Nuclear magnetic resonance study of dihydrofolate reductase labeled with [gamma-13C]tryptophan

Dihydrofolate reductase isozyme 2 of Streptococcus faecium has been labeled with 13C in the C gamma position of tryptophan residues by growing the organism on a defined medium containing L-[gamma-13C]tryptophan (90% 13C). The 13C nuclear magnetic resonance (NMR) spectrum of the enzyme shows four well-resolved resonances which have nuclear Overhauser enhancements of 1.1-1.3. Values of T1 (spin-lattice relaxation time) and T2 (spin-spin relaxation time) are significantly less than predicted for an isotropically rotating, rigid sphere with no intermolecular dipole-dipole interactions. Three of the resonances have chemical shifts downfield from the 13C resonance of urea-denatured enzyme by amounts up to 1.43 ppm. The chemical shift of resonance 4 in the spectrum is 4.0 ppm upfield from Trp C gamma of urea-denatured enzyme. This large upfield shift is attributed to electric field effects generated by polar side chains. The two more upfield peaks both provide evidence that the corresponding tryptophan residues, WC and WD, each undergo chemical exchange between alternative microenvironments. In the case of WC, which gives a resonance with two components, exchange is slow (ve, exchange rate much less than 55 s-1), and the relative populations of the two stable states are in the ratio 2:3. WD is apparently in intermediate to fast exchange on the NMR time scale. With a two-state model, ve increases from approximately 90 to 150 s-1 as the temperature is increased from 5 to 25 degrees C. This increases in temperature is also accompanied by an increase in the fractional population of the minor downfield state(s), from about 0.062 at 5 degrees C to 0.24 at 25 degrees C. However, the data may also be interpreted as a temperature-dependent equilibrium between a continuum of many states. WD is tentatively identified with Trp-22 since comparison of the sequences of Lactobacillus casei dihydrofolate reductase and S. faecium dihydrofolate reductase and inspection of the crystal structure of the L. casei enzyme indicate that Trp-6, Trp-115, and Trp-160 are probably all involved in regions of beta sheet whereas Trp-22 is in a loop joining beta A to alpha B. Earlier crystallographic evidence for the Escherichia coli reductase suggests that in the methotrexate complex with this enzyme the corresponding loop has a good deal of flexibility. It is probable that in the uncomplexed S. faecium reductase the motion of this loop is the major mechanism for the exchange process involving Trp-22. The upfield chemical shift of resonance 4 is attributed to electric field effects on C gamma of Trp-22 produced by the carboxylate groups of Asp-27 and Asp-9. On the basis of the small difference between the chemical shift of resonance 3 and that of tryptophan C gamma in urea-denatured reductase, it is suggested that WC may be identified with Trp-6.