Stereospecific assignments of the leucine methyl resonances in the ¹H NMR spectrum of Lactobacillus casei dihydrofolate reductase

A general method is described for the stereospecific assignment of methyl resonances in protein NMR spectra based on selective deuteration procedures. A selectively deuterated dihydrofolate reductase from L. casei was prepared by incorporating stereoselectively deuterated L-leucine, (2S,4R)[5,5,5-2H3]leucine. By comparing the COSY spectra of the dihydrofolate reductase-methotrexate complexes formed using deuterated and non-deuterated enzyme the stereospecific assignments for resonances of all 13 leucine residues were obtained by noting the absence of cross-peaks in spectra from the deuterated proteins.