Decarboxylation and transmethylation
Decarboxylation removes CO₂ from amino acids to make biogenic amines (PLP needed); transmethylation moves methyl groups from SAM, made from methionine.
Decarboxylation: amino acid → amine
| Amino acid | Amine formed | Enzyme(s) | Role |
|---|---|---|---|
| Histidine | Histamine | Histidine decarboxylase | Allergy and inflammation; widens blood vessels; stimulates stomach acid |
| Glutamate | GABA (γ-aminobutyric acid) | Glutamate decarboxylase | Main calming (inhibitory) signal in the brain |
| Tryptophan | Serotonin (5-hydroxytryptamine) | Tryptophan hydroxylase, then aromatic amino acid decarboxylase | Mood, sleep and appetite; forms melatonin |
| Tyrosine | Dopamine | Tyrosine hydroxylase (→ DOPA), then DOPA decarboxylase | Brain signal for movement (low in Parkinson's disease); forms noradrenaline and adrenaline |
Transmethylation and the methionine cycle
Transmethylation means moving a methyl group (–CH₃) from one molecule to another. Methionine is first activated to SAM, the body's main methyl donor.
- 1★Methionine + ATP → S-adenosylmethionine (SAM)Methionine adenosyltransferase · uses all three phosphates of ATP
- 2★SAM + acceptor → methylated product + S-adenosylhomocysteine (SAH)Methyltransferases · the methyl transfer itself
- 3SAH + H₂O → homocysteine + adenosineSAH hydrolase
- 4★Homocysteine + methyl-folate → methionine + folate (THF)Methionine synthase · needs vitamin B12; the cycle is complete
★ one-way, controls the pace
What SAM methylates
- Guanidoacetate → creatine, the muscle's energy store
- Noradrenaline → adrenaline
- Phosphatidylethanolamine → phosphatidylcholine (lecithin)
- N-acetylserotonin → melatonin, the sleep hormone
- DNA bases, which helps switch genes on and off
Decarboxylases need PLP (vitamin B6), so B6 deficiency lowers GABA and can cause convulsions in infants. The methionine cycle needs folate (gives the methyl group) and vitamin B12 (methionine synthase). Low folate or B12 raises blood homocysteine, a marker of heart disease risk. Homocysteine can also be turned into cysteine, which needs B6. Green leafy vegetables and pulses give folate; B12 comes mainly from milk, curd, eggs and meat, so strict vegetarians are at risk.
2 marks: “What is SAM? Give its function.” S-adenosylmethionine, the activated form of methionine; the main methyl donor, e.g., for creatine and adrenaline synthesis.
5 marks: “Write short notes on decarboxylation and transmethylation of amino acids.” Define each; a table of four amines with PLP; then SAM and the methionine cycle with folate and B12.
10 marks: “Discuss the general reactions of amino acids: transamination, deamination, decarboxylation and transmethylation.” For each: definition, one example equation, the coenzyme (PLP, NAD⁺, SAM) and significance. This topic gives the last two.
"SAM gives the methyl, SAH is what is left": SAM → SAH → homocysteine → back to methionine, with folate and B12.