triggers an extensive, probably co-operative, transition to a proteinase-resistant conformation, and that carbamoyl phosphate similarly triggers a transition to an alternative, proteinase-sensitive, conformation. These antagonistic conformational changes may account for the regulatory kinetic effects reported elsewhere [Yon (1984) Biochem. J. 221, 281-287]. The protective effect by the ligand pairs aspartate/phosphate and succinate/carbamoyl phosphate, which operates only against trypsin, is concluded to be due to local shielding of essential lysine or arginine residues in the aspartate-binding pocket of the active site, to which aspartate (or its analogue, succinate) can only bind as part of a ternary complex.
mesh_terms: D001221:Aspartate Carbamoyltransferase / Q000037:antagonists & inhibitors*; D004591:Electrophoresis, Polyacrylamide Gel; D007700:Kinetics; D008024:Ligands; D010446:Peptide Fragments / Q000032:analysis; D010447:Peptide Hydrolases / Q000494:pharmacology*; D010710:Phosphates / Q000494:pharmacology; D010944:Plants / Q000201:enzymology*; D011487:Protein Conformation; D014908:Triticum / Q000201:enzymology; D014357:Trypsin / Q000494:pharmacology
chemical_list: D008024:Ligands; D010446:Peptide Fragments; D010710:Phosphates; D001221:Aspartate Carbamoyltransferase; D010447:Peptide Hydrolases; D014357:Trypsin