Covalent tethering of the dimer interface annuls aggregation in thymidylate synthase

Thymidylate synthase (TS), a dimeric enzyme, forms large soluble aggregates at concentrations of urea (3.3-5M), well below that required for complete denaturation, as established by fluorescence and size-exclusion chromatography. In contrast to the wild-type enzyme, an engineered mutant of TS (T155C/E188C/C244T), TSMox, in which two subunits are crosslinked by disulfide bridges between residues 155-188' and 188-155' does not show this behavior. Aggregation behavior is restored upon disulfide bond reduction in the mutant protein, indicating the involvement of interface segments in forming soluble associated species. Intermolecular disulfide crosslinking has been used as a probe to investigate the formation of larger non-native aggregates. The studies argue for the formation of large multimeric species via a sticky patch of polypeptide from the dimer interface region that becomes exposed on partial unfolding. Covalent reinforcement of relatively fragile protein-protein interfaces may be a useful strategy in minimizing aggregation of non-native structures in multimeric proteins.

Expression and characterization of the Trypanosoma cruzi dihydrofolate reductase domain

We have cloned and expressed in Escherichia coli a 702-base pair gene coding for the dihydrofolate reductase (DHFR) domain of the bifunctional dihydrofolate reductase-thymidylate synthase (DHFR-TS) from Trypanosoma cruzi. The DHFR domain was purified to homogeneity by methotrexate-Sepharose chromatography followed by an anion-exchange chromatography step in a mono Q column, and displayed a single 27-kDa band on SDS-PAGE. Gel filtration showed that the catalytic domain was expressed as a monomer. Kinetic parameters were similar to those reported for the wild-type bifunctional enzyme with Km values of 0.75 microM for dihydrofolate and 16 microM for NADPH and a kcat value of 16.5 s-1. T. cruzi DHFR is poorly inhibited by trimethoprim and pyrimethamine and the inhibition constants were always lower for the bifunctional enzyme. The binding of methotrexate was characteristic of a class of inhibitors that form an initial complex which isomerizes slowly to a tighter complex and are referred to as 'slow, tight-binding' inhibitors. While the slow-binding step of inhibition was apparently unaffected in the individually expressed DHFR domain, the overall inhibition constant was two-fold higher as a consequence of the superior inhibition constant value obtained for the initial inhibitory complex.

Contribution of a salt bridge to binding affinity and dUMP orientation to catalytic rate: mutation of a substrate-binding arginine in thymidylate synthase

Invariant arginine 179, one of four arginines that are conserved in all thymidylate synthases (TS) and that bind the phosphate moiety of the substrate 2'-deoxyuridine-5'-monophosphate (dUMP), can be altered even to a negatively charged glutamic acid with little effect on kcat. In the mutant structures, ordered water or the other phosphate-binding arginines compensate for the hydrogen bonds made by Arg179 in the wild-type enzyme and there is almost no change in the conformation or binding site of dUMP. Correlation of dUMP Kds for TS R179A and TS R179K with the structures of their binary complexes shows, that the positive charge on Arg179 contributes significantly to dUMP binding affinity. kcat/K(m) for dUMP measures the rate of dUMP binding to TS during the ordered bi-substrate reaction, and in the ternary complex dUMP provides a binding surface for the cofactor. kcat/K(m) reflects the ability of the enzyme to accept a properly oriented dUMP for catalysis and is less sensitive than is Kd to the changes in electrostatics at the phosphate binding site.

Crystal structure of human thymidylate synthase: a structural mechanism for guiding substrates into the active site

The crystal structure of human thymidylate synthase, a target for anti-cancer drugs, is determined to 3.0 A resolution and refined to a crystallographic residual of 17.8%. The structure implicates the enzyme in a mechanism for facilitating the docking of substrates into the active site. This mechanism involves a twist of approximately 180 degrees of the active site loop, pivoted around the neighboring residues 184 and 204, and implicates ordering of external, eukaryote specific loops along with the well-characterized closure of the active site upon substrate binding. The highly conserved, but eukaryote-specific insertion of twelve residues 90-101 (h117-128), and of eight residues between 156 and 157 (h146-h153) are known to be alpha-helical in other eukaryotes, and lie close together on the outside of the protein in regions of disordered electron density in this crystal form. Two cysteines [cys 202 (h199) and 213 (h210)] are close enough to form a disulfide bond within each subunit, and a third cysteine [cys 183 (h180)] is positioned to form a disulfide bond with the active site cysteine [cys 198 (h195)] in its unliganded conformation. The amino terminal 27 residues, unique to human TS, contains 8 proline residues, is also in a region of disordered electron density, and is likely to be flexible prior to substrate binding. The drug resistance mutation, Y6H, confers a 4-fold reduction in FdUMP affinity and 8-fold reduction in kcat for the dUMP reaction. Though indirectly connected to the active site, the structure suggests a mechanism of resistance that possibly involves a change in structure. This structure offers a unique opportunity for structure-based drug design aimed at the unliganded form of the human enzyme.

