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Role of Folic Acid

*Acidum folicum*

For medical students2 min readUpdated 2026-10-10

Folic acid is a vital metabolic participant that acts as an intermediate carrier of one-carbon units. After activation in the liver, it provides cells with essential carbon radicals required for the synthesis of nucleotides and other compounds.

Site of activationConversion of folate into its active form occurs in liver tissue
Main functionTransfer of one-carbon fragments (neither acyl nor amino groups)
Reaction coenzymeNADPH is required for the activity of folate reductase enzymes
Key targetSynthesis of nucleotides via the addition of one-carbon groups

Formation of the Active Form

Folic acid (folate) itself is a precursor. To participate in biochemical processes, it must be converted into its active form—tetrahydrofolate ($H_4$-folate).

This process takes place in the liver and requires specific enzymes:

To carry out reduction reactions, both enzymes strictly require the coenzyme NADPH, which acts as a donor of reducing equivalents. Sequential reactions yield $H_4$-folate, which is ready to perform its transport functions.

Transport of One-Carbon Groups

The primary biological role of $H_4$-folate is serving as an intermediate carrier of one-carbon groups.

It is important to clearly understand what $H_4$-folate does not do in biochemical reactions. It cannot act as a:

Its specialization is strictly limited to capturing, holding, and transferring fragments containing exactly one carbon atom.

Forms of One-Carbon Fragments

One-carbon radicals bound to $H_4$-folate are not static. The basic methylene group ($-CH_2-$) within the methylene-$H_4$-folate molecule undergoes redox transformations, converting into other functional forms.

The main forms of one-carbon groups transported by folate include:

  1. Methylene ($-CH_2-$) — the initial form for many transformations.
  2. Methyl ($-CH_3$) — the most reduced group.
  3. Methenyl ($-CH=$) — a form with a double bond.
  4. Formyl ($-CHO$) — the most oxidized group.

This plasticity allows folate to supply various biochemical pathways with the required types of carbon residues.

Biological Significance

All processes of transfer and interconversion of one-carbon fragments are directed toward supporting vital synthetic processes within the cell.

First and foremost, one-carbon groups are utilized for the synthesis of nucleotides—the structural building blocks of nucleic acids. Without adequate function of the tetrahydrofolate system, the cell loses the ability to form complete nucleotides and a series of other life-dependent metabolites that rely on the addition of a single carbon atom.

Mnemonic

To remember the carbon group forms on folate, use the rule of three "M"s and one "F": Methylene, Methyl, Methenyl, Formyl.

Frequently asked questions

In the synthesis of which specific nucleotides (purine or pyrimidine) do tetrahydrofolate derivatives participate?

Tetrahydrofolate derivatives participate in the synthesis of both purine and pyrimidine nucleotides.

  • $N^5,N^{10}$-methylene-$H_4$-folate — ensures the synthesis of pyrimidine nucleotides (specifically, thymidylate or TMP).
  • $N^5,N^{10}$-methenyl-$H_4$-folate — required for purine nucleotide synthesis (incorporates a carbon atom into position 8 of the purine ring).
  • $N^{10}$-formyl-$H_4$-folate — also participates in purine nucleotide synthesis (incorporates a carbon atom into position 2 of the purine ring).
In the metabolism of which amino acids does tetrahydrofolate take part?

Tetrahydrofolate takes part in the interconversions and metabolism of several amino acids.

  • Serine — donates a one-carbon group to tetrahydrofolate, converting into glycine.
  • Glycine — is formed from serine with the participation of tetrahydrofolic acid; its catabolism also proceeds via $H_4$-folate.
  • Methionine — is regenerated from homocysteine during remethylation, where methyl-$H_4$-folate serves as the primary methyl group donor.
  • Homocysteine — uses the methyl group from $N^5$-methyl-THF to convert back into methionine.
Which drugs are competitive inhibitors of dihydrofolate reductase?

Competitive inhibitors of dihydrofolate reductase are drugs that act as folic acid analogues.

  • Methotrexate — inhibits dihydrofolate reductase, disrupts the reduction of $H_2$-folate into active $H_4$-folate, and is used in tumor therapy.
  • Aminopterin — alongside methotrexate, is a potent inhibitor of cell proliferation that blocks this enzyme.
What structural components make up the folic acid molecule?

The folic acid molecule consists of three main components:

  • Pteridine ring (pterin).
  • Para-aminobenzoic acid (PABA).
  • Glutamic acid.

The core structure of folic acid is pteroylmonoglutamic acid; other folates differ by the number of glutamic acid residues attached as a $\gamma$-glutamyl peptide.

Where in the body is $H_4$-folate produced?

The process of forming active tetrahydrofolate from folic acid takes place in the liver.

Which enzymes and coenzymes are required for folate activation?

The conversion involves folate reductase and dihydrofolate reductase enzymes, with NADPH serving as their coenzyme.

Can $H_4$-folate transfer amino groups or phosphates?

No, tetrahydrofolate is strictly an intermediate carrier of one-carbon fragments.

What are one-carbon fragments used for?

They are primarily needed for nucleotide synthesis and the construction of various other cellular compounds.

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