Proteins – Biochemistry · FCPS Part 1

Amino Acids

Essential amino acids

Essential amino acids cannot be synthesized in sufficient amounts by the body and must be supplied through the diet.

Amino acid	High-yield association
Histidine	Growth and tissue maintenance
Isoleucine	Branched-chain amino acid
Leucine	Branched-chain amino acid; stimulates protein synthesis
Lysine	Collagen cross-linking and protein synthesis
Methionine	Sulfur-containing; precursor of SAM
Phenylalanine	Precursor of tyrosine
Threonine	Protein synthesis
Tryptophan	Precursor of serotonin, melatonin and niacin
Valine	Branched-chain amino acid

Memory aid: PVT TIM HALL is one commonly used mnemonic for the essential amino acids: phenylalanine, valine, threonine, tryptophan, isoleucine, methionine, histidine, arginine, leucine and lysine. Arginine is conditionally essential, especially during growth.

Other useful classifications

Group	Examples
Branched-chain amino acids	Valine, leucine, isoleucine
Aromatic amino acids	Phenylalanine, tyrosine, tryptophan
Sulfur-containing	Methionine, cysteine
Positively charged/basic	Lysine, arginine, histidine
Negatively charged/acidic	Aspartate, glutamate
Ketogenic only	Leucine, lysine
Both ketogenic and glucogenic	Isoleucine, phenylalanine, tyrosine, tryptophan, threonine

Important Amino Acid Derivatives

Amino acid precursor	Product	Important function
Tyrosine	Dopamine, norepinephrine, epinephrine	Catecholamine neurotransmission and stress response
Tyrosine	Thyroid hormones	Regulation of metabolic activity
Tryptophan	Serotonin	Mood, GI motility and other functions
Tryptophan	Melatonin	Circadian rhythm regulation
Tryptophan	Niacin	NAD/NADP synthesis
Histidine	Histamine	Inflammation and gastric acid regulation
Glutamate	GABA	Major inhibitory neurotransmitter in the CNS
Arginine	Nitric oxide	Vasodilation and cell signaling
Glycine	Heme and purine synthesis	Important biosynthetic precursor and inhibitory neurotransmitter
Cysteine	Glutathione	Antioxidant defense
Methionine	S-adenosylmethionine (SAM)	Major methyl-group donor

Important metabolic pathways

Phenylalanine → tyrosine: requires phenylalanine hydroxylase and tetrahydrobiopterin (BH₄).
Tyrosine → catecholamines: tyrosine → DOPA → dopamine → norepinephrine → epinephrine.
Tryptophan → serotonin: requires hydroxylation and decarboxylation.
Glutamate → GABA: catalyzed by glutamate decarboxylase; requires vitamin B6 (PLP).
Arginine → nitric oxide: catalyzed by nitric oxide synthase.

Clinical correlations

Disorder	Defect or association	Key clue
Phenylketonuria (PKU)	Usually phenylalanine hydroxylase deficiency	Increased phenylalanine
Albinism	Impaired melanin synthesis, classically tyrosinase deficiency	Reduced pigmentation
Homocystinuria	Often cystathionine β-synthase deficiency	Thrombosis, lens displacement and skeletal features
Maple syrup urine disease	Defect in branched-chain α-ketoacid dehydrogenase	Elevated branched-chain amino acids and ketoacids
Cystinuria	Defective renal and intestinal transport of cystine and certain dibasic amino acids	Cystine kidney stones

Exam tip: Leucine and lysine are exclusively ketogenic. This distinction is frequently tested in biochemistry MCQs.