Protein

• Amino acids


  Building blocks for proteins, provide C and N for synthesis of other biomolecules, sources of energy (4 Cal/g)
• Amino acid utilization


  Most important function is to provide building blocks for protein synthesis in the body (about 75%)
• Amino acid pool


  Amino acids come from: proteins eaten and hydrolyzed during digestion, the body's own degraded tissues, synthesis in the liver of certain amino acids

  
• Protein turnover


  The process in which body proteins are continuously hydrolyzed and resynthesized
• Protein turnover rate/half-life


  • Liver proteins: 10 days
  • Plasma proteins: 10 days
  • Hemoglobin: 120 days
  • Muscle protein: 180 days
  • Collagen: as high as 1000 days
  • Enzyme and polypeptide hormones: as short as a few minutes (Insulin: 7-10 minutes)
• The frequent turnover of proteins allows the body to continually renew important molecules and respond to changing needs
• There is a constant draw on the amino acid pool for the synthesis of other N-containing biomolecules
• Amino acid metabolic pathways


  1. N atoms are converted to ammonium ions, urea, or uric acid and excreted

  2. Carbon skeletons are converted to pyruvate, acetyl CoA, or citric acid cycle intermediates and used for energy production, gluconeogenesis, or conversion to triglycerides

  
• Nitrogen catabolism


  1. Stage 1: Transamination

  2. Stage 2: Deamination

  3. Stage 3: Urea Formation

  
• Transamination


  Amino groups freely move from one amino acid to another, under the influence of enzymes called amino transferases or transaminases

  
• Transamination examples


  • Transfer of amino group to α-ketoglutarate
  • Production of aspartate from transfer of amino group to oxaloacetate

  
• Deamination


  Removal of amino group as ammonium ion by glutamate dehydrogenase, regenerating α-ketoglutarate

  
• Deamination is the principal source of NH4+ in humans
• Urea formation


  NH4+ is converted to urea, which is less toxic and can be excreted in urine

  
• Normal urine from an adult usually contains about 23-30 g of urea daily

  
• Excretion of ammonium along with acidic ions helps the kidneys control the acid-base balance of body fluids
• Fate of amino acid carbon skeletons


  Converted into pyruvate, acetyl CoA, acetoacetyl CoA, or citric acid cycle intermediates

  
• Essential and nonessential amino acids

  Nonessential amino acids can be made in the amounts needed by the body, essential amino acids cannot be made in large enough amounts and must be obtained from the diet
• Biosynthesis of nonessential amino acids


  1. Key starting materials are intermediates in glycolysis and the citric acid cycle

  2. Tyrosine is produced from the essential amino acid phenylalanine

  3. Glutamate, alanine, and aspartate are synthesized from α-keto acids via transamination

  4. Asparagine and glutamine are formed from aspartate and glutamate by reaction with ammonium ions

  5. Synthesis of arginine, cysteine, glycine, proline, and serine are more complex

  
• The transaminases adjust the relative proportions of amino acids to meet the needs of the body, since most diets do not contain amino acids in the exact proportions needed by the body