1cla
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(New page: 200px<br /><applet load="1cla" size="450" color="white" frame="true" align="right" spinBox="true" caption="1cla, resolution 2.34Å" /> '''EVIDENCE FOR TRANSIT...)
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Revision as of 10:26, 20 November 2007
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EVIDENCE FOR TRANSITION-STATE STABILIZATION BY SERINE-148 IN THE CATALYTIC MECHANISM OF CHLORAMPHENICOL ACETYLTRANSFERASE
Overview
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 Modeling studies, (P. C. E. Moody and A. G. W. Leslie, unpublished results) suggested that, the hydroxyl group of Ser-148 could be involved in transition-state, stabilization via a hydrogen bond to the oxyanion of the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).
About this Structure
1CLA is a Single protein structure of sequence from Escherichia coli with CO and CLM as ligands. Active as Chloramphenicol O-acetyltransferase, with EC number 2.3.1.28 Full crystallographic information is available from OCA.
Reference
Evidence for transition-state stabilization by serine-148 in the catalytic mechanism of chloramphenicol acetyltransferase., Lewendon A, Murray IA, Shaw WV, Gibbs MR, Leslie AG, Biochemistry. 1990 Feb 27;29(8):2075-80. PMID:2109633
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