DIHYDROFOLATE-REDUCTASE FROM LACTOBACILLUS-CASEI - STEREOCHEMISTRY OF NADPH BINDING

Abstract

The NADPH molecule binds to dihydrofolate reductase in an extended conformation. Several of the individual dihedral angles, especially in the adenine mononucleotide portion of the coenzyme, differ from their minimum energy conformations. The ribose phosphate portions of the coenzyme are involved in numerous specific H-bonded and charge-charge interactions. The adenine ring resides in an apparently nonspecific hydrophobic cleft and the nicotinamide ring is bound within an intricately constructed cavity, 1 wall of which includes the pyrazine ring of bound methotrexate. Two rather extended loops (residues 10-24 and 117-135) connecting .beta.A to .alpha.B and .beta.F to .beta.G, respectively, move 2-3 .ANG. when NADPH binds to dihydrofolate reductase. No overall structural homology is evident between the dinucleotide binding domains of dihydrofolate reductase on 1 and the 4 NAD+-dependent dehydrogenases of known structure. Binding does occur in both cases at the carboxyl edge of a region of parallel .beta. sheet flanked by a pair of .alpha. helices.