The synthesis of blocked triplet-phosphoramidites and their use in mutagenesis

A general approach for the synthesis of oligonucleotide-triplet phosphoramidites and the synthesis of four such blocks are described. A strategy was devised to minimize the number of dimer precursors needed for synthesis of a complete set of triplet-amidite blocks encoding all 20 amino acids. Whereas synthesis of 20 triplet-amidite blocks consisting of codon sequences requires 16 dimer blocks, just seven dimer blocks are required to synthesize all required antisense sequences. The antisense sequences are then converted to codons in template mediated replication. Using a mixture of four triplet-amidites and conventional automated solid-phase DNA synthesis, short (6mer) and medium length (30mer) oligonucleotide mixtures were synthesized and analyzed. The latter was replicated in vitro and used as a mutagenic cassette to produce four mutants of Asp 221 in the enzyme thymidylate synthase. The method establishes the direction and utility for the production and use of triplet-amidite blocks in DNA synthesis.

Thermodynamic analysis of the binding of 5-fluoro-2'-deoxyuridine 5'-monophosphate to thymidylate synthase over a range of temperatures

The binding of 5-fluoro-2'-deoxyuridine 5'-monophosphate (FdUMP) to Lactobacillus casei recombinant thymidylate synthase has been studied by isothermal titration microcalorimetry at pH 7.1 over the temperature range 16-35 degrees C. Calorimetric measurements in various buffer systems with different heats of ionization suggest that a proton uptake is involved in the binding process of the nucleotide. In the temperature range investigated, the mol protons bound/mol nucleotide increases as the temperature decreases. A model of two equal and independent sites fits well with the binding isotherms for thymidylate synthase. The binding constants, the changes in Gibbs energy, enthalpy, and entropy/site for FdUMP binding were calculated at each temperature. The results show that the binding is driven by both enthalpy and entropy contributions in the range 16-35 degrees C. The enthalpy changes become more negative as the temperature increases, with delta Cp = -170 +/- 20 J.K-1.(mol FdUMP bound)-1. The behavior of the system supports the observation that FdUMP binds to thymidylate synthase without producing profound conformational changes in the protein dimer.

Role of the conserved tryptophan 82 of Lactobacillus casei thymidylate synthase

BACKGROUND

Thymidylate synthase (TS; EC 2.1.1.45) catalyzes the reductive methylation of 2'-deoxyuridine-5'-monophosphate (dUMP) by 5,10-methylene-5,6,7,8-tetrahydrofolate (CH2H4folate) to produce 2'-deoxythymidine-5'-monophosphate (dTMP) and 7,8-dihydrofolate (H2folate). Major advances in the understanding of the mechanism of TS have been made by studying site-specific mutants of the enzyme. Trp82 is completely conserved in all of the 20 TS sequences known. It forms part of the CH2H4folate binding pocket, is reported to be a component of a catalytically important H-bond network, and is suspected to be the source of an unusual absorbance change at 330 nm when TS forms a ternary complex with 5-fluoro-dTMP and CH2H4folate. We therefore prepared and characterized a set of 12 mutants at position 82 of Lactobacillus casei TS.

RESULTS

Eight Trp82 mutants were active enough for us to determine their kinetic constants for dTMP production, while four were inactive. The active mutants had higher Km values for dUMP (2- to 10-fold) and CH2H4folate (2- to 27-fold), and lower kcat values (12- to 250-fold) than wild-type TS. The most active mutants were those containing the aromatic side chains Phe and His at position 82. All of the Trp82 mutants catalyzed the debromination of 5-bromo-dUMP with kinetic parameters similar to those of wild-type TS, and all formed ternary complexes with 5-fluoro-dUMP and CH2H4folate. The absence of Trp82 did not prevent the absorbance change at 330 nm on ternary complex formation.

