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28 AAs - Digestion-2 - Amino Acids and Proteins Digestion...

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Amino Acids and Proteins: Digestion HNFE 3025 Fall 2009
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Proteins must be broken down into their individual constituent amino acids (or to small di- and tripeptides) to be absorbed. Exogenous sources Animal products - except fats Plant products - grains, legumes, vegetables Endogenous proteins Desquamated mucosal cells Digestive enzymes and glycoproteins Protein Sources 2
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Protein digestion = Proteolysis : hydrolysis of the peptide bonds of a protein Partial hydrolysis yields various polypeptides Complete hydrolysis yields (free) amino acids (AAs) Protein Digestion 3 H 3 N + – CH – C R1 O N – CH – COO - R2 H Peptide bond that is hydrolyzed – insertion of H O A H 3 N + – CH – C R1 O O - N + – CH – COO - R2 H H H +
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Zymogen = Proenzyme Inactive precursor form of a digestive enzyme that is later activated by cleavage and other modifications. Storage and release of zymogens that are later activated protects against inappropriate protein hydrolysis. Peptidase = Protease = Proteinase Enzyme that hydrolyzes peptide bonds Varied specificity and different methods of function Digestion Definitions 4
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2 basic categories based on whether cleavage is at an internal location or on the end of a polypeptide Endopeptidases cleave internal peptide bonds Exopeptidases cleave external peptide bonds from N- or C-terminus Either will be specific for the side of the AA it will cleave Types of Peptidases 5 Met - Lys - Met - Val - Glu - Phe - Cys - Leu - His- Ile - Asn (C-side) Endopeptidase N-Terminus C-Terminus (C-side) Exopeptidase (N-side) Exopeptidase
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Digestion begins in the stomach with gastric acid secretion A complex process based on 3 major inputs Neural: vagus nerve releases acetylcholine (Ach), in response to anticipation, sight, smell and taste of food as well as stomach distention Endocrine: gastrin release based on multiple factors Paracrine: histamine released and acts on H 2 receptors Protein Digestion 6
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Gastric Secretions Acidic gastric juice (pH ~1-2) due to HCl (secreted by parietal cells) Key in acid secretion is a H + /K + ATPase or "proton pump” Gastric juice also contains pepsinogen (secreted by chief cells) Pepsinogen is activated by cleavage of short peptides from the N-terminus to form pepsin. Aided by low pH and autoactivation.
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