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Chapter 6

Chapter 6. Protein. Functional Categories. Catalysts - enzymes Hydrolases - cleave compounds Isomerases - transfer atoms in a molecule Ligases (synthases) - join compounds Oxidoreductases - transfer electrons Transferases - move functional groups. Functional Categories. Messengers

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Chapter 6

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  1. Chapter 6 Protein 2009 Cengage-Wadsworth

  2. Functional Categories • Catalysts - enzymes • Hydrolases - cleave compounds • Isomerases - transfer atoms in a molecule • Ligases (synthases) - join compounds • Oxidoreductases - transfer electrons • Transferases - move functional groups 2009 Cengage-Wadsworth

  3. Functional Categories • Messengers • Hormones • Structural elements • Contractile proteins • Fibrous proteins • Globular proteins • Immunoprotectors • Immunoproteins (antibodies) 2009 Cengage-Wadsworth

  4. Functional Categories • Transporters • Albumin • Transthyretin (prealbumin) • Transferrin • Ceruloplasmin • Lipoproteins 2009 Cengage-Wadsworth

  5. Functional Categories • Buffers • Regulation of acid-base balance • Fluid balancers • Proteins attract water to blood • Other roles • Adhesion, signaling, receptors, storage • Conjugated proteins • Glycoproteins • Proteoglycans 2009 Cengage-Wadsworth

  6. Protein Structure & Organization • Primary structure • Sequence of covalent bonds among amino acids • Secondary structure • Hydrogen bonding • -helix • -conformation or -pleated sheet • Random coil 2009 Cengage-Wadsworth

  7. Protein Structure & Organization • Tertiary structure • Clustering of hydrophobic AAs toward center • Electrostatic (ionic) attraction • Strong covalent bonding between cysteine residues - disulfide bridges • Quaternary structure • Interactions between 2 or more polypeptide chains • Oligomers 2009 Cengage-Wadsworth

  8. Amino Acid Classification • Structure • Central C • At least 1 amino group (NH2) • At least 1 carboxy (acid) group (COOH) • Side chain (R group) • Makes AA unique 2009 Cengage-Wadsworth

  9. Amino Acid Classification • Net electrical charge • Zwitterions have none • Polarity • Polar or nonpolar • Determined by R group • Essentiality • Lysine, threonine & histidine totally indispensable 2009 Cengage-Wadsworth

  10. Sources of Protein • Exogenous sources • Animal products - except fats • Plant products - grains/grain products, legumes, vegetables • Endogenous proteins • Desquamated mucoasal cells • Digestive enzymes & glycoproteins 2009 Cengage-Wadsworth

  11. Digestion & Absorption • Protein digestion • Mouth & esophagus - none • Stomach • HCl denatures • Pepsin hydrolyzes peptide bonds 2009 Cengage-Wadsworth

  12. Digestion & Absorption • Small intestine • Pancreatic enzymes • Trypsinogen  trypsin • Chymotrypsinogen  chymotrypsin • Procarboxypeptidases A & B  carboxypeptidases • Proelastase • Collagenase • Brush border peptidases • Aminopeptidases, dipeptdylaminopeptidases, tripeptidases • Tripeptides hydrolyzed or absorbed at brush border 2009 Cengage-Wadsworth

  13. Digestion & Absorption • Intestinal brush border membrane amino acid & peptide absorption • Amino acid transport • Carriers required - passive & active transporters • Peptide transport • PEPT1 • Co-movement of protons (H+) 2009 Cengage-Wadsworth

  14. Digestion & Absorption • Intestinal basolateral membrane transport of amino acids • Diffusion & sodium-independent transport are main modes • Intestinal cell amino acid use • Cells use or partially metabolize for release into blood 2009 Cengage-Wadsworth

  15. Digestion & Absorption • Intestinal glutamine metabolism • Primary energy source for enterocytes • Intestinal glutamate metabolism • Intestinal aspartame metabolism • Intestinal arginine metabolism • Intestinal methionine & cysteine metabolism 2009 Cengage-Wadsworth

  16. Digestion & Absorption • Amino acid absorption into extraintestinal tissues • AAs enter portal vein to liver • Transport into hepatocytes • Transport into other cells • -glutamyl cycle 2009 Cengage-Wadsworth

  17. Amino Acid Metabolism • Metabolism of AAs includes: • Protein synthesis • Amino acid catabolism • Hepatic catabolism • Uses of aromatic amino acids • Uses of sulfur-containing amino acids • Uses of branched-chain amino acids • Uses of other amino acids • Plasma amino acids & pools 2009 Cengage-Wadsworth

  18. Synthesis of Plasma Proteins, Nitrogen-Containing Nonprotein Compounds, & Purine & Pyrimidine Bases • Plasma proteins • Albumin • Transthyretin (prealbumin) • Retinol-binding protein • Blood clotting proteins • Immunoproteins • Transport proteins • Acute phase proteins • Stress (heat) shock proteins (hsp) 2009 Cengage-Wadsworth

