Digestion and Absorption of Protein and Carbohydrates PDF

Digestion and Absorption of Protein and Carbohydrates Module : GIHEp Class : MedYear2 semester 1 Lecturer : Paul O’Farrell Date : 10 September 2023 1 Learning objectives Describe t...

Digestion and Absorption of Protein and Carbohydrates Module : GIHEp Class : MedYear2 semester 1 Lecturer : Paul O’Farrell Date : 10 September 2023 1 Learning objectives Describe the sources of the digestive enzymes involved in the digestion of protein and carbohydrates in the body Discuss the enzymatic processes by which proteins and carbohydrate are digested Describe the products of protein and carbohydrate digestion Explain the basic mechanisms of absorption Describe the absorption of the products of protein and carbohydrate digestion in the body 2  Digestion and absorption Digestion : Polymeric nutrient molecules are broken down to monomers/small oligomers before absorption into the body proper Most digestion and absorption occurs in the small intestine Passive and active mechanisms of absorption Absorbed by enterocytes – absorptive cells lining the lumen Absorbed through enterocytes ie absorbed into enterocytes, then released from enterocytes into the body 3 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 4 Absorption of different materials happens throughout the GIT Smith et al 8.1 5 Surface area for absorption 6 Villus structure Guyton and Hall 11 ed. fig 65.6 7 Digestive enzymes FOOD TYPE ENZYME SOURCE PRODUCTS CARBOHYDRATES Salivary amylase Salivary glands Maltose Pancreatic amylase Pancreas Maltose Maltase Small intestine Glucose PROTEINS Pepsin Stomach mucosa Peptides Trypsin Pancreas Peptides Peptidases Intestinal mucosa Amino acids FATS Lipase Pancreas Fatty acids (bile salts) (Liver) and monoacylglycerol 8 Carbohydrates Major sources of energy in the diet Starch is major source– plant polymer of glucose monomers Amylose and Amylopectin Linear and branched polymers Section of amylopectin structure α-1,4 bonds along length of chain α-1,6 bonds at branch points Glycogen – animal sources: branched polymer of glucose, branch points closer together than in amylopectin As humans cannot cleave the β-1,4 bonds that join the glucose monomers in cellulose, this abundant plant polysaccharide is not useful as an energy source 9 Carbohydrates digestion Major enzymes are α-amylases secreted from salivary glands and pancreas. Break α-1,4 linkages. Endoglycosidase, strictly breaks bonds within polysaccharide chains Products: Branched oligosaccharides, dextrins, maltose, glucose (intestinal “isomaltase” - α-1,6 glycosidase) As humans cannot cleave the β-1,4 bonds that join the glucose monomers in cellulose, this abundant plant polysaccharide is not useful as an energy source 10 Carbohydrates digestion Digestion begins in the mouth with salivary amylase, pH optimum 6.5-7.0 Digestion continues in stomach within bolus, eventually low pH stops action Digestion continues in duodenum by pancreatic amylase 11 Brush border enzymes - embedded in microvilli of brush border - Further breakdown of starch and other sugars Isomaltase : isomaltose  glucose part of a bifunctional enzyme with sucrase activity also Glucoamylase : small oligosaccharides  glucose Exoglycosidase Part of as bifunctional enzyme with maltase activity also Maltase : maltose glucose Lactase : lactose  glucose and galactose Immunofluorescent staining for sucrase-isomaltase on intestinal villi Sucrase : sucrose  glucose and fructose Mellitzer et al. J Clin Invest. 2010; 120(5):1708–1721 12 Absorption of monosaccharides into enterocyte D-glucose, D-galactose, D-fructose Passive diffusion possible but slow – membrane relatively impermeable to polar molecules Hexose transporters Na+/glucose cotransporter (SGLT1) Na+ independent hexose transporters (GLUT5) 13 TRANSPORT OF MONOSACCHARIDES INTO AND OUT OF THE ENTEROCYTE 14  Proteins Polymers of amino acids joined together by peptide bonds Diverse in sequence Digested by endopeptidases and exopeptidases Provide: Fixed nitrogen Essential amino acids V, L, I, T, M, K, F, W, H*, R* 15 Proteins : digestion in stomach Low pH helps denature proteins – but not low/strong enough for (chemical) hydrolysis Pepsin: secreted by stomach in inactive form (zymogen) – pepsinogen, activated by acid conditions Pepsin is an endopeptidase; favours peptide bonds of aromatic residues Catalytic mechanism: aspartate protease Responsible for digestion of ~15% of dietary protein 16  Proteins: digestion in small intestine Pancreatic juice contains: 3 endopeptidases : Pancreas Trypsin C-terminal side of basic residues Chymotrypsin C-terminal side of aromatic residue Elastase small hydrophobic sidechains (Elastin) All are serine proteases 2 carboxypeptidases carboxypeptidase B – basic C-term residues carboxypeptidase A – branched, hydrophobic, proline, at C-terminus Both are metalloproteases (Zn2+) All produced as inactive zymogens Products are oligo-, tetra-, tri-peptides etc 17  Activation of pancreatic proteases Trypsinogen cleaved by brush border enteropeptidase (enterokinase) to give trypsin Trypsin acts other zymogens (including trypsinogen) to produce active enzymes Enteropeptidase is only available in the GIT, hence this process helps ensure that the pancreas is not damaged by digestive enzymes 18 Brush border enzymes Most abundant in jejunum Attached to membrane. Active sites face lumen - act on contact with chyme Also act within lumen (present on cells shed from villi) Oligopeptidases, dipeptidyl aminopeptidase; leucine aminopeptidase Further degrade products of pancreatic enzymes to absorbable forms Smith 8.10 19 Protein absorption - peptides Di- and tripeptides are absorbed by a co-transport mechanism Faster rate of absorption than for free amino acids H+ dependant co-transporter – gradient generated by Na+/H+ exchanger Internalized peptides hydrolysed by intracellular peptidases This is the major route for Smith 8.12 absorption of amino acids 20 Protein absorption – amino acids Passive diffusion – relatively hydrophobic amino acids can be absorbed to an appreciable extent Specific carrier systems exist in brush border and basolateral border 7 in brush border, 5 Na+- dependent co-transporters 3 in basolateral border – Na+- Smith 8.11 independent 21  More detail than you need to know Amino-acid carriers in enterocyte Smith Table 8.2  AAs with acidic sidechains (aspartate and glutamate) are used as energy substrates within the enterocyte and do not appear to be transported out of the cell by specific mechanisms 22 PROTEIN ABSORPTION SUMMARY Enterocytes Interstitial Fluid 23 Reading Medical Sciences by Jeannette Naish, Chapter 15 Other reading Smith & Morton – Systems of the Body “The Digestive System” esp. Ch. 5, 8. Lippencotts 4th ed – Ch 15; Ch 7 Meisenberg and Simmons 2nd ed Ch. 19 24