Amino Acid Biosynthesis

Amino Acid Biosynthesis

Humans can synthesize only non-essential amino acids (11 of the 20) from metabolic intermediates. Essential amino acids (9) must be supplied in the diet; their deficiency causes specific diseases and growth failure.

Essential vs Non-Essential Amino Acids

Mnemonic for EAA (Phe, Val, Thr, Trp, Ile, Met, His, Arg*, Leu, Lys) — "PVT TIM HALL"

*Arg is conditionally essential (needed in growth; adults can synthesize via urea cycle but not in sufficient amounts).

Non-essential: Ala, Asp, Asn, Glu, Gln, Gly, Pro, Ser, Cys*, Tyr*, Sel. (*Conditionally essential — Cys from Met; Tyr from Phe)

Biosynthetic Routes from Metabolic Intermediates

  • From Pyruvate: Alanine (transamination of pyruvate with Glu), Cysteine (from homocysteine + Serine)
  • From Oxaloacetate (OAA): Aspartate, Asparagine
  • From α-Ketoglutarate: Glutamate (GDH: NH₃ + α-KG → Glu), Glutamine (Glutamine Synthetase: Glu + NH₃ + ATP → Gln — key for ammonia detoxification)
  • From 3-Phosphoglycerate (glycolysis): Serine → Glycine (serine hydroxymethyltransferase, B6, THF) → Cysteine
  • From Glutamate: Proline, Arginine (via partial urea cycle)

Important Amino Acid Derivatives

  • Tyrosine (from Phe): Precursor to Catecholamines (DOPA → Dopamine → Norepinephrine → Epinephrine), Thyroid hormone (T3/T4), Melanin (via DOPA)
  • Tryptophan: Precursor to Serotonin (→ Melatonin), Niacin (B3; 60mg Trp = 1mg Niacin)
  • Histidine: Precursor to Histamine (via histidine decarboxylase, B6)
  • Glycine: Precursor to Heme (with Succinyl-CoA), Purine, Creatine, Bile acids
  • Glutamate: Precursor to GABA (Glutamate decarboxylase, B6)
  • Serine: Precursor to Sphingolipids, Phosphatidylserine, Glycine
  • Arginine: Precursor to Nitric Oxide (NO) via NOS; also Creatine, Polyamines

Creatine Synthesis

Step 1: Arginine + Glycine → Guanidinoacetate [in Kidney]. Step 2: Guanidinoacetate + SAM (methyl donor) → Creatine [in Liver]. Creatine → Creatine Phosphate (phosphocreatine) in muscle — immediate energy buffer. Creatinine = non-enzymatic breakdown product of phosphocreatine; excreted by kidney at constant rate → used to assess GFR.

Inborn Errors of Amino Acid Metabolism

  • PKU (Phenylketonuria): Phenylalanine Hydroxylase deficiency → Phe accumulates → converted to Phenylketones (phenylpyruvate, phenylacetate, phenyllactate). → Fair skin, blue eyes, mousy odor, intellectual disability, eczema. Treat: Phe-restricted diet (avoid aspartame!), BH4 supplementation (for BH4-responsive PKU).
  • Tyrosinemia: Fumarylacetoacetate hydrolase deficiency (Type I) → liver failure, renal tubular dysfunction, cabbage-like odor, hepatocellular carcinoma risk.
  • Alkaptonuria: Homogentisate oxidase deficiency → Homogentisic acid accumulates → darkens in urine (ochronosis), arthritis.
  • Albinism: Tyrosinase deficiency → no Melanin → photosensitivity, vision problems.
  • MSUD (Maple Syrup Urine Disease): Branched-chain α-keto acid dehydrogenase deficiency → Leu, Ile, Val accumulate. Maple syrup odor, encephalopathy, death if untreated. Treatment: Diet low in BCAAs.
  • Homocystinuria: CBS (Cystathionine β-Synthase) deficiency → Homocysteine accumulates. Features: Lens dislocation (downward, ectopia lentis), thromboembolism, intellectual disability, Marfanoid habitus. Elevated homocysteine → atherosclerosis risk. Treatment: B6, folate, B12, methionine restriction.

Quiz - Exam Preparation Strategy

When studying Quiz for your final board exams, it is critical to focus on the core concepts and fundamental formulas. Relying strictly on NCERT textbook solutions and practicing previous year questions (PYQs) is the proven methodology for scoring high marks. Avoid rote memorization and instead focus on the logical application of the theories presented in this chapter.

⚠️ Common Mistakes to Avoid

❓ Frequently Asked Questions

How can I quickly memorize the concepts of Quiz?

The most effective way is to create short, handwritten revision notes and continuously test your knowledge using our interactive Mock Tests. Spaced repetition and active recall are much better than passive reading.

What type of questions are most commonly asked from Quiz?

Board exams tend to favor conceptual application questions and direct formula-based derivations from the NCERT syllabus. Ensure you have solved every single exercise in the official textbook.

Is reading the NCERT book enough for this chapter?

Yes, the NCERT textbook is the absolute gold standard for board exams. However, to improve your speed and accuracy during the actual exam, you must supplement your reading by solving timed mock tests and objective questions.