  19. Synthesis of Plasma Proteins, Nitrogen-Containing Nonprotein Compounds, & Purine & Pyrimidine Bases • Nitrogen-containing nonprotein compounds • Glutathione - antioxidant, reacts with H2O2, AA transport, conversion of prostaglandin H2 to D2 & E2 • Carnitine - FA transport • Creatine - part of phosphocreatine (high-energy compound) 2009 Cengage-Wadsworth

  20. Synthesis of Plasma Proteins, Nitrogen-Containing Nonprotein Compounds, & Purine & Pyrimidine Bases • Carnosine - may be antioxidant • Choline - methyl donor, part of acetylcholine & lecithin & sphingomyelin 2009 Cengage-Wadsworth

  21. Synthesis of Plasma Proteins, Nitrogen-Containing Nonprotein Compounds, & Purine & Pyrimidine Bases • Purine & pyrimidine bases • Main constituents of DNA & RNA • Pyrimidines • 6-membered rings containing N in positions 1 & 3 • Uracil, cytosine & thymidine • Purines • 2 fused rings, N in positions 1, 3, 7, 9 • Adenine & guanine 2009 Cengage-Wadsworth

  22. Protein Synthesis Overview • Insulin & glucagon • Rate of protein digestion • Leucine • Fed vs. fasted state 2009 Cengage-Wadsworth

  23. Amino Acid Catabolism Overview • Transamination &/or deamination of amino acids • Deamination = removal of amino group • Transamination = transfer of amino group from one AA to AA carbon skeleton or -keto acid • Catalyzed by aminotransferases 2009 Cengage-Wadsworth

  24. Amino Acid Catabolism Overview • Disposal of ammonia--the urea cycle • NH3 combines with CO2 or HCO3- to form carbamoyl phosphate • Carbamoyl phosphate reacts with ornithine transcarbamoylase (OTC) to form citruline • Aspartate reacts with citruline to form argininosuccinate • Arginosuccinate is cleaved to form fumarate & arginine • Urea is formed and ornithine is re-formed from cleavage of arginine 2009 Cengage-Wadsworth

  25. Amino Acid Catabolism Overview • An overview of metabolism of the carbon skeleton/-keto acid • Energy generation • Glucose & ketone body production • Cholesterol production • Fatty acid production 2009 Cengage-Wadsworth

  26. Hepatic Catabolism & Uses of Aromatic Amino Acids • Phenylalanine & tyrosine • Phenylalanine converted to tyrosine by phenylalanine hydroxylase • Tyrosine • Degradation begins with transamination to p-hydroxyphenylpyruvate • Tyrosine used in other tissues for synthesis of L-dopa & catecholamines • Melanin, thyroid hormones • Disorders of phenylalanine & tyrosine metabolism 2009 Cengage-Wadsworth

  27. Hepatic Catabolism & Uses of Aromatic Amino Acids • Tryptophan • Catabolized to N-formylkynurenine • This is catabolized to formate & kynurenine • Used for: • Protein synthesis • Energy, glucose, & ketone body production • Synthesis of serotonin & melatonin • Disorders of tryptophan metabolism. 2009 Cengage-Wadsworth

  28. Hepatic Catabolism & Uses of Sulfur (S)-Containing Amino Acids • Methionine • Converted to S-adenosyl methionine • SAM is principal methyl donor • Removal of methyl group yields S-adenosyl homocysteine (SAH) • SAH converted to homocysteine • Homocysteine reacts with serine to form cystathionine • Cystathionine cleaved to form cysteine & -ketobutyrate • Propionyl CoA (made from -ketobutyrate) converted to D-methylmalonyl CoA • Disorders of methionine metabolism 2009 Cengage-Wadsworth

  29. Hepatic Catabolism & Uses of Sulfur (S)-Containing Amino Acids • Cysteine • Used for protein & glutathione synthesis • Converted to cysteine sulfinate, used to produce taurine • Taurine important in retina, functions as bile salt & inhibitory neurotransmitter • Cysteine degradation yields pyruvate & sulfite 2009 Cengage-Wadsworth

  30. Hepatic Catabolism & Uses of the Branched-Chain Amino Acids • Isoleucine, leucine, & valine • Taken up & transaminated primarily in muscles 2009 Cengage-Wadsworth

  31. Hepatic Catabolism & Uses of Other Amino Acids • Lysine • Ketogenic - catabolism yields acetyl CoA • Disorders of lysine metabolism • Threonine • 3 pathways • Disorders of threonine metabolism 2009 Cengage-Wadsworth

