Stereochemistry of reduction of the vitamin folic acid by dihydrofolate reductase

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Abstract
Reduction of the vitamin folic acid (6) to the coenzyme 5,6,7,8-tetrahydrofolic acid (1) by the enzyme dihydrofolate reductase is shown to involve transfer of the 4-pro R hydrogen of NADPH to the same face at both C-6 and C-7 of the pteridine system (the re face at C-6 and the si face at C-7). The orientations of the pteridine system of folic acid (6) and of dihydrofolic acid (5) when bound to the enzyme are different from the orientation of the pteridine ring of the anti-cancer drug methotrexate (11) when bound to this enzyme.