2 Amino acids

• Amino acids


  Nitrogen-containing macromolecules consisting of L-a-amino acids as the repeating units
• Types of amino acids

  • Essential amino acids
  • Non-essential amino acids
• Of the 20 amino acids found in proteins, half can be synthesized by the body (non-essential) while the rest have to be provided in the diet (essential amino acids)
• Proteolysis


  Release individual amino acids
• Amino acids are not just the structural components of proteins
• Each one of the 20 naturally occurring amino acids undergoes its own metabolism and performs specific functions
• Biologically important compounds synthesized from amino acids

  • Melanin
  • Serotonin
  • Creatine
• Certain amino acids may directly act as neurotransmitters (e.g. glycine, aspartate, glutamate)
• Amino acid pool


  The amount of free amino acids distributed throughout the body
• An adult has about 100 g of free amino acids which represent the amino acid pool of the body
• Glutamate and glutamine together constitute about 50%, and essential amino acids about 10% of the body pool (100 g)
• The concentration of intracellular amino acids is always higher than the extracellular amino acids
• The amino acid pool of the body is maintained by the sources that contribute (input) and the metabolic pathways that utilize (output)
• Sources of amino acid pool

  • Turnover of body protein
  • Intake of dietary protein
  • Synthesis of non-essential amino acids
• Protein turnover


  1. Balance between protein synthesis

  2. Protein degradation
• The protein present in the body is in a dynamic state
• It is estimated that about 300-400 g of protein per day is constantly degraded and synthesized which represents body protein turnover
• There is a wide variation in the turnover of individual proteins
• The plasma proteins and digestive enzymes are rapidly degraded, their half-lives being in hours or days
• The structural proteins (e.g. collagen) have long half-lives, often in months and years
• Control of protein turnover


  Influenced by many factors
• A small polypeptide called ubiquitin tags with the proteins and facilitates degradation
• Certain proteins with amino acid sequence proline, glutamine, serine, and threonine (PEST sequence) are rapidly degraded
• In healthy adults, it is estimated that about 30-50 g of protein is lost every day from the body
• This amount of protein (30-50 g/day) must, therefore, be supplied daily in the diet for maintaining nitrogen balance
• The purpose of dietary protein is to supply amino acids (particularly the essential ones) for the synthesis of proteins and other nitrogen compounds
• There is no storage form of amino acids as is the case for carbohydrates (glycogen) and lipids (triacylglycerols)
• The excess intake of amino acids is metabolized-oxidized to provide energy, converted to glucose or fat
• The amino groups are lost as urea and excreted
• The protein consumption in developed countries is much higher than the recommended dietary allowance (i.e. 1 g/kg body weight/day)
• The daily protein intake by an adult in most countries is 40-100 g
• Protein digestion


  1. Digested by proteolytic enzymes

  2. Absorbed in the intestine

  3. Enter the body pool of amino acids
• Synthesis of non-essential amino acids

  • Ten out of the 20 naturally occurring amino acids can be synthesized by the body
• Most of the body proteins (300-400 g/day) degraded are synthesized from the amino acid pool
• Important nitrogenous compounds produced from amino acids

  • Porphyriens
  • Purines
  • Pyrimidines
• About 30 g of protein is daily utilized for the production of nitrogenous compounds
• Generally, about 10-15% of body energy requirements are met from the amino acids
• The amino acids are converted to carbohydrates and fats
• This becomes predominant when the protein consumption is in excess of the body requirements