Harvey's Garden

Amino acids

3 min read

Classification & Groups

  • Essential Amino Acids: Must be obtained from the diet.

    • Mnemonic: PVT TIM HALL
    • Phenylalanine, Valine, Threonine, Tryptophan, Isoleucine, Methionine, Histidine, Arginine*, Leucine, Lysine.
    • *Arginine is semi-essential (essential in children, not adults).

  • Glucogenic vs. Ketogenic: Refers to the metabolic fate of the carbon skeleton.

    • Exclusively Ketogenic: Can be degraded directly into Acetyl-CoA or Acetoacetate.
      • Leucine & Lysine (the two L’s).
    • Both Glucogenic & Ketogenic:
      • Phenylalanine, Isoleucine, Threonine, Tryptophan, Tyrosine (PITTT).
    • Exclusively Glucogenic: All remaining amino acids. Can be converted to glucose via gluconeogenesis.

  • Acidic vs. Basic:

    • Acidic (negatively charged at body pH): Aspartate, Glutamate.
    • Basic (positively charged at body pH): Arginine, Lysine, Histidine. Arginine is the most basic. Histones are rich in arginine and lysine.

  • Branched-Chain Amino Acids (BCAAs):

    • Isoleucine, Leucine, Valine (I Love Vermont maple syrup from Branched trees).
    • 支链氨基酸：支姐，亮一亮［缬氨酸，亮氨酸，异亮氨酸］
    • Metabolized in muscle. Defect causes Maple Syrup Urine Disease.
  • 非极性疏水性氨基酸：非姐，脯亮一亮，（给你）本饼干［缬氨酸，脯氨酸，亮氨酸，异亮氨酸，苯丙氨酸，丙氨酸，甘氨酸］

High-Yield Derivatives & Functions

  • Tryptophan (Trp):

  • Phenylalanine (Phe):

    • → Tyrosine (via phenylalanine hydroxylase; requires BH4).
    • Deficiency leads to Phenylketonuria (PKU).

  • Tyrosine (Tyr):

    • → Dopa → Melanin (via tyrosinase). Deficiency → Albinism.
    • → Dopa → Dopamine → Norepinephrine (NE) → Epinephrine (Epi) (Catecholamines).
    • → Thyroxine (Thyroid hormone).

  • Histidine (His):

    • → Histamine (via histidine decarboxylase). Requires B6.

  • Glycine (Gly):

    • → Porphyrin → Heme (precursor, with Succinyl-CoA). Requires B6.
    • Major inhibitory neurotransmitter in the spinal cord.

  • Glutamate (Glu):

    • → GABA (γ-aminobutyric acid; major inhibitory neurotransmitter in the brain). Requires B6.
    • → Glutathione (important antioxidant).

  • Arginine (Arg):

    • → Creatine.
    • → Urea.
    • → Nitric Oxide (NO) (vasodilator). Requires BH4.

Key Associated Disorders

  • Phenylketonuria (PKU):

    • Patho: AR; Deficient phenylalanine hydroxylase or BH4 cofactor.
    • Presentation: Intellectual disability, seizures, fair skin/hair, mousy/musty body odor.
    • Dx: Newborn screen (↑ phenylalanine, ↓ tyrosine).
    • Tx: Lifelong dietary restriction of phenylalanine; supplement tyrosine.

  • Maple Syrup Urine Disease (MSUD):

    • Patho: AR; Deficient α-ketoacid dehydrogenase → buildup of BCAAs (Isoleucine, Leucine, Valine).
    • Presentation: Sweet-smelling urine/cerumen, poor feeding, vomiting, severe CNS defects, intellectual disability.
    • Tx: Restriction of BCAAs in diet.

  • Homocystinuria:

    • Patho: AR; Several causes, most common is cystathionine synthase deficiency.
    • Presentation: Marfanoid habitus, DOWNWARD lens dislocation (vs. upward in Marfan), intellectual disability, thromboembolism, osteoporosis.
    • Tx: ↓ Methionine, ↑ Cysteine, ↑ B6, ↑ B12, and ↑ folate in diet.

  • Hartnup Disease:

    • Patho: AR; Defective neutral amino acid transporter in kidneys and intestine.
    • Presentation: Failure to absorb tryptophan, leading to niacin deficiency. Pellagra-like symptoms (dermatitis, diarrhea, dementia/ataxia).
    • Tx: High-protein diet, Niacin supplementation.

  • Cystinuria:

    • Patho: AR; Defective dibasic amino acid transporter (Cystine, Ornithine, Lysine, Arginine - COLA).
    • Presentation: Recurrent hexagonal cystine kidney stones.
    • Dx: Urinary cyanide-nitroprusside test is diagnostic.
    • Tx: Urinary alkalinization (e.g., potassium citrate), chelating agents (penicillamine), hydration.

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