Identification of the 1H resonances of valine and leucine residues in dihydrofolate reductase by using a combination of selective deuteration and two-dimensional correlation spectroscopy.

Lactobacillus casei dihydrofolate reductase (Mr 18 500) contains 16 valine and 14 leucine residues. By comparing the 2D COSY NMR spectra of normal and [gamma-2H6]valine enzyme we have been able to identify all 60 methyl resonances from these residues, and to connect the pairs arising from the same residue. This pairing of the methyl resonances was aided by the examination of the 2D RELAY spectrum which also allowed the C alpha H resonances (and hence the complete spin systems) of 14 of the valine residues to be identified. The combination of selective deuteration with 2D NMR techniques is shown to be a powerful general method for resolving 1H resonances in the complex spectra of proteins and for assigning them to amino-acid type.