Day Date Title Instructor 5 th Ed 6 th Ed. Protein digestion and AA absorption

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1 Day Date Title Instructor 5 th Ed 6 th Ed 1 Tuesday 18 April 2017 Protein digestion and AA absorption D S Jairajpuri Wednesday 19 April 2017 Removal of nitrogen and urea cycle D S Jairajpuri Monday 24 April 2017 Catabolism of amino acids D S Jairajpuri Tuesday 25 April 2017 Synthesis of non-essential amino acids D S Jairajpuri Wednesday 26 April 2017 Porphyrin & bilirubin metabolism D S Jairajpuri May 2015 LABOR DAY 6 Tuesday 02 May 2017 Specialized products from amino acids D S Jairajpuri Wednesday 03 May 2017 Principle of nutrition D S Jairajpuri Monday 08 May 2017 Macronutrients D S Jairajpuri Tuesday 09 May 2017 Water-soluble vitamins D S Jairajpuri Wednesday 10 May 2017 Fat-soluble vitamins D S Jairajpuri

2 PROTEIN DIGESTION AND AMINO ACIDS ABSORPTION [LIPPINCOTT S ] Deeba S. Jairajpuri

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4 Two major enzymes systems are responsible for degrading proteins: ATP-dependent ubiquitin-proteosome system of the cytosol, which selectively degrade damage or short-lived proteins. ATP-independent degradative enzyme system of the lysosomes, which use acid hydrolases to nonselectively degrade intracellular proteins and extracellular proteins.

5 DIGESTION OF DIETARY PROTEINS Proteins are generally too large to be absorbed by the intestine. They must, therefore, be hydrolyzed to yield di- and tripeptides as well as individual amino acids, which can be absorbed. Proteolytic enzymes responsible for degrading proteins are produced by three different organs: the stomach, the pancreas, the small intestine.

6 DIGESTION OF PROTEINS BY GASTRIC SECRETION Hydrochloric acid: It is secreted by the parietal cells, Denatures proteins [making them more susceptible to subsequent hydrolysis by proteases]. Pepsin: It is acid-stable An endopeptidase It is secreted by the chief cells inactive proenzyme, pepsinogen which is activated to pepsin by: HCl by activated pepsin molecules Pepsin releases peptides and a few free amino acids from dietary proteins.

7 DIGESTION OF PROTEINS BY PANCREATIC ENZYMES Large polypeptides are cleaved to oligopeptides and amino acids by pancreatic proteases. Pancreatic proteases of each type have different specificity for the amino acid R-groups adjacent to the susceptible peptide bond. The release and activation of pancreatic zymogens is mediated by the secretion of cholecystokinin and secretin Enteropeptidase secreted by intestinal mucosal brush border cells activates the pancreatic trypsinogen to trypsin. Trypsin is the common activator of all the pancreatic zymogens.

8 DIGESTION OF OLIGOPEPTIDES BY ENZYMES OF THE SMALL INTESTINE The luminal surface of the intestine contains aminopeptidase, an exopeptidase that repeatedly cleaves the N-terminal residue from oligopeptides to produce even smaller peptides and free amino acids.

9 ABSORPTION OF AMINO ACIDS AND SMALL PEPTIDES Free amino acids are taken into the enterocytes by a sodium linked secondary transport system. Di- and tri -peptides, are taken up by a hydrogen linked transport system. The peptides are hydrolyzed in the cytosol to amino acids that are released into the portal system by facilitated diffusion. Free amino acids in the portal vein are: Metabolized by the liver and released into the general circulation. Branched-chain aminoacids are not metabolized by the liver and are sent from the liver to muscle via blood.

10 ABNORMALITIES IN PROTEIN DIGESTION Deficiency in pancreatic secretions can occur in: chronic pancreatitis or cystic fibrosis As a result fat and protein digestion and absorption are incomplete leading to abnormal appearance in the feces of lipids and undigested protein. Cystinuria: One transport system is responsible for the uptake of cystine and the dibasic amino acids, ornithine, arginine, and lysine. In the inherited disorder, cystinuria, this carrier system is defective. As a result all four amino acids appear in the urine. 1 in 7,000 individuals is affected(the most common genetic error of aminoacid transport). Clinically disease appear as kidney stones