Kinetic parameters from progress curves of competing substrates. Application to beta-lactamases

Interactions of human acetylcholinesterase with phenyl valerate and acetylthiocholine: Thiocholine as an enhancer of phenyl valerate esterase activity

Phenyl valerate (PV) is a neutral substrate for measuring the PVase activity of neuropathy target esterase (NTE), a key molecular event of organophosphorus-induced delayed neuropathy. This substrate has been used to discriminate and identify other proteins with esterase activity and potential targets of organophosphorus (OP) binding. A protein with PVase activity in chicken (model for delayed neurotoxicity) was identified as butyrylcholinesterase (BChE). Further studies in human BChE suggest that other sites might be involved in PVase activity. From the theoretical docking analysis, other more favorable sites for binding PV related to the Asn289 residue located far from the catalytic site ("PVsite") were deduced.In this paper, we demonstrate that acetylcholinesterase is also able to hydrolyze PV. Robust kinetic studies of interactions between substrates PV and acetylthiocholine (AtCh) were performed. The kinetics did not fit the classic competition models among substrates. While PV interacts as a competitive inhibitor in AChE activity, AtCh at low concentrations enhances PVase activity and inhibits this activity at high concentrations. Kinetic behavior suggests that the potentiation effect is caused by thiocholine released at the active site, where AtCh could act as a Trojan Horse. We conclude that the products released at the active site could play an important role in the hydrolysis reactions of different substrates in biological systems.

Assay for rapid analysis of the tri-peptidase activity of LTA4 hydrolase

Leukotriene A4 hydrolase is a bifunctional zinc metalloenzyme with an epoxide hydrolase activity as well as an arginyl tri-peptidase activity. Detailed enzymological and mechanistic investigations of the latter activity have been hampered by the lack of a rapid and convenient enzyme assay. Here we have developed a new method allowing direct spectrophotometric assessment of the tri-peptide cleaving activity of leukotriene A4 hydrolase, as well as other peptidases. The method utilizes two competing substrates, one chromogenic reference substrate together with the tri-peptide substrate of interest, and relies on computer-assisted analysis of progress curves. The chromogenic reference substrate serves to disclose the "invisible" tri-peptide substrate for kinetic analysis. The method is fast and simple and will allow detailed kinetic studies and screening for natural peptide substrates of leukotriene A4 hydrolase as well as other members of the M1 family of aminopeptidases.

Site-directed mutagenesis of beta-lactamase I: role of Glu-166

Two Glu-166 mutants of beta-lactamase I from Bacillus cereus 569/H were constructed: one with a lengthened side chain (E166Cmc, the S-carboxymethylcysteine mutant) and the other with the side chain shortened and made non-polar (E166A). Their kinetic properties were studied and compared with those of the wild-type and the E166D mutant (with a shortened side chain) previously made by Gibson, Christensen and Waley (1990) (Biochem. J. 272, 613-619). Surprisingly, with good penicillin substrates, Km, kcat. and kcat./Km of the two conservative mutants (E166Cmc and E166D) are similar to those of the non-conservative mutant E166A. Their kcat. values are 3000-fold lower than that of the wild-type enzyme, showing that Glu-166 is a very important residue. The acylenzyme intermediate of E166A and a good substrate, penicillin V, was trapped by acid-quench and observed by electrospray ionization mass spectrometry, suggesting that Glu-166 is more important in catalysing the deacylation step than the acylation step. The beta-lactamase I E166A mutant is about 200-fold more active than the Bacillus licheniformis E166A mutant with nitrocefin or 6 beta-furylacryloyl-amidopenicillanic acid as substrate. This suggested that other groups in the active site of the beta-lactamase I mutant may activate the catalytic water molecule for deacylation.

Site-directed mutagenesis and substrate-induced inactivation of beta-lactamase I

The substrate-induced inactivation of beta-lactamase I from Bacillus cereus 569/H has been studied. Both the wild-type enzyme and mutants have been used. The kinetics follow a branched pathway of the type recently analysed [Waley (1991) Biochem. J. 279, 87-94]. The substrate cloxacillin (a penicillin) formed an acyl-enzyme (characterized by m.s.), and it was probably the instability of this intermediate that brought about inactivation. A disulphide bond was introduced into beta-lactamase I (the wild-type enzyme lacks this bond) by site-directed mutagenesis: Ala-77 and Ala-123 were replaced by cysteine. Spontaneous oxidation yielded the disulphide. The activity of this newly cross-linked enzyme was a little diminished, but the stability towards inactivation by cloxacillin was not increased. A second mutant of beta-lactamase I was studied: this mutant lacked the first 17 residues, i.e. the first alpha-helix. The mutant had reduced activity towards ordinary (non-inactivating) substrates and no hydrolysis of cloxacillin could be detected. These mutant enzymes were expressed in Bacillus subtilis, and were purified from the extracellular medium.

