Folic Acid

Folic acid was first recognized as the yeast or liver factor that could cure a severe megoblastic anemia in chicks, monkeys, and humans. Later proof that the active substance was a growth factor for certain bacteria such as Lactobacillus casei and Streptococcus faecalis provided a rapid bioassay for isolating, identifying, and eventually synthesizing the vitamin and its coenzymes. The name folic acid was given in 1941 in recognition of its abundance in leafy green vegetables or 'foliage' and its structure was confirmed as monopterylglutamic acid in 1946. Today, we recognize folic acid as one of our most complex vitamin coenzymes because of its presence in many biochemical forms. Despite such enormous complexity, however, the biochemical role of folic acid narrows down to a specific set of synthetic reactions whose common denominator is one-carbon units.

The structure of folic acid (N-pteroyl-L-glutamic acid) can be pictured as a composite of three cova-lently linked molecules: a methylated pteridine ring attached to p-aminobenzoic acid (PABA), which in turn is linked via the carboxyl group to the a nitrogen of glutamic acid (Figure 4A). The coenzyme form is tetrahydrofolate (FH4) formed in mammals by adding four electrons and four hydrogens to the pteridine ring (Figure 4B). The reduction is catalyzed by dihydrofolate reductase with NAPDH as the electron donor. The addition of one or more glutamic acid residues completes the structure. In the reductive step, a new asymmetric center is generated at C-6 and appears to be critical to the biological role since only one stereoisomer of this center

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