CONCLUSIONS

Trp82, a completely conserved residue that was shown by X-ray crystallography to interact directly with CH2H4folate and indirectly with dUMP, does not appear to be essential for binding or catalysis. We do, however, find a preference for an aromatic side chain at position 82. Trp82 does not contribute to the unique spectral change at 330 nm that accompanies TS ternary complex formation.

Cloning, expression, purification, and characterization of 2'-deoxyuridylate hydroxymethylase from phage SPO1

2'-Deoxyuridylate hydroxymethylase (dUMP-hmase) from phage SPO1 has been cloned and expressed in Escherichia coli. In crude extracts, the enzyme represents about 25% of the soluble protein and has a higher specific activity than the most purified preparation yet reported. The enzyme was purified to homogeneity by ion-exchange and hydrophobic chromatography. The subunits of dUMP-hmase are 45 kDa by SDS-PAGE and form dimers with a molecular mass of 89.2 kDa by analytical centrifugation. In addition to the normal reaction, dUMP-hmase catalyzes the 5,10-methylene-5,6,7,8-tetrahydrofolate (CH2H4folate)-independent tritium exchange of [5-3H]dUMP for protons of water and dehalogenation of 5-bromo-2'-deoxy-uridine-5'-monophosphate; the enzyme also forms a covalent binary adduct with pyridoxal 5'-monophosphate and a covalent ternary complex with 5-fluoro-2'-deoxyuridine-5'-monophosphate and CH2H4folate. Folic acid inhibits the tritium release catalyzed by dUMP-hmase in the presence of cofactor but has no effect on the catalysis of cofactor-independent tritium exchange.

Peptide aldehyde inhibitors of hepatitis A virus 3C proteinase

Picornaviral 3C proteinases are a group of closely related thiol proteinases responsible for processing of the viral polyprotein into its component proteins. These proteinases adopt a chymotrypsin-like fold [Allaire et al. (1994) Nature 369, 72-77; Matthews et al. (1994) Cell 77, 761-771] and a display an active-site configuration like those of the serine proteinases. Peptide-aldehydes based on the preferred peptide substrates for hepatitis A virus (HAV) 3C proteinase were synthesized by reduction of a thioester precursor. Acetyl-Leu-Ala-Ala-(N,N'-dimethylglutaminal) was found to be a reversible, slow-binding inhibitor for HAV 3C with a Ki* of (4.2 +/- 0.8) x 10(-8) M. This inhibitor showed 50-fold less activity against the highly homologous human rhinovirus (strain 14) 3C proteinase, whose peptide substrate specificity is slightly different, suggesting a high degree of selectivity. NMR spectrometry of the adduct of the 13C-labeled inhibitor with the HAV-3C proteinase indicate that a thiohemiacetal is formed between the enzyme and the aldehyde carbon as previously noted for peptide-aldehyde inhibitors of papain [Lewis & Wolfenden (1977) Biochemistry 16,4890-4894; Gamcsik et al. (1983) J. Am. Chem. Soc. 105, 6324-6325]. The adduct can also be observed by electrospray mass spectrometry.

The catalytic mechanism and structure of thymidylate synthase

Thymidylate synthase (TS, EC 2.1.1.45) catalyzes the reductive methylation of dUMP by CH2H4folate to produce dTMP and H2folate. Knowledge of the catalytic mechanism and structure of TS has increased substantially over recent years. Major advances were derived from crystal structures of TS bound to various ligands, the ability to overexpress TS in heterologous hosts, and the numerous mutants that have been prepared and analyzed. These advances, coupled with previous knowledge, have culminated in an in-depth understanding of many important molecular details of the reaction. We review aspects of TS catalysis that are most pertinent to understanding the current status of the structure and catalytic mechanism of the enzyme. Included is a discussion of available sources and assays for TS, a description of the enzyme's chemical mechanism and crystal structure, and a summary of data obtained from mutagenesis experiments.

Quantitative structure-activity relationships of the inhibition of Pneumocystis carinii dihydrofolate reductase by 4,6-diamino-1,2-dihydro-2,2-dimethyl-1-(X-phenyl)-s-triazines

The inhibitory activities of 60 4,6-diamino-1,2-dihydro-2,2-dimethyl-1- (X-phenyl)-s-triazines versus purified, recombinant Pneumocystis carinii (Pc) dihydrofolate reductase (DHFR) have been determined at pH 7.0. Utilization of these Kiapp values has led to the formulation of appropriate quantitative structure-activity relationships (QSAR's) for both meta- and parasubstituted derivatives. The QSAR's from Pc are compared with other triazine QSAR's derived versus chicken, murine tumor, Escherichia coli, and particularly human DHFR. Selectivity indices indicate that hydrophobic triazines are particularly effective versus Pc DHFR; they have lower Ki values for Pc DHFR than for human DHFR.

