Ultraviolet difference-spectroscopic studies of substrate and inhibitor binding to Lactobacillus casei dihydrofolate reductase.

The u.v. difference spectra generated when methotrexate, trimethoprim or folate bind to Lactobacillus casei dihydrofolate reductase were analysed. The difference spectrum producted by methotrexate binding is shown to consist of three components: (a) one closely resembling that observed on protonation of methotrexate, reflecting an increased degree of protonation on binding; (b) a pH-independent contribution corresponding to a 40 nm shift to longer wavelengths of a single absorption band of methotrexate: (c) a component arising from perturbation of tryptophan residue(s) of the enzyme. Quantitative analysis of the pH-dependence of component (a) shows that pK of methotrexate is increased from 5.35 to 8.55 (+/-0.10) on binding. In contrast, folate is not protonated when bound to the enzyme at neutral pH. At pH7.5, where methotrexate is bound 2000 times more tightly than folate, one-third of the difference in binding energy between the two compounds arises from the difference in chaarge stage. A similar analysis of the difference spectra generated on trimethoprim binding demonstrates that this compound, too, shows an increase in pK on binding but only from 7.22 to 7.90 (+/-0.10), suggesting that its 2,4-diaminopyrimidine ring does not bind to the enzyme in precisely the same way as the corresponding moiety of methotrexate.