STRUCTURE OF THE HEXAPEPTIDE XENOBIOTIC ACETYLTRANSFERASE FROM PSEUDOMONAS-AERUGINOSA

The crystal structure of the xenobiotic acetyltransferase from Pseudom onas aeruginosa PA103 (PaXAT) has been determined, as well as that of its complex with the substrate chloramphenicol and the cofactor analog ue desulfo-coenzyme A, PaXAT is a member of the large hexapeptide acyl transferase family of enzymes that display tandem repeated copies of a six-residue hexapeptide repeat sequence motif encoding a left-handed parallel beta helix (L beta H) structural domain. The xenobiotic acety ltransferase class of hexapeptide acyltransferases is composed of micr obial enzymes that utilize acetyl-CoA to acylate a variety of hydroxyl -bearing accepters. The active site of trimeric PaXAT is a short tunne l into which chloramphenicol and the cofactor analogue desulfo-CoA pro ject from opposite ends. This tunnel is formed by the flat parallel be ta sheets of two separate L beta H domains and an extended 39-residue loop. His 79 of the extended loop forms hydrogen bonds from its imidaz ole NE2 atom to the 3-hydroxyl group of chloramphenicol and from its N D1 group to the peptide oxygen of Thr 86, The interactions of this his tidine residue are similar to those found in the structurally unrelate d type III chloramphenicol acetyltransferase and suggest that His 79 o f PaXAT maybe similarly positioned and tautomerically stabilized to se rve as a general base catalyst.