STEROID RECOGNITION BY CHLORAMPHENICOL ACETYLTRANSFERASE - ENGINEERING AND STRUCTURAL-ANALYSIS OF A HIGH-AFFINITY FUSIDIC ACID-BINDING SITE

The antibiotic fusidic acid and certain closely related steroidal comp ounds are potent competitive inhibitors of the type I variant of chlor amphenicol acetyltransferase (CAT(I)). Ln the absence of crystallograp hic data for CAT(I), the structural determinants of steroid binding we re identified by (1) construction in vitro of genes encoding chimaeric enzymes containing segments of CAT(I) and the related type III varian t (CAT(III)) and (2) site-directed mutagenesis of the gene encoding CA T(III), followed by kinetic characterisation of the substituted varian ts. Replacement of four residues of CAT(III) (Gln92, Asn146, Tyr168 an d Ile172) by their equivalents from CATI yields an enzyme variant that is susceptible to competitive inhibition by fusidate with respect to chloramphenicol (K-i = 5.4 mu M). The structure of the complex of fusi date and the Q92C/N146F/Y168F/I172V variant, determined at 2.2 fi reso lution by X-ray crystallography, reveals the inhibitor bound deep with in the chloramphenicol binding site and in close proximity to the side -chain of His195, an essential catalytic residue. The aromatic side-ch ain of Phe146 provides a critical hydrophobic surface which interacts with non-polar substituents of the steroid. The remaining three substi tutions act in concert both to maintain the appropriate orientation of Phe 146 and via additional interactions with the bound inhibitor. The substitution of Gln92 by Cys eliminates a critical hydrogen bond inte raction which constrains a surface loop (residues 137 to 142) of wild- type CAT(III) which must move in order for fusidate to bind to the enz yme. Only two hydrogen bonds are observed in the CAT-fusidate complex, involving the 3-alpha-hydroxyl of the A-ring and both hydroxyl of Tyr 25 and NE2 of His195, both of which are also involved in hydrogen bond s with substrate in the CAT(III)-chloramphenicol complex. In the acety l transfer reaction catalysed by CAT, NE2, of His195 serves as a gener al base in the abstraction of a proton from the 3-hydroxyl of chloramp henicol as the first chemical step in catalysis. The structure of the CAT-inhibitor complex suggests that deprotonation of the 3-alpha-hydro xyl of bound fusidate by this mechanism could produce an oxyanion nucl eophile analogous to that seen with chloramphenicol, but one which is incorrectly positioned to attack the thioester carbonyl of acetyl-CoA, accounting for the observed failure of CAT to acetylate fusidate. (C) 1995 Academic Press Limited