Ligand interaction energies and molecular recognition by chloramphenicol acetyltransferase

Replacement of catalytic histidine-195 of chloramphenicol acetyltransferase: evidence for a general base role for glutamate

The imidazole N epsilon 2 of His-195 plays an essential part in the proposed general base mechanism of chloramphenicol acetyltransferase (CAT), hydrogen bonding to and a abstracting a proton from the primary hydroxyl group of chloramphenicol. Replacement of His-195 by alanine or glutamine results in apparent decreases in kcat of (9 x 10(5)- and (3 x 10(5))-fold, respectively, whereas Km values for both substrates (chloramphenicol and acetyl-CoA) are similar to those of wild-type CAT. The structure of Gln-195 CAT has been solved at 2.5-A resolution and is largely isosteric with that of wild-type CAT. Substitution of His-195 by glutamate resulted in a (5 x 10(4))-fold decrease in kcat together with a 3-fold increase in the Km for chloramphenicol. Direct determination of binding constants for both substrates demonstrated that these substitutions result in only small decreases in the affinity of CAT for acetyl-CoA (Kd values increased 2- to 3-fold), whereas chloramphenicol Kd values are elevated 26-, 20-, and 53-fold for Ala-195 CAT, Gln-195 CAT and Glu-195 CAT, respectively. The pH dependence of kcat/Km yields apparent pKa values of 6.5 and 6.7 for Ala-195 CAT and Gln-195 CAT, respectively, which are very similar to that (6.6) determined for the ionization of His-195 in wild-type CAT. In contrast, the pH dependence of kcat/Km for Glu-195 CAT (pKa = 8.3) is very different from that of wild-type CAT.(ABSTRACT TRUNCATED AT 250 WORDS)

Intrinsic fluorescence of chloramphenicol acetyltransferase: responses to ligand binding and assignment of the contributions of tryptophan residues by site-directed mutagenesis

Replacement by tyrosine or phenylalanine was used to assign the additive contributions of each of the three tryptophan residues of chloramphenicol acetyltransferase (CAT) to its intrinsic fluorescence on excitation at 295 nm. During the assessment of the fluorescence responses of the wild-type enzyme to the binding of ligands, it was found that the overlapping absorption spectra of chloramphenicol and tryptophan, with an attendant inner filter effect, required the use of a displacement technique involving an alternative substrate (the p-cyano analogue of chloramphenicol) without significant absorption at 295 nm. By the use of two-Trp, one-Trp, and Trp-less variants, in combination with this displacement technique, it was possible to demonstrate that Trp-86 and Trp-152 are involved in the fluorescence quenching associated with the binding of chloramphenicol, most likely via nonradiative energy transfer from these residues to the bound substrate. Trp-152 is mainly responsible for the fluorescence enhancement accompanying the binding of acetyl-CoA (and CoA) through proximity effects and solvent exclusion on substrate association.

Substrate binding to chloramphenicol acetyltransferase: evidence for negative cooperativity from equilibrium and kinetic constants for binary and ternary complexes

Chloramphenicol acetyltransferase (CAT) catalyzes the acetyl-CoA-dependent acetylation of chloramphenicol by a ternary complex mechanism with a rapid equilibrium and essentially random order of addition of substrates. Such a kinetic mechanism for a two-substrate reaction provides an opportunity to compare the affinity of enzyme for each substrate in the binary complexes (1/Kd) with corresponding values (1/Km) for affinities in the ternary complex where any effect of the other substrate should be manifest. The pursuit of such information for CAT involved the use of four independent methods to determine the dissociation constant (Kd) for chloramphenicol in the binary complex, techniques which included stopped-flow measurements of on and off rates, and a novel fluorometric titration method. The binary complex dissociation constant (Kd) for acetyl-CoA was measured by fluorescence enhancement and steady-state kinetic analysis. The ternary complex dissociation constant (Km) for each substrate (in the presence of the other) was determined by kinetic and fluorometric methods, using CoA or ethyl-CoA to form nonproductive ternary complexes. The results demonstrate an unequivocal decrease in affinity of CAT for each of its substrates on progression from the binary to the ternary complex, a phenomenon most economically described as negative cooperativity. The binary complex dissociation constants (Kd) for chloramphenicol and acetyl-CoA are 4 microM and 30 microM whereas the corresponding dissociation constants in the ternary complex (Km) are 12 microM and 90 microM, respectively.(ABSTRACT TRUNCATED AT 250 WORDS)