Stereochemistry of tRNA(m5U54)-methyltransferase catalysis: 19F NMR spectroscopy of an enzyme-FUraRNA covalent complex

The catalytic mechanism of tRNA(m5U54)-methyltransferase (RUMT) involves the formation of a covalent adduct between Cys324 of RUMT and C6 of Ura54 in tRNA. The covalent adduct is subsequently methylated at C5 by S-adenosyl-L-methionine (AdoMet). We used an RNA substrate analog containing 5-fluorouracil (FUra) in place of Ura54 to trap the covalent complex and analyzed the adduct by 19F NMR spectroscopy. The 19F NMR spectrum of the adduct consisted of an overlapping doublet of quartets, with an H6-F coupling constant of 4 Hz and a CH3-F coupling constant of 22.4 Hz. On the basis of the magnitude of the H6-F coupling constant, we determined that Cys324 of RUMT and the methyl moiety from AdoMet added across the 5,6-double bond of FUra54 in cis fashion. We deduced that the nucleophilic addition was also cis in the normal enzymatic reaction and that the subsequent beta-elimination of the 5-H and catalytic cysteine was trans. Further, on the basis of chemical considerations, we proposed several conformational adaptations of enzyme-substrate complexes that must occur on the reaction pathway. Together with previous studies, this study enables the proposal of the complete stereochemical pathway for the RUMT-catalyzed methylation of Ura54 in tRNA.

Mechanism-based inhibition of thymidylate synthase by 5-(trifluoromethyl)-2'-deoxyuridine 5'-monophosphate

Thymidylate synthase (TS) from Lactobacillus casei is inhibited by 5-(trifluoromethyl)-2'-deoxyuridine 5'-monophosphate (CF3dUMP). CF3dUMP binds to the active site of TS in the absence of 5,10-methylenetetrahydrofolate, and attack of the catalytic nucleophile cysteine 198 at C6 of the pyrimidine leads to activation of the trifluoromethyl group and release of fluoride ion. Subsequently, the activated heterocycle reacts with a nucleophile of the enzyme to form a moderately stable covalent complex. Proteolytic digestion of TS treated with [2'-3H]CF3dUMP, followed by sequencing of the labeled peptides, revealed that tyrosine 146 and cysteine 198 are covalently bound to the inhibitor in the enzyme-inhibitor complex. The presence of dithiothreitol (DTT) or beta-mercaptoethanol resulted in the breakdown of the covalent complex, and products from the breakdown of the complex were isolated and characterized. The three-dimensional structure of the enzyme-inhibitor complex was determined by X-ray crystallography, clearly demonstrating covalent attachment of the nucleotide to tyrosine 146. A chemical reaction mechanism for the inhibition of TS by CF3dUMP is presented that is consistent with the kinetic, biochemical, and structural results.

Isolation of a covalent steady-state intermediate in glutamate 60 mutants of thymidylate synthase

Glutamate 60 of thymidylate synthase coordinates a hydrogen bond network important in proton transfer reactions to and from the substrate dUMP. The E60A and E60L mutants of Lactobacillus casei thymidylate synthase catalyzed tritium exchange from [5-3H]dUMP for solvent protons faster than dTMP formation, indicating accumulation of a steady-state intermediate and a change in partitioning of the intermediate. A covalent complex consisting of E60A or E60L thymidylate synthase, dUMP, and the cofactor CH2H4 folate was isolated on SDS-polyacrylamide gel electrophoresis and shown to be chemically and kinetically competent to form dTMP. These results provide proof of the formation of a covalent steady-state intermediate in the reaction pathway of thymidylate synthase and demonstrate that the rate-determining step in the mutants occurs during conversion of the covalent intermediate to dTMP.

