Nitrogen Metabolism

• Nitrogen metabolism

  Synthesis and degradation of amino acids and proteins

  
• Nitrogen is too unreactive to take part in most biochemical reactions
• Nitrogen is 'fixed' to biochemically available nitrogen compounds by a few microorganisms

  
• Nitrogen is required for the amino group in amino acids
• Amino acids and proteins

  Not stored like carbohydrates and fats
• Amino acid degradation

  Releases ammonia, which is toxic. This is converted to urea for excretion
• Amino acid pool

  Free amino acids - very low concentrations inside cells or in the blood stream. Mixing and exchange with other free amino acids distributed throughout the body
• Protein requirements

  No 'storage' form of protein in the body to replace proteins and other N containing compounds. Protein is needed in the diet to replace lost amino acids and allow for tissue repair. Recommendation: 50-80g protein per day. High protein intake in well-fed individual is wasteful - surplus of amino acids are rapidly catabolized, and the nitrogen excreted as urea in the urine
• Positive nitrogen balance

  N intake > N excretion. Protein synthesis exceeds the rate of breakdown. During normal growth in children, in convalescence after serious illness, after immobilization after an accident, in pregnancy

  
• Negative nitrogen balance

  N intake < N excretion. Protein breakdown exceeds the rate of synthesis. In starvation, during serious illness, in late stages of some cancers, in injury and trauma. If not corrected and becomes prolonged, there will be irreversible loss of essential body tissue - will eventually lead to death
• Protein degradation pathways

  Most cellular proteins: Recognized as 'old' or damaged, removed by the ubiquitin breakdown system, give a mixture of the 20 amino acids. Foreign 'exogenous' proteins: 'Old' or damaged sub cellular organelles, taken into vesicles by endocytosis/autophagocytosis, vesicle fuse with lysosome, proteolytic enzymes degrade proteins into amino acids. Protein degradation regulated by: Starvation, Hormones (e.g., cortisol increases rates of protein breakdown in muscle)
• Deamination - Removal of a-amino group

  Glutamate -> a-Ketoglutarate + NH4+. Takes place in liver mitochondrial matrix, catalyzed by glutamate dehydrogenase, only happens to Glutamate in humans. Ammonia is toxic - Its converted to a non-toxic compound in the Urea Cycle. Urea is transported via the blood to the kidney for extraction
• Transamination - Conversion of one amino acid to another

  Amino Acid + a-Ketoglutarate -> Glutamate + Keto Acid. Catalyzed by aminotransferases, which are specific for one amino acid. a-amino group is transferred from an amino acid to a-Ketoglutarate to produce Glutamate and a Keto Acid
• Transdeamination - Deamination + Transamination

  Combined action of aminotransferases and glutamate dehydrogenase allows a-amino groups from different amino acids to enter the Urea Cycle, using glutamate as an intermediate
• Glutamine - Transport of Ammonia in the blood

  In extra-hepatic tissues, ammonia is added to glutamate to produce glutamine. Glutamine is safely transported in the bloodstream to the liver. Glutamine is converted back to glutamate, and the ammonia released enters the Urea Cycle for safe disposal
• Alanine - Generated in skeletal muscle during exercise

  Vigorously exercising muscle uses protein as well as carbohydrate for energy. The carbon skeletons of amino acids can be used in the TCA cycle to generate ATP but the ammonia must be safety removed. Skeletal muscles generated lots of pyruvate (and lactate). Transamination of pyruvate to alanine allows the ammonia to be safety transported to the liver. The glucose-alanine cycle allows the liver to regenerate glucose from this alanine

  
• Carbon skeletons produced from degradation of 20 amino acids

  • Pyruvate
  • Oxaloacetate
  • Fumarate
  • Succinyl-CoA
  • a-Ketoglutarate
  • Acetoacetayl-CoA
  • Acetyl-CoA
• Glucogenic amino acids

  Amino acids producing carbon skeletons that can enter the TCA cycle to generate ATP or enter gluconeogenesis to release glucose into the blood

  
• Ketogenic amino acids

  Amino acids producing carbon skeletons that produce ketone bodies. Only Lysine and Leucine are strictly ketogenic
• Essential amino acids

  • Cannot be synthesized in humans - must be obtained via diet. Glutamate, Glutamine, Alanine & Aspartate are readily generated and degraded in humans and play important roles in metabolism
• Amino acid catabolism in the liver - summary

  Glutamine transports ammonia to the liver where it is converted to Glutamate + Ammonia. Alanine from skeletal muscle is converted to Glutamate + Pyruvate. Excess amino acids are converted to Glutamate by transamination. Glutamate is deaminated to generate a-Ketoglutarate an ammonia. Ammonia is converted to Urea in the Urea Cycle. Urea is transported to the kidney for excretion