Clinical significance of folic acid during pregnancy
According to current clinical guidelines, pregnant patients are advised to take folic acid throughout the first 12 weeks of gestation. The recommendation has a level of certainty of 1a, which is the highest. This prescription is due to the critical role of the vitamin in embryonic cell division and the prevention of neural tube defects.
Vitamin deficiency during active organogenesis leads to fetal growth retardation and impaired formation of tissues consisting of rapidly dividing cells. This is precisely why the biochemical rationale for folate administration is brought up in biochemistry colloquia and credit tests.
Structure and sources of vitamin B9
Folic acid (vitamin B9) is ingested with food: meat, liver, kidneys, yeast, and leafy greens. Part of the vitamin is synthesized by the intestinal microflora, but this amount is usually insufficient to cover physiological needs, especially during pregnancy.
The chemical structure of folic acid includes three mandatory components:
- pteridine ring — the heterocyclic part of the molecule;
- para-aminobenzoic acid (PABA) — the central fragment;
- glutamic acid residue.
Recognition of these fragments in the general formula is required in test assignments for bioorganic chemistry and amino acid metabolism.
Coenzyme form and transfer of one-carbon units
Like other vitamins, B9 is converted in the liver into its active coenzyme form — tetrahydrofolic acid (THFA, H₄-folate). The conversion consists of adding four hydrogen atoms to the pteridine part of the molecule.
The main biochemical function of THFA is the transfer of single-carbon fragments. Two nitrogen atoms in its structure are capable of binding −CH₃, −CH₂−, −CHO, and similar groups. Depending on the attached fragment, derivatives are formed: methyl-THFA, methylene-THFA, formyl-THFA, and others.
In addition to transferring C₁-fragments, THFA is involved in:
- the synthesis of purine and pyrimidine bases;
- the synthesis of the amino acids glycine and serine;
- the regeneration of methionine (via methyl-THFA).
The link between folate deficiency and megaloblastic anemia
The biochemical pathogenesis of vitamin B9 hypovitaminosis consequences forms a logical chain: without THFA, the synthesis of purines and pyrimidines is impossible → nucleotides are not synthesized → DNA and RNA synthesis is disrupted → rapidly dividing cells suffer.
Clinical manifestations reflect precisely this impairment of proliferation:
- megaloblastic (macrocytic) anemia: erythrocytes are unable to divide fully, so their count decreases while their size compensatorily increases to preserve gas transport function;
- damage to mucous membranes and impaired wound healing as a consequence of suppressed epithelial division;
- fetal growth restriction and risk of developmental anomalies, which determines the necessity of prescribing the vitamin in the first trimester.
Sulfonamides as structural analogs of PABA
Sulfonamide drugs are used to treat bacterial infections. Their general formula is structurally close to para-aminobenzoic acid, the central fragment of folic acid. Such similarity allows sulfonamides to act as competitive inhibitors of bacterial folate synthesis enzymes.
The mechanism of selective action is explained by differences in metabolism:
- Bacteria synthesize folic acid independently from pteridine, PABA, and glutamate using their own enzymes. The sulfonamide inserts into the active center of the enzyme instead of PABA, blocking molecule assembly. Without folate, the bacterium cannot synthesize DNA and dies.
- Humans obtain ready-made folic acid from food and do not possess the enzymes that assemble it from individual components. Therefore, sulfonamides do not exert a cytostatic effect on human cells.
This example illustrates a general principle of pharmacology: the selective toxicity of a drug is based on the differences in biochemical pathways between the pathogen and the host.
Key points
- Folic acid (vitamin B9) is prescribed in the first 12 weeks of pregnancy with a level of certainty of 1a.
- The molecule consists of pteridine, para-aminobenzoic acid, and glutamate.
- The active form is tetrahydrofolic acid (THFA, H₄-folate).
- THF transfers one-carbon units and ensures the synthesis of purines and pyrimidines.
- Folate deficiency disrupts DNA and RNA synthesis, damaging rapidly dividing cells.
- Clinically, this manifests as megaloblastic anemia, mucosal damage, and fetal developmental delay.
- Sulfonamides are structural analogs of PABA and competitive inhibitors of bacterial folate synthesis.
- The selectivity of sulfonamides is due to the fact that humans obtain folic acid from food rather than synthesizing it.