Cloning, expression and characterization of thymidylate synthase from Cryptococcus neoformans

The thymidylate synthase (TS)-encoding gene from Cryptococcus neoformans (Cn) has been isolated from cDNA and genomic libraries. The 1127-bp gene contains three introns and a 951-bp open reading frame encoding a 35,844-Da protein. The cDNA clones lack 324 bp of the 5' coding region of the gene. The complete coding sequence was assembled as an expression cassette in pUC19 using parts of the coding sequence from the cDNA and genomic DNA and completing the sequence using synthetic DNA. Production of active TS from Cn (CnTS) was first demonstrated by complementation of a thymine(Thy)-requiring Escherichia coli strain. The expression cassette was subsequently subcloned into the T7 polymerase vector pET15-b. In this construct, CnTS is produced as approximately 10% of the total soluble protein in E. coli. Homogeneous enzyme was obtained at a 36% yield after consecutive chromatography on DEAE-cellulose, Q-Sepharose, phenyl-Sepharose and Affi-Gel Blue. Steady-state kinetic analysis showed that the Km values for dUMP and CH2H4.folate were 2.7 +/- 0.5 microM and 38.2 +/- 2.5 microM, respectively, and the kcat was 5.1 s-1. The enzyme was stable upon storage at -80 degrees C in Tris.HCl pH 7.4 and thiol.

5-Fluoro-2'-deoxycytidine 5'-monophosphate is a mechanism-based inhibitor of thymidylate synthase

Thymidylate synthase (TS) is inhibited by 5-fluoro-2'-deoxycytidine 5'-monophosphate (FdCMP). From initial velocity measurements, the apparent Ki for the binary FdCMP-enzyme complex was about 20 microM. In the presence of 5,10-methylene-5,6,7,8-tetrahydrofolate (CH2H4folate), FdCMP causes a time-dependent inactivation of the enzyme and formation of a TS-FdCMP-CH2H4 folate complex. The ternary complex contains one mol of inhibitor per monomer of enzyme, and can be readily isolated on nitrocellulose filters. Dissociation of the ternary complex is quite slow (t1/2 approximately 16 h), and yields unchanged FdCMP. As with the corresponding complex formed with 5-fluoro-2'-deoxyuridine 5'-monophosphate (FdUMP), the TS-FdCMP-CH2H4 folate complex shows a differential absorbance maximum at 326 nm, and is stable to SDS-PAGE. Taken together, these results indicated that FdCMP is a slow, tight binding inhibitor of TS and has a mechanism of inhibition similar to that of FdUMP.

Cloning and expression of the dihydrofolate reductase-thymidylate synthase gene from Trypanosoma cruzi

We have cloned, sequenced and expressed the Trypanosoma cruzi gene encoding the bifunctional protein dihydrofolate reductase-thymidylate synthase (DHFR-TS). The strategy followed for the isolation of positive clones from a genomic library was based on the construction of a probe by the amplification of highly conserved sequences of the TS domain by the polymerase chain reaction. Translation of the open reading frame of 1563 bp yields a polypeptide of 521 amino acids with a molecular mass of 58829 Da. For heterologous expression of T. cruzi DHFR-TS in Escherichia coli, the entire coding sequence was amplified by polymerase chain reaction and cloned into the plasmid vector pKK223.3. The presence of catalytically active DHFR-TS was demonstrated by complementation of the Thy- E. coli strain chi 2913 and the DHFR- Thy- E. coli strain PA414. The gene is expressed as an active protein which constitutes approximately 2% of the total cell soluble protein. Recombinant bifunctional enzyme and the DHFR domain have been purified by methotrexate-Sepharose chromatography to yield 1-2 mg of active DHFR-TS per litre of culture. Southern and electrophoretic analyses using the coding sequence as probe indicated that the T. cruzi enzyme is encoded by a single copy gene which maps to two bands of approximately 990 kb and 1047 kb. It appears that T. cruzi is diploid for the DHFR-TS gene which is located on two different-sized homologous chromosomes.

