Importance of residues Arg-167 and Gln-231 in both the allosteric and catalytic mechanisms of Escherichia coli aspartate transcarbamoylase.
Site-specific mutagenesis has been used to create two mutant versions of aspartate transcarbamoylase. Arg-167 and Gln-231, both previously identified as interacting with the portion of the bisubstrate analogue N-(phosphonoacetyl)-L-aspartate (PALA) that corresponds to aspartate [Krause, K. L., Voltz, K. W., & Lipscomb, W. N. (1987) J. Mol. Biol. 193, 527-553], were replaced by glutamine and leucine, respectively. The Arg-167----Gln and the Gln-231----Leu enzymes show approximately 900-fold and 1500-fold reductions in the maximal observed specific activity, respectively. The aspartate concentration at half the maximal observed specific activity is increased 18-fold for the Gln-231----Leu enzyme compared to the value for the wild-type enzyme, but is altered little in the case of the Arg-167----Gln enzyme. The carbamoyl phosphate concentration at half the maximal activity is unchanged by either mutation, suggesting that these mutations result in only local changes to the aparatate binding site. Both mutations eliminate homotropic cooperativity; however, the Gln-231----Leu enzyme also has altered heterotropic interactions and no longer exhibits substrate inhibition. At relatively low concentrations of aspartate and saturating carbamoyl phosphate, PALA is able to activate the Gln-231----Leu enzyme, whereas the Arg-167----Gln enzyme is inhibited at PALA concentrations that normally activate the wild-type enzyme. Equilibrium binding measurements indicate that the Gln-231----Leu enzyme binds CTP approximately 10-fold more weakly than the wild-type enzyme, even though the mutation is some 70 A from the regulatory binding site.(ABSTRACT TRUNCATED AT 250 WORDS)
Department of Chemistry, Boston College, Chestnut Hill, Massachusetts 02167.
J W
JW
Stebbins
Y
Y
Zhang
E R
ER
Kantrowitz
D000255:Adenosine Triphosphate / Q000494:pharmacology; D000494:Allosteric Regulation; D001221:Aspartate Carbamoyltransferase / Q000235:genetics / Q000378:metabolism*; D001224:Aspartic Acid / Q000031:analogs & derivatives / Q000378:metabolism / Q000493:pharmacokinetics; D001665:Binding Sites; D002384:Catalysis; D003570:Cytidine Triphosphate / Q000494:pharmacology; D004926:Escherichia coli / Q000187:drug effects / Q000201:enzymology* / Q000235:genetics; D005973:Glutamine / Q000378:metabolism; D007700:Kinetics; D009154:Mutation; D010746:Phosphonoacetic Acid / Q000031:analogs & derivatives / Q000493:pharmacokinetics; D011487:Protein Conformation
D005973:Glutamine; D001224:Aspartic Acid; D003570:Cytidine Triphosphate; C013195:sparfosic acid; D000255:Adenosine Triphosphate; D001221:Aspartate Carbamoyltransferase; D010746:Phosphonoacetic Acid