PROTEINS

• Protein Metabolism


  The chemical cycle of breaking down protein (catabolism) and using the components to synthesize new molecules (anabolism) to be used in the body
• Protein Metabolism


  • Occurs in the liver
• Protein Catabolism


  1. Amino Acids Metabolism

  2. Amino Acids Catabolism

  3. Amino Acids Biosynthesis

  4. Specialized Products

  5. Replication

  6. Transcription

  7. Translation
• Amino Acids


  The building blocks for proteins, they provide C and N for the synthesis of other biomolecules, and they are also sources of energy (4 Cal/g)
• Amino Acid Pool


  The maintenance of body proteins must occur constantly because tissue proteins break down from normal wear and tear, from injuries, and from diseases. These amino acids can come from: proteins that are eaten and hydrolyzed during digestion, the body's own degraded tissues, and the synthesis in the liver of certain amino acids

  
• Protein Turnover


  The process in which body proteins are continuously hydrolyzed and resynthesized
• Protein Half-Life


  The turnover rate, or life expectancy, of body proteins expressed as a half-life. 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 also 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 Metabolic Pathways

  Amino acids in excess of immediate body requirements cannot be stored for later use. The N atoms are converted to either ammonium ions, urea, or uric acid (depending on the organism), and excreted. Their carbon skeletons are converted to pyruvate, acetyl CoA, or one of the intermediates in the citric acid cycle and used for energy production, the synthesis of glucose through gluconeogenesis, or conversion to triglycerides

  
• Amino Acid Catabolism


  1. Stage 1: Transamination

  2. Stage 2: Deamination

  3. Stage 3: Urea Formation

  
• Transamination


  In the tissues, amino groups freely move from one amino acid to another, under the influence of enzymes called amino tranferases or trans aminases. A key reaction for amino acids undergoing catabolism is a transamination involving the transfer of amino groups to α-ketoglutarate

  
• Deamination


  The enzyme glutamate dehydrogenase catalyzes the removal of the amino group as an ammonium ion and regenerates α-ketoglutarate, which can participate in transamination again. This reaction is the principal source of NH4+ (ammonium) in humans

  
• Urea Formation


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

  
• Fate of the Carbon Skeleton

  After the amino group is removed by transamination or oxidative deamination, the remaining amino acid carbon skeleton undergoes catabolism and is converted into one of several products: pyruvate, acetyl CoA, acetoacetyl CoA, or various substances that are intermediates in the citric acid cycle
• Nonessential Amino Acids


  • Amino acids that can be made in the amounts needed by the body

  
• Essential Amino Acids


  • Amino acids that cannot be made in large enough amounts and must be obtained from the diet
• Biosynthesis of Nonessential Amino Acids


  The key starting materials are intermediates in glycolysis and the citric acid cycle. Tyrosine is produced from the essential amino acid phenylalanine. Three nonessential amino acids (glutamate, alanine, and aspartate) are synthesized from a-keto acids via transamination. The transaminases adjust the relative proportions of amino acids to meet the needs of the body

  
• Biosynthesis of Asparagine and Glutamine

  Asparagine and glutamine are formed from aspartate and glutamate by reaction of the side-chain carboxylate groups with ammonium ions