Partial restoration of activity to Lactobacillus casei thymidylate synthase following inactivation by domain deletion

Thymidylate synthase (TS) from Lactobacillus casei has a 50 amino acid insert (residues 90-139) in the small domain that is found in only one other TS. A deletion mutant was constructed which lacked the entire insert, thereby reducing the small domain to the size found in Escherichia coli TS. This mutant did not catalyze the formation of dTMP. From the crystal structure of L. casei TS, we surmised that the loss of activity might have resulted from the exposure of residues of helices C and D, which were previously buried by the insert. To restore the local structure of helices C and D in the deletion mutants, we replaced several residues in this region by the corresponding residues found in E. coli TS. The mutant whose sequence most closely resembled that of E. coli TS carried six mutations and possessed partially restored TS activity. The mutant which had all those mutations except F87D did not catalyze any dTMP formation. The crucial role of F87D was proven in a deletion mutant which had only this change and showed greatly increased activity. All of the mutants catalyzed the debromination of BrdUMP in the absence of cofactor about as well as wild type TS. The kinetic parameters for dTMP formation of the active mutants show that the deletion has its major effect on kcat and binding of cofactor CH2H4folate, with less effect on binding of the substrate dUMP. Removal of residues 90-139 is believed to disorder helices C and D, which in turn decreases cofactor binding and catalysis.

Identification of biologically active mutants by combinatorial cassette mutagenesis: exclusion of wild-type codon from degenerate codons

A degenerate codon (N)(N)(G+C) is often used in cassette mutagenesis to encode all 20 natural amino acids at the target mutation site. However, the presence of the wild-type codon in the degenerate codon presents some inconvenience in screening and identification of catalytically active mutants. The wild-type enzyme will always be identified as catalytically active in a screening process and in most cases can only be distinguished from active mutants by DNA sequencing. Sequencing of background wild-type enzyme represents wasted effort in the identification of active mutants. This paper describes a simple approach for exclusion of the wild-type codon in degenerate codons through the synthesis of two or three oligonucleotide mixtures. The minimum number of individual colonies required to achieve a high degree of certainty of including all possible codons for screening of catalytic activity can be estimated using a statistical procedure. The use of degenerate codons that exclude the wild-type amino acid facilitates the screening process and saves time and expense in DNA sequencing.

Heterodimeric thymidylate synthases with C-terminal deletion on one subunit

We have combined site-directed mutagenesis with the technique of reversible unfolding and subunit dissociation to construct heterodimeric thymidylate synthases that lack the C-terminal valine from only one subunit of the dimer. Removal of this residue either from both subunits of the dimer by mutagenesis (V316Am mutation) or from only one subunit by treatment with carboxypeptidase has been reported to result in an inactive enzyme (Carreras, C. W., Climie, S. C., and Santi, D. V. (1992) Biochemistry 31, 6038-6044; Aull, J.L., Loeble, R.B., and Dunlap. R.B. (1974) J. Biol. Chem. 249, 1167-1172). Arg-178 is an essential active site residue of thymidylate synthase that is donated from the opposing subunit of the dimer. The R178F-V316Am heterodimer was formed by the unfolding and refolding of a mixture of inactive R178F and V316Am mutants. This enzyme has one intact active site and was found to have half of the activity and the same Km values as wild-type thymidylate synthase that was unfolded and refolded as a control. We have also formed the V316Am-WT heterodimer and report that this heterodimeric enzyme is also active, has a kcat value that is approximately half of that of the wild-type thymidylate synthase dimer, and binds substrate and cofactor with Km values similar to those of the wild-type enzyme.

Isolation and characterization of a mutant dihydrofolate reductase-thymidylate synthase from methotrexate-resistant Leishmania cells

The MTX-resistant Leishmania major promastigote cell line D7BR1000 displays extrachromosomal amplified R-region DNA, which contains the gene for dihydrofolate reductase-thymidylate synthase (DHFR-TS) (Garvey, E. P., and Santi, D. V. (1986) Science 233, 535-540). Now we report that these methotrexate (MTX)-resistant cells also possessed a structurally altered DHFR-TS. We have performed the cloning, expression, and characterization of the altered DHFR-TS gene. The DNA sequence of the altered DHFR-TS gene revealed a single base change in position 158 which resulted in the substitution of a methionine in position 53 of DHFR for an arginine. Steady-state measurements of the purified recombinant enzyme indicated that the mutation did not cause significant modifications in the Km for DHFR or TS substrates but lowered the kcat by 4-fold. Of greater interest, there was a modification in the effect on MTX inhibition of DHFR. The initial inhibition complex appeared to have been unaffected by the alteration, but the subsequent slow-binding step of inhibition in the wild-type enzyme is absent in the altered enzyme. Consequently, the overall Ki for MTX was 30-fold greater for the mutant than for the wild-type enzyme. Transfection of L. major with the mutant DHFR-TS gene gives parasites that are capable of growing in medium containing 10 mM methotrexate, showing that the altered DHFR gene is in itself capable of conferring MTX resistance in Leishmania.