NMR DETECTION OF ARGININE-LIGAND INTERACTIONS IN COMPLEXES OF LACTOBACILLUS-CASEI DIHYDROFOLATE-REDUCTASE

H-1-NMR and N-15-NMR signal assignments have been made for the eight a rginine residues in Lactobacillus casei dihydrofolate reductase in its binary complex with methotrexate and in its ternary complex with meth otrexate and NADPH. H-1-NMR chemical shifts for the guanidino groups o f two of the arginines (Arg57 and Arg43) were sensitive to different m odes of binding of the guanidino groups with charged oxygen atoms of t he ligands, In the complexes formed with methotrexate, Arg57 showed fo ur nonequivalent NHeta proton signals indicating hindered rotation abo ut the N-epsilon-C-zeta and C-zeta-N-eta bonds. The NHeta 12 and NHeta 22 protons showed large downfield shifts, which would be expected for a symmetric end-on interaction of these protons with the charged oxyg en atoms of a carboxylate group in methotrexate. These effects were no t observed for the complex formed with trimethoprim, which does not co ntain any carboxylate groups, In the complex formed with NADPH present , Arg43 showed a large downfield chemical shift for its NHepsilon prot on and a retardation of its rate of exchange with water. This pattern of deshielding contrasts with that detected for Arg57 and is that expe cted for a side-on interaction of the guanidino group protons with cha rged oxygen atoms of the ribose 2'-phosphate group of NADPH.