From Proteopediaproteopedia link
Crystal structure of a methotrexate-resistant mutant of human dihydrofolate reductase
[DYR_HUMAN] Defects in DHFR are the cause of megaloblastic anemia due to dihydrofolate reductase deficiency (DHFRD) [MIM:613839]. DHFRD is an inborn error of metabolism, characterized by megaloblastic anemia and/or pancytopenia, severe cerebral folate deficiency, and cerebral tetrahydrobiopterin deficiency. Clinical features include variable neurologic symptoms, ranging from severe developmental delay and generalized seizures in infancy, to childhood absence epilepsy with learning difficulties, to lack of symptoms. 
[DYR_HUMAN] Key enzyme in folate metabolism. Contributes to the de novo mitochondrial thymidylate biosynthesis pathway. Catalyzes an essential reaction for de novo glycine and purine synthesis, and for DNA precursor synthesis. Binds its own mRNA and that of DHFRL1. 
Publication Abstract from PubMed
Methotrexate is a slow, tight-binding, competitive inhibitor of human dihydrofolate reductase (hDHFR), an enzyme that provides key metabolites for nucleotide biosynthesis. In an effort to better characterize ligand binding in drug resistance, we have previously engineered hDHFR variant F31R/Q35E. This variant displays a >650-fold decrease in methotrexate affinity, while maintaining catalytic activity comparable to the native enzyme. To elucidate the molecular basis of decreased methotrexate affinity in the doubly substituted variant, we determined kinetic and inhibitory parameters for the simple variants F31R and Q35E. This demonstrated that the important decrease of methotrexate affinity in variant F31R/Q35E is a result of synergistic effects of the combined substitutions. To better understand the structural cause of this synergy, we obtained the crystal structure of hDHFR variant F31R/Q35E complexed with methotrexate at 1.7-A resolution. The mutated residue Arg-31 was observed in multiple conformers. In addition, seven native active-site residues were observed in more than one conformation, which is not characteristic of the wild-type enzyme. This suggests that increased residue disorder underlies the observed methotrexate resistance. We observe a considerable loss of van der Waals and polar contacts with the p-aminobenzoic acid and glutamate moieties. The multiple conformers of Arg-31 further suggest that the amino acid substitutions may decrease the isomerization step required for tight binding of methotrexate. Molecular docking with folate corroborates this hypothesis.
Multiple conformers in active site of human dihydrofolate reductase F31R/Q35E double mutant suggest structural basis for methotrexate resistance.,Volpato JP, Yachnin BJ, Blanchet J, Guerrero V, Poulin L, Fossati E, Berghuis AM, Pelletier JN J Biol Chem. 2009 Jul 24;284(30):20079-89. Epub 2009 May 28. PMID:19478082
From MEDLINE®/PubMed®, a database of the U.S. National Library of Medicine.