Dihydrofolate reductase from Lactobacillus casei. Stereochemistry of NADPH binding.

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 hydrogen-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, one wall of which includes the pyrazine ring of bound methotrexate. Two rather extended loops (residues 10 to 24 and 117 to 135) connecting beta A to alpha B and beta F to beta G, respectively, move 2 to 3 A when NADPH binds to dihydrofolate reductase. No overall structural homology is evident between the dinucleotide binding domains of dihydrofolate reductase on the one hand and the four NAD+-dependent dehydrogenases of known structure on the other. However, binding does occur in both cases at the carboxyl edge of a region of parallel beta sheet flanked by a pair of alpha helices.