Amino Acid Disorders - NEET PG Biochemistry

Amino acid disorders are important topics for NEET PG because they are a group of inherited genetic disorders that affect the metabolism of amino acids, leading to a variety of health problems. These disorders are significant because they are potentially life-threatening and can lead to intellectual disability, developmental delays, and other serious health issues. Knowledge of amino acid disorders is crucial for NEET PG exam preparation. 

Read this blog further and get an overview of this important biochemistry topic.

Glycine disorders

Phenylketonuria (PKU)

• Enzyme deficient: Phenylalanine hydroxylase

[Phenyl-keto-uria → phenyl- keto acid (phenyl Pyruvate) - in urine]

• C/F
  • Mousy/musty body odor (due to phenylacetate)
  • Severe MR (due to Phenylalanine)
• Diagnosis
  • Fecl₃ urine test → Green color (d/t Phenyl pyruvate)
• Rx
  1. Lifelong Restriction of phenylalanine in diet 
     • Aspartame (artificial sweetener) is contraindicated as aspartame is made of Aspartate + Phenylalanine
  2. Give tyrosine & tryptophan
  3. Tetra hydro Biopterin Supplementation helps in some patients

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Maple Syrup Urine Disease (MSUD)

• Enzyme deficient: Branched Chain Keto Acid (BCKA) dehydrogenase or decarboxylase
• Accumulation of Branched chain amino acids and keto acids occurs
• Urine odor is of Burnt sugar / Maple syrup odor
• Clinical features
  • Mental Retardation
  • Ketosis
  • If untreated, coma & death can occur
  • Has high mortality rate 
• Rx
  1. Vitamin B1 supplementation (coenzyme for BCKA dehydrogenase)
  2. dietary restriction of branched chain amino acids

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Alkaptonuria/Black bone disease

• Enzyme defect: Homogentisate dioxygenase (previously k/a Homogentisate oxidase)
• Homogentisic acid accumulates
• C/F
  • On standing color of urine turns black due to oxidation of Homogentisic acid
  • Age of onset: 30 - 40 years
  • no MR
  • Homogentisic acid gets polymerized in to alkapton bodies which accumulates in:
    • Cartilages (Nose, Ear Pinna, IV discs): Arthritis
    • Connective tissue: Bluish black color
    • Intervertebral discs: lower back pain (due to 
• Collectively, accumulation of homogentisic acid in all body parts is termed as Ochronosis and arthritis as Ochronotic arthritis.

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Albinism

• Enzyme deficient: Tyrosinase (makes melanin from tyrosine)
• Tyrosine is an oxidase enzyme & requires copper for its action. 
• c/f
• Hypopigmentation (milky white skin, Blonde hair, and Red eye color)

Important Information

Vitiligo 

• Patchy hypopigmentation
• Tyrosinase is normal 

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Homocystinuria (HCU)

• Two types:
  • Acquired
    • Due to vitamin B6, B9 and B12 deficiency
  • Genetic
    • Due to Enzyme Cystathionine beta Synthase defect 
    • Here cysteine becomes essential.
    • c/f
      • ↑ Serum Homocysteine (has SH - group)
      • ↑ urine Homocystine (made of two homocysteine by S-S bond)

Cyanide nitroprusside test is positive.

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