Tetrahydrofolate (FH₄), Vitamin B₁₂, and S-Adenosylmethionine (SAM)

Overview: Integrated Biochemical System

FH₄, Vitamin B₁₂, and SAM function as an integrated system essential for cellular metabolism. Their primary purposes include:

• DNA synthesis

• Cell division

• Methylation reactions

Core pathways:

• Folate cycle

• Methionine/SAM cycle

• Failure of any part leads to impaired DNA synthesis and anemia.

Primary Function of Folate

One-Carbon Metabolism

Folate's main biochemical role is the transfer of one-carbon units between molecules. These units include:

• –CH₃ (methyl)

• –CH₂– (methylene)

• –CHO (formyl)

One-carbon units are carried exclusively by reduced folate derivatives, especially tetrahydrofolate (FH₄) and its derivatives.

Biological Importance

• FH₄ carries one-carbon units in multiple oxidation states.

• Required for:

  • DNA synthesis

  • RNA synthesis

  • Amino acid metabolism

• Essential for rapidly dividing cells:

  • Bone marrow

  • GI epithelium

  • Fetal tissues

Folate vs Folic Acid: Structural Difference

Folic Acid (Oxidized, Synthetic Form)

• Synthetic form used in supplements and food fortification.

• Fully oxidized pteridine ring; not biologically active.

• Must be reduced by dihydrofolate reductase (DHFR) twice to become active.

• DHFR activity is slow and low-capacity in humans, but faster in mice/rodents.

Folate (Reduced, Active Form)

• Naturally occurring in foods.

• Reduced pteridine ring.

• Exists as:

  • Tetrahydrofolate (FH₄)

  • Methylene-FH₄

  • Formyl-FH₄

• Biologically active; can accept and donate one-carbon units.

FH₄ Derivatives in DNA Synthesis

Thymine and Purine Synthesis

• Thymine synthesis requires 5,10-methylene-FH₄.

• Purine synthesis requires 10-formyl-FH₄.

• Impaired FH₄ leads to DNA replication arrest and cell cycle arrest.

Thymidylate Synthase — Role in DNA Synthesis

Function and Clinical Correlation

• Synthesizes thymidine (dTMP) from uracil (dUMP) for DNA replication.

• Requires 5,10-methylene-tetrahydrofolate (FH₄ derivative).

• dTMP is required only for DNA, not RNA.

• Rate-limiting step in DNA synthesis; essential for rapidly dividing cells.

• Inhibition leads to impaired DNA synthesis and megaloblastic anemia.

• 5-fluorouracil (5-FU) is a cancer drug targeting thymidylate synthase.

• Methotrexate inhibits recycling of FH₂ to FH₄.

Folate & Purine Metabolism

De Novo Purine Synthesis

• Purine nucleotides (AMP, GMP) are synthesized de novo.

• Two steps in purine synthesis require one-carbon units supplied by formyl-tetrahydrofolate (FH₄).

Key Enzymes in One-Carbon Metabolism

Serine Hydroxymethyltransferase (SHMT)

• Reaction: Serine → Glycine

• One-carbon unit transferred to FH₄, producing 5,10-methylene-FH₄.

• Cofactor: Pyridoxal phosphate (PLP)

Glycine Cleavage Enzyme

• Reaction: Glycine + FH₄ → CO₂ + NH₃ + 5,10-methylene-FH₄

• Methylene carbon of glycine is transferred to FH₄.

Methylation Cycle

Biochemical Role

• Methylation is the transfer of a –CH₃ (methyl) group.

• Occurs throughout the body.

• Essential for:

  • Gene regulation

  • Neurotransmitter metabolism

  • Lipid & membrane biology (phosphatidylcholine synthesis)

  • Detoxification (Phase II detox)

Methylene-FH₄ Reductase (MTHFR)

• Converts 5,10-methylene-FH₄ → methyl-FH₄

• Methyl-FH₄ is used only for methionine synthase.

• Irreversible reaction.

S-Adenosylmethionine (SAM)

• Primary methyl donor.

• Formed from Methionine + ATP.

• Methyl donation converts SAM to SAH (S-adenosylhomocysteine).

Role of Folate & Vitamin B₁₂ in Methylation

Methionine Synthase

• 5-methyl-FH₄ provides methyl group.

• Vitamin B₁₂ (cobalamin) receives methyl group, becoming methyl-cobalamin.

• Methyl group transferred to homocysteine via methionine synthase.

• Regenerates methionine and FH₄.

Vitamin B₁₂ Structure

• Also called cobalamin.

• Contains cobalt at center of corrin ring.

• Active cofactor forms:

  • Methylcobalamin

  • Adenosylcobalamin

SAH: A Critical Regulator

• SAH is a potent inhibitor of methyltransferases.

• Must be rapidly removed by SAH hydrolase.

• Accumulation decreases methylation capacity.

Homocysteine & Methylation Balance

• Homocysteine sits at a metabolic branch point.

• Fates:

  • Remethylation → Methionine

  • Transsulfuration → Cysteine

• Elevated homocysteine indicates impaired methylation.

Alternative Methylation Pathway: Betaine

• Betaine (Trimethylglycine) donates methyl group to homocysteine.

• Enzyme: Betaine-homocysteine methyltransferase

• Independent of folate and Vitamin B₁₂; derived from choline.

• Minimal to absent in CNS; not a major methylation pathway in the brain.

Vitamins That Lower Homocysteine

• Folate: Provides methyl groups.

• Vitamin B₁₂: Transfers methyl group to homocysteine.

• Vitamin B₆: Required for transsulfuration pathway.

Vitamin B₁₂-Dependent Reactions

• Methionine Synthase: Homocysteine → Methionine; requires methyl-FH₄ and methylcobalamin; regenerates FH₄.

• Methylmalonyl-CoA Mutase: Methylmalonyl-CoA → Succinyl-CoA; requires adenosylcobalamin.

Methyl-Trap Hypothesis

• B₁₂ deficiency: Methionine synthase inactive.

• Methyl-FH₄ accumulates; FH₄ unavailable.

• Results in functional folate deficiency, lack of nucleotide synthesis.

• Causes megaloblastic anemia.

Megaloblastic Anemia

Pathophysiology

• A macrocytic anemia caused by impaired DNA synthesis.

• Characterized by large, immature erythroid precursors (megaloblasts) and nuclear–cytoplasmic asynchrony.

• Most commonly due to folate or Vitamin B₁₂ deficiency.

• Impaired thymidine (dTMP) and purine synthesis.

• Impaired DNA replication; cell cycle arrest in S phase.

• Cytoplasmic maturation continues, resulting in enlarged cells.

Summary Table: Folate and Vitamin B₁₂ Pathways

Key Equations

• Serine Hydroxymethyltransferase:

• Glycine Cleavage:

• Methionine Synthase:

• SAM Formation:

• Thymidylate Synthase:

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