Synthesis of Biologically Important Compounds from Amino Acids

Heme Synthesis (from Glycine) - Bloody Building Blocks

• Initial Step (Mitochondria):
  • $Glycine + Succinyl CoA \xrightarrow{ALA Synthase, PLP} \delta-Aminolevulinate (ALA) + CO_2 + CoA$.
  • Rate-limiting: ALA Synthase (PLP-dependent).
• Pathway Overview & Key Intermediates:

-   Intermediates: ALA, Porphobilinogen, Uroporphyrinogen, Coproporphyrinogen, Protoporphyrinogen, Protoporphyrin IX.

• Key Deficiencies:
  • ALA Dehydratase (Lead sensitive): ↑ ALA.
  • Uroporphyrinogen I Synthase: Acute Intermittent Porphyria (AIP). ↑ PBG, ALA. No photosensitivity.
• Regulation:
  • Heme/Hemin: Feedback inhibits ALA synthase.
  • Glucose: Represses ALA synthase.
• 📌 Porphyria (5 Ps): Painful abdomen, Polyneuropathy, Psychological issues, Precipitated by drugs, Purple pee.

⭐ Lead poisoning inhibits ALA dehydratase & Ferrochelatase → ↑ ALA & ↑ Protoporphyrin IX. Causes anemia, neurotoxicity.

Tyrosine Derivatives - Mood & Pigment Power

Tyrosine, a non-essential amino acid (conditionally essential in PKU), is a precursor to vital catecholamines, melanin, and thyroid hormones.

• Catecholamine Synthesis Pathway:

  ⭐ Tyrosine hydroxylase is the rate-limiting enzyme in catecholamine synthesis and requires tetrahydrobiopterin (THB or BH4) as a cofactor.

• Melanin Synthesis (Pigment):

  • Tyrosine $\xrightarrow{\text{Tyrosinase (Cu-dependent)}}$ L-DOPA $\rightarrow$ Dopaquinone $\rightarrow$ Melanin.

• Thyroid Hormones (T3/T4): Synthesized from iodination and coupling of tyrosine residues in thyroglobulin.

• Clinical Significance:

  • Parkinson's Disease: ↓ Dopamine.
  • Pheochromocytoma: ↑ Catecholamines.
  • Albinism: Tyrosinase deficiency $\rightarrow$ ↓ Melanin.
  • Phenylketonuria (PKU): Phenylalanine Hydroxylase deficiency makes Tyrosine essential.

Tryptophan Derivatives - Serene Nights & Energy

• Key Products: Serotonin (neurotransmitter), Melatonin (sleep hormone), Niacin (Vitamin B3).
• Serotonin (5-HT): Tryptophan $\xrightarrow{\text{Tryptophan hydroxylase (THB, rate-limiting)}}$ 5-Hydroxytryptophan $\xrightarrow{\text{Decarboxylase (PLP)}}$ Serotonin.
• Melatonin: Serotonin $\xrightarrow{\text{Pineal gland, Acetylation, Methylation (SAM)}}$ Melatonin.
• Niacin (B3): Tryptophan $\xrightarrow{\text{Kynurenine pathway (PLP, FAD, Niacin)}}$ Quinolinic acid $\rightarrow$ NAD+/NADP+. Inefficient.
• Clinical Notes:
  • Depression (↓Serotonin; SSRIs target reuptake).
  • Sleep disorders (↓Melatonin).
  • Pellagra (Niacin deficiency): 📌 3 D's: Diarrhea, Dermatitis, Dementia (+ Death).
  • Hartnup disease (defective Tryptophan absorption).
  • Carcinoid syndrome (↑Serotonin).

⭐ Tryptophan is a precursor for serotonin, melatonin, and niacin (Vitamin B3); approximately 60 mg of tryptophan yields 1 mg of niacin.

Other AA Derivatives - Vital Variety Pack

• Histamine: Histidine $\rightarrow$ Histamine (Histidine decarboxylase, PLP).
  • Sources: Mast cells, basophils.
  • Functions: Allergic reactions, gastric acid secretion.
• GABA ($\ ext{gamma}$-Aminobutyric acid): Glutamate $\rightarrow$ GABA (Glutamate decarboxylase, PLP).
  • Function: Major inhibitory CNS neurotransmitter.
• Creatine/Phosphocreatine:
  • Synthesis: Arginine + Glycine $\rightarrow$ Guanidinoacetate (kidney); Guanidinoacetate + SAM $\rightarrow$ Creatine (liver).
  • Storage: Creatine $\rightarrow$ Phosphocreatine (muscle, brain energy reserve).
  • Excretion: Creatinine (from non-enzymatic cyclization of creatine phosphate).
• Glutathione (GSH): Glutamate + Cysteine $\rightarrow$ $\gamma$-Glutamylcysteine; then + Glycine $\rightarrow$ GSH.

  ⭐ Glutathione (GSH), a tripeptide of glutamate, cysteine, and glycine, is a major intracellular antioxidant, protecting cells from oxidative damage.

• Nitric Oxide (NO): Arginine $\rightarrow$ NO + Citrulline (NO synthase; requires NADPH, THB).
  • Functions: Vasodilator, neurotransmitter.

High‑Yield Points - ⚡ Biggest Takeaways

• Tyrosine is a precursor for catecholamines (Dopamine, Epinephrine), thyroid hormones, and melanin.
• Tryptophan forms serotonin, melatonin, and is a source for niacin (B3).
• Glycine is essential for heme, purine rings, and creatine synthesis.
• Arginine synthesizes nitric oxide (NO), urea, and creatine.
• Histidine is decarboxylated to histamine, a key inflammatory mediator.
• Glutamate is converted to GABA (inhibitory neurotransmitter) and glutathione.
• S-Adenosylmethionine (SAM) from methionine is the primary methyl donor.