Site-directed mutagenesis of beta-lactamase I. Single and double mutants of Glu-166 and Lys-73

Two single mutants and the corresponding double mutant of beta-lactamase I from Bacillus cereus 569/H were constructed and their kinetics investigated. The mutants have Lys-73 replaced by arginine (K73R), or Glu-166 replaced by aspartic acid (E166D), or both (K73R + E166D). All four rate constants in the acyl-enzyme mechanism were determined for the E166D mutant by the methods described by Christensen, Martin & Waley [(1990) Biochem. J. 266, 853-861]. Both the rate constants for acylation and deacylation for the hydrolysis of benzylpenicillin were decreased about 2000-fold in this mutant. In the K73R mutant, and in the double mutant, the rate constants for acylation were decreased about 100-fold and 10,000-fold respectively. All three mutants also had lowered values for the rate constants for the formation and dissociation of the non-covalent enzyme-substrate complex. The specificities of the mutants did not differ greatly from those of wild-type beta-lactamase, but the hydrolysis of cephalosporin C by the K73R mutant gave 'burst' kinetics.

The Michaelis constants of a nonchromogenic substrate may be determined using a chromogenic substrate

A general method is presented for the determination of the KM and Vmax for a nonchromogenic substrate in an experimental system where a chromogenic and a nonchromogenic substrate compete for the active site of a single enzyme. Entire progress curves of absorbance versus time for the transformation of the chromogenic substrate must be obtained in the absence and presence of the nonchromgenic substrate. Two quantities may then be extracted from the entire kinetic curves: the value of a delta Area, the area bounded by the kinetic traces of absorbance versus time in the presence and absence of the nonchromogenic substrate; and the value of delta t(5%), the time required to transform 5% of the chromogenic substrate in the presence of the nonchromogenic substrate minus the corresponding time required in its absence. The values of KM and Vmax for the nonchromogenic substrate may be obtained from the dependencies of delta Area and delta t(5%) upon the concentration of the nonchromogenic substrate. The ability of this procedure to yield the correct values of KM and Vmax was demonstrated using beta-lactamase, beta-galactosidase, alkaline phosphatase, and 19 chromogenic/nonchromogenic substrate pairs. This method is equally valid in the absence or presence of competitive product inhibition and should be applicable to any enzyme-catalyzed, strongly exergonic reaction.

Imipenem as substrate and inhibitor of beta-lactamases

The interaction between imipenem, a carbapenem antibiotic, and two representative beta-lactamases has been studied. The first enzyme was beta-lactamase I, a class-A beta-lactamase from Bacillus cereus; imipenem behaved as a slow substrate (kcat. 6.7 min-1, Km 0.4 mM at 30 degrees C and at pH 7) that reacted by a branched pathway. There was transient formation of an altered species formed in a reversible reaction; this species was probably an acyl-enzyme in a slightly altered, but considerably more labile, conformation. The kinetics of the reaction were investigated by measuring both the concentration of the substrate and the activity of the enzyme, which fell and then rose again more slowly. The second enzyme was the chromosomal class-C beta-lactamase from Pseudomonas aeruginosa; imipenem was a substrate with a low kcat. (0.8 min-1) and a low Km (0.7 microM). Possible implications for the clinical use of imipenem are considered.

The pH-dependence of class B and class C beta-lactamases

The classification by structure allots beta-lactamases to (at present) three classes, A, B and C. The pH-dependence of the kinetic parameters for class B and class C have been determined. They differ from each other and from class A beta-lactamases. The class B enzyme was beta-lactamase II from Bacillus cereus 569/H/9. The plots of kcat against pH for the hydrolysis of benzylpenicillin by Zn(II)-requiring beta-lactamase II and Co(II)-requiring beta-lactamase II were not symmetrical, but those of kcat/Km were. A similar feature was observed for the hydrolysis of both benzylpenicillin and cephalosporin C by a class C beta-lactamase from Pseudomonas aeruginosa. The results have been interpreted by a scheme in which two ionic forms of an intermediate can give product, but do so at differing rates.