Dihydrofolate reductases of Escherichia coli and bacteriophage Tr. A spectrofluorometric study

Improving protein solubility: the use of the Escherichia coli dihydrofolate reductase gene as a fusion reporter

We have devised a strategy for screening mutant libraries for enzyme variants with enhanced solubility. The method is based on the observation that Escherichia coli can become insensitive to the antibiotic trimethoprim (TMP) if dihydrofolate reductase (DHFR) is expressed at an appropriate level. DHFR is a very soluble protein and can be expressed at levels that exceed normally lethal concentrations of TMP. In our approach, the gene encoding an insoluble target protein is placed in a vector so that the translated protein will be fused to DHFR. The resulting fusion protein will form inclusion bodies and inactivate DHFR-the cells will be susceptible to TMP. Mutations to the target protein that make it more soluble will also make the fusion protein more soluble so that DHFR will be at least partially active-the cells will be resistant to TMP. As the solubility of the target protein increases, the cells will become more resistant to TMP. The system was tested with a putative acetyltransferase (ACE) from a strain of the marine bacterium Vibrio fischerii. The gene encoding this protein was of interest since it is part of a mobile gene cassette within an integron array of the strain in question. After multiple rounds of shuffling and selection, ACE mutants were produced that had significantly improved solubility.

Consideration of the pH-dependent inhibition of dihydrofolate reductase by methotrexate

Poisson-Boltzmann calculations were used to determine the pKa of protein functional groups in the unliganded dihydrofolate reductase enzyme, and the pKa of protein and ligand groups in methotrexate-enzyme complexes. The results reported here are in conflict with two fundamental tenets of dihydrofolate reductase inhibition by methotrexate: (1) Asp27 is not expected to be protonated near pH 6.5 in the apoenzyme as previously proposed based on fitting of empirical equations to binding data, and (2) the calculated pKa for the pteridine N1 of the inhibitor while bound to the protein is significantly lower than that estimated for this group from interpretation of NMR data (>10). In fact, the electrostatic calculations and complementary quantum chemical calculations indicate that Asp27 is likely protonated when methotrexate is bound, resulting in a neutral dipole-dipole interaction rather than a salt-bridge between the enzyme and the inhibitor. Reasons for this discrepancy with the experimental data are discussed. Furthermore, His45 and Glu17 in the Escherichia coli enzyme are proposed to be in part responsible for the pH dependence of the conformational degeneracy in the inhibitor-enzyme complex.

Thermal stability of dihydrofolate reductase and its fused proteins with oligopeptides

Two fused proteins of dihydrofolate reductase (DHFR) with oligopeptides were prepared by a recombinant DNA method. One of these, DHFR-IQI, has three (Ile-Gln-Ile) and the other, DHFR-lek, has eight (Ile-Arg-Met-Tyr-Gly-Gly-Phe-Leu) additional amino acid residues at the C terminals; in both proteins, Cys152 of wild DHFR is replaced by Glu. The thermal transition of the proteins was measured by CD and DSC at pH 7.0 and compared with that of wild DHFR. The results show that the thermal stability of DHFR-IQI is the same as that of the wild DHFR and that of DHFR-lek is less than that of the former two DHFRs. Analysis of the DSC data of DHFR-IQI indicates that the thermal transition is a three-state one. Data from both DSC and CD measurements suggest the association of DHFR-lek molecules.

Interactions between inhibitors of dihydrofolate reductase

The binding of substrates and inhibitors to dihydrofolate reductase was studied by steady-state kinetics and high-field 1H-n.m.r. spectroscopy. A series of 5-substituted 2,4-diaminopyrimidines were examined and were found to be 'tightly binding' inhibitors of the enzyme (Ki less than 10(-9) M). Studies on the binding of 4-substituted benzenesulphonamides and benzenesulphonic acids also established the existence of a 'sulphonamide-binding site' on the enzyme. Subsequent n.m.r. experiments showed that there are two binding sites for the sulphonamides on the enzyme, one of which overlaps the coenzyme (NADPH) adenine-ring-binding site. An examination of the pH-dependence of the binding of sulphonamides to the enzyme indicated the influence of an ionizable group on the enzyme that was not directly involved in the sulphonamide binding. The change in pKa value from 6.7 to 7.2 observed on sulphonamide binding suggests the involvement of a histidine residue, which could be histidine-28.

