Biochemistry · MU Pleven

44. Essential and Nonessential and Conditionally Essential Аmino Аcids. Biosynthesis of Nonessential Аmino Acids.

Explains the three categories of amino acids: essential ones that must come from the diet, nonessential ones the body can make, and conditionally essential ones that become necessary in prematurity, severe illness or liver disease. It then traces how several nonessential amino acids are made from glycolytic and TCA cycle intermediates, and closes with clinical links to phenylketonuria, homocystinuria and growth or trauma.

Inside: 4 sections · 9 labelled pictures · 13 flashcards · 2 min

Topic 44 of 76 in the MU Pleven syllabus for Biochemistry (Medicine, Year 2). The free AceMed note covers it in 4 sections, about 318 words (2 min of reading), with 13 flashcards.

In this note

  1. Essential and Nonessential and Conditionally Essential Аmino Acids. Biosynthesis of Nonessential Аmino Acids.
  2. Essential vs Nonessential vs Conditionally Essential Amino Acids
  3. Biosynthesis of Nonessential Amino Acids
  4. Key Clinical Connections

Essential vs Nonessential vs Conditionally Essential Amino Acids

  • Essential Amino Acids
  • These cannot be synthesized by the human body in sufficient amounts.
  • Must be obtained through diet.
  • Include: Valine, Leucine, Isoleucine, Methionine, Threonine, Tryptophan, Phenylalanine, Lysine, Histidine.
  • Arginine is essential in children (due to high growth demand).
  • Nonessential Amino Acids
  • These can be synthesized in the body from metabolic intermediates.
  • Their synthesis requires: carbon skeleton + ammonia/glutamate for amino group.
  • Include: Alanine, Aspartate, Asparagine, Glutamate, Serine, Glycine, Proline, Cysteine, Tyrosine, Glutamine, Arginine (in adults).
  • Conditionally Essential Amino Acids
  • Normally synthesized in adequate amounts but become essential in:
  • Prematurity
  • Severe illness
  • Liver disease
  • Examples:
  • Tyrosine becomes essential if Phenylalanine is deficient (like in PKU).
  • Cysteine becomes essential if Methionine is deficient.
  • Arginine, Glutamine become essential during severe stress (trauma, sepsis, burns).

Biosynthesis of Nonessential Amino Acids

(Think of them forming mostly from glycolysis, TCA cycle intermediates, or other amino acids.)

  • From α-Ketoglutarate (TCA Cycle Intermediate)
  • Glutamate is formed by reductive amination of α-ketoglutarate.
  • Glutamine is formed from glutamate (via glutamine synthetase, requires ATP).
  • Proline and Arginine are synthesized from glutamate derivatives.
  • From Oxaloacetate (TCA Cycle Intermediate)
  • Aspartate is made by transamination of oxaloacetate.
  • Asparagine is made from aspartate (via asparagine synthetase, requires ATP and glutamine as N donor).
  • From Pyruvate (Glycolysis End Product)
  • Alanine is made from transamination of pyruvate.
  • From 3-Phosphoglycerate (Glycolysis Intermediate)
  • Serine is synthesized from 3-phosphoglycerate.
  • Glycine is formed from serine by removal of a methylene group (via serine hydroxymethyltransferase, needs folate).
  • Cysteine is formed from serine + homocysteine (the latter comes from methionine).

Meaning: cysteine synthesis depends on dietary methionine.

  • From Phenylalanine
  • Tyrosine is synthesized by phenylalanine hydroxylase (requires tetrahydrobiopterin BH4).
  • In Phenylketonuria (PKU) this step is impaired, making tyrosine essential.

2-Methionine participates with serine in the synthesis of cysteine amino acid as follow

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