  32. Hepatic Catabolism & Uses of Other Amino Acids • Glycine & serine • Produced from one another in reversible reaction requiring folate • Disorders of glycine metabolism • Arginine • Kidney - creatine synthesis • Liver - generation of urea & ornithine • Histidine • Glutamate, carnosine, histamine 2009 Cengage-Wadsworth

  33. Amino Acids Not Taken Up by the Liver: Plasma Amino Acids & Amino Acid Pool(s) • Plasma concentrations rise after a meal • Pool of about 150 g of endogenous + exogenous AAs • Re-use thought to be primary source of AAs for protein synthesis • More nonessential than essential in pool 2009 Cengage-Wadsworth

  34. Interorgan “Flow” of Amino Acids & Organ-Specific Metabolism • Glutamine & the liver, kidneys, & intestine • Ammonia transport • Hypercatabolic conditions • Alanine & the liver & muscle • Inter-tissue transfer of amino groups • Liver: converted to glutamate or glucose 2009 Cengage-Wadsworth

  35. Interorgan “Flow” of Amino Acids & Organ-Specific Metabolism • Skeletal muscle • Isoleucine, leucine, & valine • Nitrogen-containing compounds as indicators of muscle mass & muscle/ protein catabolism 2009 Cengage-Wadsworth

  36. Interorgan “Flow” of Amino Acids & Organ-Specific Metabolism • Kidneys • Serine synthesis from glycine • Glycine catabolism to ammonia • Histidine generation from carnosine degradation • Arginine synthesis from citruline • Tyrosine synthesis from phenylalanine • Guanidoacetate formation from arginine & glycine for creatine synthesis 2009 Cengage-Wadsworth

  37. Brain & Accessory Tissues • Biogenic amines & neurotransmitters/hormones • Tryptophan - melatonin & serotonin • Tyrosine - dopamine, norepinephrine, epinephrine • Glysine - inhibitory neurotransmitter • Taurine - inhibitory neurotransmitter • Aspartate - excitatory neurotransmitter • Glutamate - excitatory neurotransmitter or converted to -amino butyric acid (GABA) 2009 Cengage-Wadsworth

  38. Brain & Accessory Tissues • Neuropeptides • Hormone-releasing factors • Endocrine effects • Modulatory actions on transmitter functions, mood or behavior • Neurosecretory cells of hypothalamus secrete • Synthesized from AAs via DNA codes 2009 Cengage-Wadsworth

  39. Protein Turnover: Synthesis & Catabolism of Tissue Proteins • Food intake & nutritional status • Hormonal mediation • AA pools connect 2 cycles of N metabolism: • Protein turnover • Nitrogen balance • Protein synthesis & degradation controlled separately 2009 Cengage-Wadsworth

  40. Protein Turnover: Synthesis & Catabolism of Tissue Proteins • Cellular protein degradation systems • Lysosomal degradation • Proteasomal degradation • Calcium or calcium-activated proteolytic degradation 2009 Cengage-Wadsworth

  41. Changes in Body Mass with Age • Lean body mass increases throughout childhood • Changes in total fluid & ECF/ICF • Gender differences develop during adolescence • Greater increase in males • After 25, weight gain = fat gain • Lean mass decreases with increasing age • More so in women than men • Body water declines too 2009 Cengage-Wadsworth

  42. Protein Quality & Protein Intake • Foods can be categorized as: • High-quality or complete proteins • Low-quality or incomplete proteins • Evaluation of protein quality • Nitrogen balance/nitrogen status • Chemical or amino acid score • Protein digestibility corrected amino acid score 2009 Cengage-Wadsworth

  43. Protein Quality & Protein Intake • Protein efficiency ratio • Biological value • Net protein utilization • Net dietary protein calories percentage • Protein information on food labels • % Daily Value 2009 Cengage-Wadsworth

  44. Protein Quality & Protein Intake • Recommended protein & amino acid intakes • RDA for adults = 0.8 g/kg • AI for birth-6 months • RDA for indispensible AAs • Negative effects of high protein intakes controversial (no UL) • AMDR = 10%-35% kcal 2009 Cengage-Wadsworth

  45. Protein Quality & Protein Intake • Protein deficiency/malnutrition • Kwashiorkor • Adequate energy with insufficient protein • Edema owing to loss of blood proteins • Marasmus • Wasting, emaciation • Chronic insufficiency of energy & protein 2009 Cengage-Wadsworth

  46. Perspective 6 Protein Turnover:Starvation Compared with Stress 2009 Cengage-Wadsworth

  47. Starvation vs. Stress • Starvation • Protein synthesis decreases • Hormone balance adjusts • Adaptation - muscle catabolism slows • Stress • Hypermetabolism • Lipolysis doesn’t lead to ketosis • Muscle catabolism undiminished • Protein turnover - immune response & acute phase response 2009 Cengage-Wadsworth

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