Dismutation of dihydrofolate by dihydrofolate reductase

Degradation of 7,8-dihydrofolate (H2folate) in the presence of dihydrofolate reductase (DHFR) has been shown due not to an oxygenase activity of the reductase as previously reported but to dismutation of H2folate to folate and 5,6,7,8-tetrahydrofolate (H4folate). The reaction can be followed spectrophotometrically or by analysis of the reaction mixture by high-performance liquid chromatography (HPLC). The products have also been isolated and characterized. Oxygen uptake during the reaction is much less than stoichiometric with H2folate disappearance and is attributed to autoxidation of the H4folate formed. The dismutation activity is a property of highly purified Streptococcus faecium DHFR isoenzyme 2 (but not isoenzyme 1) and of Lactobacillus casei DHFR, but not of bovine liver DHFR. The activity is dependent on tightly bound NADP+ and/or NADPH. Removal of the nucleotide results in loss of dismutation activity, which is restored by adding NADP+ or NADPH. Maximum activity is obtained when approximately 1 mol equiv of nucleotide is added per mol of DHFR. It is proposed that in the dismutation reaction bound NADP(H) is alternately reduced and oxidized by incoming molecules of H2folate with release of folate and H4folate, respectively. The relatively slow rate of folate formation presumably limits the rate of the overall reaction. The equilibrium constant for the dismutation reaction is 19.4 +/- 7.4 at 22 degrees C and pH 7.0. Calculation of standard oxidation-reduction potentials at pH 7 gave values of -0.230 V for the H2folate/H4 folate pair and -0.268 V for the folate/H2folate pair. The mechanism by which NADP+ is retained by the enzyme from some sources during purification procedures is unclear.(ABSTRACT TRUNCATED AT 250 WORDS)

Binding of coenzyme analogues to Lactobacillus casei dihydrofolate reductase: binary and ternary complexes

The binding, or association, constants of NADP+ NADPH, and a series of structural analogues to dihydrofolate reductase from Lactobacillus casei MTX/R have been determined fluorometrically. Modification of the adenine or nicotinamide rings has little effect on the binding of the oxidized coenzyme, but the thionicotinamide and acetylpyridine analogues of the reduced coenzyme bind much more weakly than NADPH itself. In the presence of the substrate, folate, or the inhibitors methotrxate or trimethoprim, the oxidized coenzymes bind appreciably more tightly to the enzyme. The magnitude of this "cooperativity", which covers a range of 1-37 fold, depends markedly on the structure of both the coenzyme and the substrate or substrate analogue; the nicotinamide ring of the coenzymes is clearly important in these effects. The binding constants of the reduced coenzymes in the presence of methotrexate or trimethoprim were too high to be measured fluorometrically. The dissociation rate constants of the coenzymes from their ternary complexes were therefore measured and compared with the values for the binary complexes reported by Dunn and co-workers [Dunn, S. M. J., Bathchelor, J. G., & King, R. W.(1978) Biochemistry 17, 2356]. The presence of the inhibitors leads to very substantial decreases in the coenzyme dissociation rate constant--by factors of 300-2200. The binding constant of methotrexate in the ternary complex is calculated to be approximately 1.3 X 10(12) M-1. The structural origins of the differences in binding constant and cooperative behavior of the various coenzymes and coenzyme analogues are discussed in the light of information from crystallography and NMR spectroscopy.

Modification of the cysteine residue of streptococcal dihydrofolate reducatse

Modification of the single cysteine residue of streptococcal dihydrofolate reducatase with 5,5'-dithiobis (2-nitrobenzoic acid) results in virtually complete inactivation of the enzyme. Reduction of the enzyme-S-nitrobenzoate mixed disulfide with dithiothreitol indicates that 1.0 cysteine has been modified and approximately 70 percent of the original enzyme activity restored. Circular dichroic and fluorescence studies suggest that a localized conformational change involving aromatic residues, possibly tryptophan, has occurred following modification. In addition, nearly stoichiometric amounts of p-hydroxymercuribenzoate lead to complete inactivation of the reducatas. It is suggested that the S-nitrobenzoate moiety bound to the cysteine residue of the enzyme may perturb one or more aromatic side chains and lead to a distortion of the hydrophobic substrate binding site.