Amino acids are valuable metabolic fuels, providing a supply of both nitrogen and carbon for intermediary metabolism and energy for growth. Controlled degradation of amino acids is important in the maintenance of the carbon–nitrogen balance. It is becoming increasingly apparent that imbalance of amino acid degradation can have important consequences for both development and disease. Generally, the first step in degradation of amino acids results in the amino group either being incorporated into other nitrogenous compounds or being excreted as ammonia or urea, whereas the carbon skeleton is catabolized to one of a few common metabolic intermediates. Thus, an understanding of amino acid degradation provides knowledge of the interrelationships between metabolic pathways and helps explain some of the clinical features when deficiencies in amino acid metabolism occur.

Key concepts

• Amino acids are important growth substrates for microorganisms.
  
• Tight control of amino acid degradation and cycling maintains the C–N balance.
  
• Glutamate is a key central amino acid in maintenance of the C–N balance.
  
• The first step in amino acid degradation is removal of the -amino group.
  
• Key steps in amino acid degradation include deamination, catalysed by pyridoxal phosphate-dependent transaminases, oxidoreductases or carbon–oxygen lyases, decarboxylase reactions and carbon skeleton rearrangements catalysed by isomerases.
  
• Carbon skeletons arising from amino acid breakdown are channelled into central metabolism.
  
• Production and excretion of urea and uric acid by animals and birds and reptiles, respectively, avoids the accumulation of toxic levels of ammonia in blood and tissues.
  
• Metabolic products derived from l-serine are essential for cell proliferation and a functional nervous system.
  
• Absence of key enzymes or imbalance in amino acid degradation leads to severe disease states, such as phenylketonuria and methylmalonic aciduria.