Folate metabolism: The enzyme folate reductase first reduces folic acid to which specific intermediate before further reduction to the fully active form?

Introduction / Context:
Folate coenzymes carry one-carbon units in amino acid and nucleotide biosynthesis. Understanding the reduction steps from folic acid to active tetrahydrofolate underlies mechanisms of action for drugs like methotrexate and trimethoprim.

Given Data / Assumptions:

• Folic acid is the oxidized vitamin form.
• Dihydrofolate reductase (folate reductase) catalyzes sequential reductions using NADPH.
• Active coenzyme form is tetrahydrofolate (THF).

Concept / Approach:
The pathway is folic acid → dihydrofolate (DHF) → tetrahydrofolate (THF). The first reduction yields DHF; the second reduction yields THF, which accepts and donates one-carbon units at various oxidation states.

Step-by-Step Solution:
Identify enzyme: dihydrofolate reductase (often termed folate reductase).First product after reduction of folic acid: dihydrofolic acid (DHF).Second reduction: DHF → THF (tetrahydrofolic acid), the biologically active cofactor.

Verification / Alternative check:
Antifolate drugs block DHFR, causing DHF accumulation and THF depletion, validating the order of intermediates.

Why Other Options Are Wrong:

• Hydrofolic/trihydrofolic acids are not standard biochemical entities.
• Tetrahydrofolic acid is the final product after a second reduction, not the first intermediate.

Common Pitfalls:
Assuming folic acid is directly active; it must be reduced first to DHF and then to THF.

Final Answer:
dihydrofolic acid