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Protein metabolism

Protein metabolism. By Dr OJ Tsotetsi. protein. A major component of foods. It is digested firstly in the stomach, and then in the duodenum to dipeptides and amino acid. Absorbed using symport active transport with sodium. Stored in liver and muscles . Uses.

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Protein metabolism

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  1. Protein metabolism By Dr OJ Tsotetsi

  2. protein • A major component of foods. It is digested firstly in the stomach, and then in the duodenum to dipeptides and amino acid. • Absorbed using symport active transport with sodium. • Stored in liver and muscles

  3. Uses • Protein synthesis -The synthesis of new proteins is very important during growth. In adults new protein synthesis is directed towards replacement of proteins as they are constantly turned over. • synthesis of a variety of other compounds - Examples of compounds synthesized from amino acids include purines and pyrimidines (components of nucleotides), catecholamines (adrenaline and noradrenalin) & neurotransmitters (serotonin)

  4. as a biological fuel - About 10% of energy production in humans is from amino acids

  5. Amino acid catabolism • The other biological fuels discussed (carbohydrates & fats) contain only the elements carbon, hydrogen and oxygen. Amino acids contain nitrogen as well. The first step in amino acid catabolism is the removal of the nitrogen (the amino group).

  6. Deamination • The removal of the amino groups of all twenty amino acids begins with the transfer of amino groups to just one amino acid - glutamic acid (or glutamate ion). This is catalyzed by transaminase enzymes which transfer the amino group from amino acids to a compound called alpha-ketoglutarate. The product is an alpha-keto acid formed from the amino acid and glutamate (formed from the addition of the amino group to alpha-ketoglutarate.

  7. Once the amino groups have all been "collected" in the form of the one amino acid, glutamate, this amino acid has its amino group removed (termed "oxidative deamination"). This reaction reforms alpha-ketoglutarate with the other product being ammonia (NH4 +). Ammonia is toxic to the nervous system and its accumulation rapidly causes death. Therefore it must be detoxified to a form which can be readily removed from the body. Ammonia is converted to urea, which is water soluble and is readily excreted via the kidneys in urine

  8. The remainder of the amino acid is referred to as the "carbon skeleton". Depending on the particular amino acid being catabolised, its carbon skeleton will be converted to : acetyl CoA Those carbon skeletons which end up as acetyl CoA are committed to energy production. They will either be immediately oxidised via the citric acid cycle or they may be converted to ketone bodies. Because the amino acids whose carbon skeletons yield acetyl CoA are potentially a source of ketone bodies they are referred to as ketogenic amino acids. or pyruvate or a citric acid cycle intermediate

  9. Amino acid synthesis • A detailed description of the processes by which amino acids are synthesised is outside the aim of this introductory module. Only a few brief relevant points are included. Amino acids are divided into two classes depending on whether they can be synthesised in the human body or whether they must be supplied in the diet. The former group are referred to as non-essential and the latter group as essential. The table below shows which of the twenty are in each group. Note that there are ten in each of the two groups

  10. Non-essential amino acids are synthesised from the products of their catabolism - i.e. acetyl CoA, pyruvate or the relevant Krebs cycle intermediate. The amino group is donated by glutamate and added by the reverse of the transamination reactions discussed above. The essential amino acids are synthesised in micro-organisms (bacteria and yeasts) and passed through the food chain until they reach us in our diet. One of the pathways essential to life which is carried out by bacteria is the "fixation" of atmospheric nitrogen initially as inorganic nitrate and ultimately as amino groups in amino acids. Higher organisms cannot perform this function.

  11. Figure 25.3 Nutrient Use in Cellular Metabolism Figure 25.3

  12. Hormone Principal metabolic actions Insulin Increases glucose uptake in peripheral tissues. Stimulates protein synthesis.Inhibits lipolysis and glycolysis Glucagon Increases cyclic AMP levels in the liver and adipose tissue, with stimulation of fatty acid mobilization, glycogenolysis, glycolysis and gluconeogenesis Catecholamines Increase cyclic AMP levels in the liver, skeletal muscle and adipose tissue, with release of glucose, free fatty acids and lactate Corticosteroids Increase gluconeogenesis. Increase amino acid mobilization from the periphery (chiefly skeletal muscle), Increase fatty acid release from extremities. Decrease glucose utilization by peripheral tissues GH AND TH

  13. Fed State • Insulin • stimulates LPL • increased uptake of FA from chylomicrons and VLDL • stimulates glycolysis • increased glycerol phosphate synthesis • increases esterification • induces HSL-phosphatase • inactivates HSL • net effect: TG storage

  14. Figure 25.15 The Absorptive State Figure 25.15

  15. Figure 25.17 The Postabsorptive State Figure 25.17

  16. Acknowlegdement • http://www.elmhurst.edu/~chm/vchembook/630proteinmet.html

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