1. METABOLISM OF AMINO ACIDS
🔹 A. Transamination
Transfer of amino group from amino acid → keto acid
Enzymes: Transaminases
⭐ Salient Features:
Requires coenzyme: Pyridoxal phosphate (PLP)
Occurs in pairs:
Alanine transaminase (ALT)
Aspartate transaminase (AST)
No free NH₃ released
Reversible process
Helps in:
Synthesis of non-essential amino acids
Gluconeogenesis
🔹 B. Deamination
Removal of amino group → helps in urea formation
Types:
1. Oxidative Deamination
Substrate: Glutamate
Enzyme: Glutamate dehydrogenase
Products:
α-ketoglutarate
NH₃
Site: Liver mitochondria
Coenzyme: NAD⁺
2. Reductive Deamination
Removal of amino group by reduction
Substrate: Keto acids / Imino acids
Enzyme: Reductase
Product: NH₃ released
Coenzyme: NADPH
2. TRYPTOPHAN METABOLISM
🔹 Features:
Essential amino acid
Contains β-indole ring
🔹 Pathways:
1. Kynurenine Pathway
Steps: Tryptophan
→ N-formylkynurenine
→ Kynurenine
→ 3-hydroxykynurenine
→ 3-hydroxyanthranilic acid
Clinical:
Pellagra → 3 D’s:
Dermatitis
Diarrhea
Dementia
Products:
Melatonin (sleep regulation)
2. Serotonin Pathway
Steps: Tryptophan
→ 5-hydroxytryptophan
→ Serotonin
Functions:
Mood regulation
Depression link
3. PHENYLALANINE & TYROSINE METABOLISM
🔹 Characteristics:
Aromatic amino acids
🔹 Conversion:
Phenylalanine
→ Tyrosine
Enzyme: Phenylalanine hydroxylase
Converts essential → non-essential amino acid
🔹 Functions of Tyrosine:
Melanin synthesis
Thyroid hormone synthesis
Catecholamine synthesis
🔹 Tyrosine Degradation Pathway:
Tyrosine
→ p-Hydroxyphenylpyruvate
→ Homogentisate
→ Maleylacetoacetate
→ Fumarylacetoacetate
→ Fumarate + Acetoacetate
Final Fate:
Fumarate → TCA cycle → Glucose
Acetoacetate → Fat
4. UREA CYCLE
🔹 Key Points:
End product of protein metabolism
80–90% nitrogen excreted via urine
🔹 Steps:
CO₂ + NH₄⁺
→ Carbamoyl phosphate
(Enzyme: Carbamoyl phosphate synthetase)
🔸 Uses 2 ATP
Carbamoyl phosphate + Ornithine
→ Citrulline
(Ornithine transcarbamoylase)
Citrulline
→ Argininosuccinate
(Argininosuccinate synthetase)
🔸 Uses 1 ATP
Argininosuccinate
→ Arginine + Fumarate
(Argininosuccinase)
Arginine
→ Urea + Ornithine
(Arginase)
🔹 Important Points:
Total energy: 4 ATP used
Fumarate enters:
TCA cycle
Gluconeogenesis
Ornithine is regenerated (cycle continues)
🔥 QUICK SUMMARY (EXAM REVISION)
Transamination → No NH₃, reversible
Deamination → NH₃ released → Urea
Tryptophan → Serotonin + Kynurenine → Pellagra
Phenylalanine → Tyrosine → Hormones & pigments
Urea cycle → Detoxifies ammonia → Uses 4 ATP
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