title: Evidence for transition-state stabilization by serine-148 in the catalytic mechanism of chloramphenicol acetyltransferase.
abstract: The function of conserved Ser-148 of chloramphenicol acetyltransferase (CAT) has been investigated by site-directed mutagenesis. Modeling studies (P. C. E. Moody and A. G. W. Leslie, unpublished results) suggested that the hydroxyl group of Ser-148 could be involved in transition-state stabilization via a hydrogen bond to the oxyanion of the putative tetrahedral intermediate. Replacement of serine by alanine results in a mutant enzyme (Ala-148 CAT) with kcat reduced 53-fold and only minor changes in Km values for chloramphenicol and acetyl-CoA. The Ser-148----Gly substitution gives rise to a mutant enzyme (Gly-148 CAT) with kcat reduced only 10-fold. A water molecule may partially replace the hydrogen-bonding potential of Ser-148 in Gly-148 CAT. The three-dimensional structure of Ala-148 CAT at 2.34-A resolution is isosteric with that of wild-type CAT with two exceptions: the absence of the Ser-148 hydroxyl group and the loss of one poorly ordered water molecule from the active site region. The results are consistent with a catalytic role for Ser-148 rather than a structural one and support the hypothesis that Ser-148 is involved in transition-state stabilization. Ser-148 has also been replaced with cysteine and asparagine; the Ser-148----Cys mutation results in a 705-fold decrease in kcat and the Ser-148----Asn substitution in a 214-fold reduction in kcat. Removing the hydrogen bond donor (Ser-148----Ala or Gly) is less deleterious than replacing Ser-148 with alternative possible hydrogen bond donors (Ser-148----Cys or Asn).
authors.affiliation: Department of Biochemistry, University of Leicester, U.K.
authors.forename: A
authors.initials: A
authors.lastname: Lewendon
authors.forename: I A
authors.initials: IA
authors.lastname: Murray
authors.forename: W V
authors.initials: WV
authors.lastname: Shaw
authors.forename: M R
authors.initials: MR
authors.lastname: Gibbs
authors.forename: A G
authors.initials: AG
authors.lastname: Leslie
mesh_terms: D000105:Acetyl Coenzyme A / Q000378:metabolism; D000595:Amino Acid Sequence; D002384:Catalysis; D015500:Chloramphenicol O-Acetyltransferase / Q000235:genetics / Q000378:metabolism*; D004926:Escherichia coli / Q000201:enzymology* / Q000235:genetics; D006868:Hydrolysis; D007700:Kinetics; D008969:Molecular Sequence Data; D009154:Mutation; D011487:Protein Conformation; D012694:Serine / Q000378:metabolism*; D013816:Thermodynamics
chemical_list: D012694:Serine; D000105:Acetyl Coenzyme A; D015500:Chloramphenicol O-Acetyltransferase