Protein Digestion & Amino Acid Absorption Lecture 2024 PDF

Protein Digestion & Amino acids Absorption Dr. Sarray Sameh Ph.D Department of Biochemistry PROTEIN DIGESTION  Proteins are generally too large to be absorbed by the intestine; therefore, proteins must be hydrolysed into di-or tripeptides or even free AA by proteolytic enzymes  Proteolytic enzymes...

Protein Digestion & Amino acids Absorption Dr. Sarray Sameh Ph.D Department of Biochemistry PROTEIN DIGESTION  Proteins are generally too large to be absorbed by the intestine; therefore, proteins must be hydrolysed into di-or tripeptides or even free AA by proteolytic enzymes  Proteolytic enzymes responsible for degrading proteins are produced by three different organs: the stomach, the pancreas, the small intestine. PROTEIN DIGESTION IN THE STOMACH  No digestion of protein takes place in the mouth ;  Protein digestion begins in the stomach which secrete gastric juice containing hydrochloric acid (HCl) and the proenzyme pepsinogen o Hydrochloric acid: gastric that help - Is secreted by the parietal cells, has HCL juice in stomach digest protein and Pepsinogen - reduces the pH of stomach to 1-2 and is become unfolded - a denaturating agent, unfolding globular proteins making proteins more accesible to protease activity. - It activates pepsinogen into pepsin, an acid stable endopeptidase which breaks the protein down into peptides of various lengths and some amino acids DIGESTION BY PANCREATIC ENZYMES duodenum  As the acidic stomach contents pass into the - secretin - cholecystokinin small intestine (duodenum); it triggers secretion of the secretin and cholecystokinin by duodenum into the blood.  Secretin stimulates the pancreas (duct cells) to secrete water and bicarbonate into the small intestine; they regulates gastric acid secretion and pH levels in the duodenum (abruptly increasing pH to about 7).  Secretin inhibits the secretion of gastrin, which triggers the initial release of hydrochloric acid into the stomach and delays gastric emptying.  Cholecystokinin : 2 functions  Trigger the release of main digestive enzymes by the pancreas: trypsinogen and chymotrypsinogen (PROENZYMES) and  Trigger the release of bile from gallbladder.  To convert trypsinogen into its active form trypsin, the duodenum secrete an enteropeptidase. In return trypsin, activates other digestive zymogens such as chymotrypsinogen, proelastase, procarboxypeptidase in the lumen of the gut. 2nd hormone DIGESTION BY PANCREATIC ENZYMES Pancreatic proteases have different substrate specificity with respect to peptide bond cleavage. Just know that the molecules are broken down to smaller peptides (Aromatic AA) (Hydrophobic AA) Carboxypeptidase A & B: Carboxy A before Ala, Ile, Leu, Val Carboxy B before Arg, Lys (basic AA) DIGESTION OF OLIGOPEPTIDES BY SMALL INTESTINE ENZYMES 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. aminopeptidase Cleaves (exopeptidase) N-terminal 1. smaller peptides 2 amino acids. to produce : ABSORPTION OF AMINO ACIDS - Free amino acids are taken into the enterocytes by a sodium linked secondary transport system. - Di- and tri -peptides, are taken up by a proton-coupled peptide transporters. The peptides are then hydrolyzed in the cytosol into free amino acids that are released into the portal vein by diffusion. Free amino acids in the portal vein are: - Metabolized by the liver and released into the general circulation. - Branched-chain amino-acids (BCAA) are not metabolized by the liver but they are sent from the liver to skeletal muscle (initial site of BCAA catabolism) and other extrahepatic tissues via blood. ABNORMALITIES IN PROTEIN DIGESTION Pancreatitis - - - Cystic fibrosis 3 proteins lipids Showing in are feces - Deficiency in pancreatic secretions can occur in chronic pancreatitis or cystic fibrosis; leading to incomplete digestion of protein and fats ; results in abnormal appearance of lipids (steatorrhea) and proteins in feces. absorption - Cystinuria, is an inherited disorder caused by a defect in the uptake system of dibasic amino acids (Cystine, Ornithine, Arginine, Lysine). As a result, all four amino acids appear in the urine. The disease is characterized by the precipitation of cystine to form kidney stones which can block the urinary tract. Hydration is an important part of treatment. - Hartunp’s disease: autosomal recessive rare disease, due to an impairment in the absorption and / or transport of tryptophan and other neutral amino acids (Ile, Leu, Val, Phe, Thr), from the intestines, renal tubes. A decrease of essential AA may cause a variety clinical disorders END!