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Amino acids & proteins

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Lecture of amino acids and proteins

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Amino acids & proteins

  1. 1. Mahmoud BalbaaProfessor of BiochemistryBeirut Arab UniversityMarch 2012
  2. 2. Dynamics of Protein And Amino Acid MetabolismDietary Proteins Digestion to Amino Acids Transport in Blood to Cells Protein Synthesis Functional Proteins Amino Acids Protein Degradation In Proteasomes Following Tagging With Ubiquitin Metabolites 2
  3. 3. Digestion of ProteinsStomach: Pepsinogen Pepsin (max. act. pH 2) EnteropeptidaseSmall Intestine: Trypsinogen Trypsin Trypsin cleaves: Chymotrypsinogen to chymotrypsin Proelastase to elastase Procarboxypeptidase to carboxypeptidase Aminopeptidases (from intestinal epithelia) 3
  4. 4. 20 Amino Acids
  5. 5. Metabolic Classification of theAmino Acids • Essential and Non-essential • Glucogenic and Ketogenic 5
  6. 6. Non-Essential Amino Acids in Humans Not required in diet Can be formed from α-keto acids by transamination and subsequent reactions• Alanine • Glycine • Asparagine • Proline • Aspartate • Serine • Glutamate • Cysteine (from Met*) • Glutamine • Tyrosine (from Phe*) * Essential amino acids 6
  7. 7. Essential Amino Acids in Humans Required in diet Humans incapable of forming requisite carbon skeleton • Arginine* • Lysine • Histidine* • Methionine • Isoleucine • Threonine • Leucine • Phenylalanine • Valine • Tryptophan * Essential in children, not in adults 7
  8. 8. Glucogenic Amino Acids Metabolized to α-ketoglutarate, pyruvate, oxaloacetate, fumarate, or succinyl CoA Phosphoenolpyruvate Glucose• Aspartate • Methionine • Alanine• Asparagine • Valine • Serine• Arginine • Glutamine • Cysteine• Phenylalanine • Glutamate • Glycine• Tyrosine • Proline • Threonine• Isoleucine • Histidine • Tryptophan 8
  9. 9. Ketogenic Amino Acids  Metabolized to acetyl CoA or acetoacetyl CoA Animals cannot convert acetyl CoA or acetoacetyl CoA to pyruvate • Isoleucine • Tryptophan • Leucine * • Phenylalanine • Lysine * • Tyrosine • Threonine * Leucine and lysine are only ketogenic 9
  10. 10. 5. Carbon Atoms Ketogenic Glucogenic Both leucine serine isoleucine lysine threonine phenylalanine aspartic acid tryptophan glutamic acid tyrosine asparagine glutamine glycine alanine valine proline histidine arginine methionine cysteine10
  11. 11. 4. The Urea Cycle11
  12. 12. Urea Formation Occurs primarily in liver; excreted by kidney Principal method for removing ammonia Hyperammonemia:  Defects in urea cycle enzymes (CPS, OTC, etc.)  Severe neurological defects in neonates  Treatment:  Stop protein intake  Dialysis  Increase ammonia excretion: Na benzoate, Na phenylbutyrate, L-arginine, L-citrulline 12
  13. 13. Blood Urea Nitrogen Normal range: 7-18 mg./dL Elevated in amino acid catabolism Glutamate N-acetylglutamate CPS-1 activation Elevated in renal insufficiency Decreased in hepatic failure 13
  14. 14. 5. Carbon Atoms14
  15. 15. Alkaptonuria • Deficiency of homogentisate dioxygenase • Urine turns dark on standing • Oxidation of homogentisic acid • Asymptomatic in childhood • Tendency toward arthritis in adulthood 15
  16. 16. Serotonin• Serotonin formed in: • Brain (neurotransmitter; regulation of sleep, mood, appetite) • Platelets (platelet aggregation, vasoconstriction) • Smooth muscle (contraction) • Gastrointestinal tract (enterochromaffin cells - major storage site)• Drugs affecting serotonin actions used to treat: • Depression •Serotonin-selective reuptake inhibitors (SSRI) • Migraine • Schizophrenia • Obsessive-compulsive disorders • Chemotherapy-induced emesis• Some hallucinogens (e.g., LSD) act as serotonin agonists 16
  17. 17. Proteins : Classification Solubility  Albumins (s. in water and salt sol.)  Globulins (s. sparingly in water, s. in salt sol.)  Prolamines (s. in 70-80% ethanol, Arg rich)  Histones (s. in salt sol., basic)  Scleroproteins (insoluble in water and salt sol., Gly, Ala, Pro rich)
  18. 18. Proteins : Classification Shape  Globular (albumins, globulins, enzymes)  Fibrous (keratin, myosin, collagen, fibrin) Function Physical properties  Electrophoretic mobility  Sedimentation (ultracentrifugation)
  19. 19. • Primary• Secondary• Tertiary• Quarternary
  20. 20. Primary structure Sequence of amino acids Peptide bond Encoded in DNA DNA mRNA protein Determines higher structures
  21. 21. Primary structure
  22. 22. Secondary structure Spatial arrangement of AA chain Most stable structure - low energy: all -NH groups bond to -CO- groups by hydrogen bonds  Within a single chain: α -helix  Between two chains: β-pleated sheet o parallel o antiparallel  Special type: collagen helix Also maintained by hydorphobic interactions
  23. 23. Alpha helix
  24. 24. Beta pleated sheet
  25. 25. Tertiary structure Overall shape and folding pattern of polypeptide chain Bonds  Disulphidic bridges  Ion interactions  Hydrophobic interactions  Hydrogen bonds Energically most efficient
  26. 26. Tertiary structure Acidic proteinase
  27. 27. Quarternary structure More polypeptide chains united by forces other than covalent bonds  Hydrogen bonds  Ion bonds  Hydrophobic interactions
  28. 28. Quarternary structure

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