protein metabolism

• Amino acids


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


  About 75% is to provide building blocks for protein synthesis in the body
• Amino acid pool


  1. Amino acids come from: Proteins eaten and hydrolyzed during digestion

  2. The body's own degraded tissues

  3. Synthesis in the liver of certain amino acids

  
• Protein turnover


  The process in which body proteins are continuously hydrolyzed and resynthesized
• The turnover rate, or life expectancy, of body proteins is a measure of how fast they are broken down and resynthesized, expressed as a halflife
• Protein half-lives


  • 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, such as the bases in DNA and RNA, the heme in hemoglobin and myoglobin, the amino alcohols in phospholipids, and neurotransmitters

  
• Amino acid catabolism


  1. Nitrogen atoms are converted to either 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 stages


  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

  
• Deamination


  The enzyme glutamate dehydrogenase catalyzes the removal of the amino group as an ammonium ion and regenerates α-ketoglutarate

  
• Urea cycle


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

  
• Normal urine from an adult usually contains about 23-30 g of urea daily, although this varies with the protein content of the diet

  
• The direct excretion of NH4+ accounts for a small but important amount of the total urinary nitrogen

  
• The excretion of ammonium along with acidic ions is a mechanism that helps the kidneys to control the acid-base balance of body fluids
• Fate of amino acid carbon skeletons

  After the amino group is removed, the skeletons are degraded 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