Digestion and Absorption of Proteins & Carbohydrates PDF

RCSI Royal College of Surgeons in Ireland Coláiste Ríoga na Máinleá in Éirinn Digestion and Absorption of Proteins and Carbohydrates Class DEM Y2 Course Gastrointestinal Biology Lectu...

RCSI Royal College of Surgeons in Ireland Coláiste Ríoga na Máinleá in Éirinn Digestion and Absorption of Proteins and Carbohydrates Class DEM Y2 Course Gastrointestinal Biology Lecturer Dr. Sarah O’Neill ([email protected]) Date September 12th 2023 LEARNING OUTCOMES At the end of this lecture you will be able to: 1. Describe the sources of the digestive enzymes involved in the digestion of proteins and carbohydrates in the body 2. Discuss the catalytic mechanisms of protein and carbohydrate digestive enzymes 3. Describe the products of protein and carbohydrate digestion 4. Describe the basic mechanisms of absorption 5. Explain the absorption of the products of protein and carbohydrate digestion in the body DIGESTION Complex dietary molecules (Protein, Fat, Carbohydrates) Hydrolytic cleavage via Digestive Enzymes Small molecules PROTEIN DIGESTION OVERVIEW Principles sites: Stomach Small intestine Proteolytic enzymes Secreted into the stomach Released from the pancreas into the small intestine PROTEIN DIGESTION IN THE STOMACH Pepsin Enzymes are proteases pH = 2.0 Released by chief cells in the form of pepsinogen Pepsinogen HCl Pepsin Pepsinogen Pepsin – Endopeptidase – Cleaves at amino side of hydrophobic aa PROTEIN DIGESTION IN THE SMALL INTESTINE Enzymes released by pancreas as zymogens Trypsinogen Enteropeptidase Trypsin Trypsin Chymotrypsinogen Chymotrypsin Trypsin Proelastase Elastase Trypsin Procarboxypeptidase Carboxypeptidase PANCREATIC ENZYME TYPE AND CATALYTIC MECHANISM Trypsin Type: Endopeptidase Chymotrypsin Mechanism: Serine protease Elastase Carboxypeptidase A Type: Carboxypeptidase Mechanism: Metalloprotease Carboxypeptidase B (Zn2+) CLEAVAGE SPECIFICITY Trypsin – Peptide bonds adjacent to basic amino acids Chymotrypsin – Peptide bonds adjacent to hydrophobic amino acids Elastase – Peptide bonds adjacent to small amino acids Carboxypeptidase A – Hydrophobic amino acids at C-terminus Carboxypeptidase B – Basic amino acids at C-terminus Refer back to lecture on Proteins in FFP1 module CLEAVAGE SPECIFICITY PRODUCTS OF PROTEIN DIGESTION Tetra-peptides Cleaved by peptidases Tri-peptides Amino acids PROTEIN DIGESTION SUMMARY Site of digestion: – Stomach Products generated: – Amino acids – Small intestine – Di-peptides – Tri-peptides – Tetra-peptides Enzymes: – Pepsin – Trypsin – Chymotrypsin – Elastase – Carboxypeptidase A and B CARBOHYDRATE DIGESTION SITES The mouth: this is where carbohydrate digestion starts The small intestine No carbohydrate digestion takes place in the stomach CARBOHYDRATE DIGESTION IN THE MOUTH Sources of carbohydrates in our diet: starch, lactose, sucrose, trehalose, cellulose Mechanical digestion: chewing Chemical digestion: – Salivary glands in the oral cavity secrete saliva – Saliva contains the enzyme salivary amylase – Salivary amylase hydrolyses only starch into maltose and dextrin – pH in mouth is ~ 6.8 – 7.0 – Only about 5% of starch is broken down in the mouth  SMALL INTESTINE CARBOHYDRATE DIGESTIVE ENZYMES Pancreatic enzyme: a-amylase Brush border enzymes: disaccharidases and oligsaccharidases  THE ACTION OF PANCREATIC ALPHA AMYLASE = glucose Pancreatic a-amylase is an endoglycosidase: does not act on terminal glycosidic bonds It is specific for a1,4 linkages and does not act on a1,6 linkages The hydrolytic products are maltose, maltotriose and a-limit dextrins THE ACTION OF BRUSH BORDER CARBOHYDRATE ENZYMES Maltase Maltose and maltotriose Glucose + glucose Sucrase Sucrose Glucose + fructose Lactase Lactose Galactose + glucose Trehalase Trehalose Glucose + glucose CARBOHYDRATE DIGESTION ENZYMES Enzyme Type of enzyme Substrate ⍺-amylase Salivary and pancreatic* Polysaccharides Disaccharidase Intestinal Disaccharides Isomaltase Branch points of oligo- Intestinal ⍺ (1,6) glucosidase and di-saccharides *Most of the digestion is due to pancreatic ⍺-amylase because food does not stay in the mouth for a long time. CARBOHYDRATE DIGESTION SUMMARY Site of digestion: – Mouth – Intestinal brush border Products generated: – Maltose – Maltotriose – a-limit dextrins Enzymes: – Glucose – (Gluco)amylase – Galactose – Isomaltase – – Fructose Sucrase – Lactase – Trehalase INTRODUCTION TO MECHANISMS OF ABSORPTION Transportation of the products of digestion and other small dissolved molecules, with ions and water across the epithelial cell membranes, mainly in the small intestine THE UNIT OF ABSORPTION The villus Length varies between 0.5 and 1.5mm Covered by simple columnar epithelium These have cytoplasmic extensions at the lumen: brush border EPITHELIAL CELLS IN INTESTINE: ENTEROCYTE SOME ENZYMES ARE ANCHORED TO THE MICROVILLI Brush border extends to 200 m2 Catalytic domains of these enzymes protrude into the intestine – Aminopeptidase – Endopeptidase 200m2 = approximately 1/2 of a FIBA basketball court – Carboxypeptidase – Dipeptidase – Disaccharidases – Oligsaccharidases ABSORPTION MECHANISMS Passive diffusion: – Down a concentration gradient Facilitated diffusion: – Uses membrane proteins in the enterocyte Active transport: – Energy dependent membrane proteins in the enterocyte REVIEW OF CO-TRANSPORT ACROSS THE MEMBRANE Click here for the link if above is not working PROTEIN DIGESTION AND ABSORPTION Source: http://www.austincc.edu/apreview/PhysText/Digestive.html PEPTIDE ABSORPTION: ENTRY INTO THE ENTEROCYTE Form in which the majority of protein is absorbed More rapid than absorption of free amino acids Coupled to H+ gradient Na+/H+ exchange maintains gradient Metabolised into free amino acids in enterocyte Only free amino acids absorbed into blood TRANSPORT ACROSS THE BASOLATERAL MEMBRANE Released into interstitial fluid by facilitated diffusion and co-transport Transported to the liver via the hepatic portal vein Glutamate and aspartate are utilised as energy in the enterocyte PROTEIN ABSORPTION SUMMARY Enterocytes Interstitial Fluid ABSORPTION OF MONOSACCHARIDES: ENTRY INTO ENTEROCYTE By passive diffusion (very slow) By membrane-associated transporters: – Na+/glucose co-transporter (SGLT1) – Na+ independent transporter (GLUT5) TRANSPORT OF MONOSACCHARIDES INTO AND OUT OF THE ENTEROCYTE  REFERENCES Medical Sciences by Jeannette Naish, Chapter 15