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1 Digestion and Absorption of Macronutrients Digestion and Absorption of Macronutrients Nutrition and Metabolism FScN4621 Xiaoli Chen, Ph.D. Food Science and Nutrition University of Minnesota Study Questions Which enzymes are involved in digestion? Where are they produced/excreted? Where in the GI tract does digestion occur? What are the starting nutrients/components, intermediates, and the final products of digestion? How and where are the final products of digestion absorbed? How are they transported, and to where? Structure of Enterocyte Intestinal lumen Unstired water Layer Brush Border Basolateral cell membrane Caplillary 1 Carbohydrate Digestion and Absorption Forms of Carbohydrate in Food • Monosaccharides: - glucose, fructose, and galactose • Disaccharides: - sucrose, lactose, maltose and trehalose • Oligosaccharides: - Naturally present in foods: -galactosides and β-fructans - Produced during food processing: maltodextrins (glucose/corn syrups) • Polysaccharides: - starches – a form of CHO stored in plant • Sugar alcohols: - sorbitol and mannitol Structure of Starches Amylose -1,4 linkages Amylopectin -1,6 linkages Carbohydrate Digestion Where does digestion of carbs occur? ◦ Mostly in the duodenum ◦ Some in the mouth What enzymes are involved in digestion of carbs? ◦ What enzyme digests starch? ___________produced by the salivary glands and the pancreas ◦ What enzymes digest oligosaccharides and disaccharides? _______________ produced by of enterocytes sucrase, -dextrinase, glucoamylase, trehalase, -galactosidase, lactase 2 Carbohydrate Digestion Starches Amylose (-1,4 linkages) -amylase maltose maltotriose Carbohydrate Digestion Starches Amylopectin (-1,6 linkages) -amylase maltose maltotriose -dextrin Carbohydrate Digestion Starches -dextrin -dextrinase maltose Glucose maltotriose maltase- glucoamylase Glucose 3 Carbohydrate Absorption What are the final products of digestion? ◦ glucose ◦ fructose ◦ galactose How final digested products are absorbed? ◦ Transport of final digested products through entrocytes requires membrane carrier proteins/transporters Transport of Carbohydrates Secondary Active Transport Glucose Enterocyte Galactose SGLT1 Na+ 2 Na+ Portal Blood GLUT2 Fructose GLUT5 Na+, K +-ATPase Facilitated Diffusion Na+ Carbohydrate Absorption Transport of final digested products ◦ glucose and galactose are carried into the enterocyte by secondary active transport via _______ ◦ Fructose is transported by facilitated diffusion via ________ ◦ Glucose, galactose and fructose are carried out of the enterocyte into the portal vein by the protein carrier _______ via facilitated diffusion 4 Questions CHO Digestion and Absorption Which enzymes are involved in digestion? Where are they produced/excreted? Where in the GI tract does digestion occur? What are the starting nutrients/components, intermediates, and the final products of digestion? How and where are the final products of digestion absorbed? How are they transported, and to where? Protein Digestion and Absorption Polypeptide chain Peptide bonds Protein Digestion What are starting molecules? ◦ ____________ What are the final products of digestion? ◦ __________, _________, __________, ____________ ◦ Oligopeptides in the _______lumen are further broken down by __________________________ and/or _________________to produce free amino acids, di- or tripeptides 5 Where does protein digestion occur? Step I in _______Stomach - hydrolysis of peptide linkages in the protein Step II in _______________________The lumen of the small intestine - digestion of polypeptides to smaller peptides Step III in _____________________________brush-border membrane of enterocytes - digestion of oligopeptides Step IV in ______________the enterocytes - digestion of di- and tripeptides Step V Transport of animo acids to enterocytes, venous capillaries and portal blood What enzymes are involved in protein digestion? Stomach ◦ ____________are secreted by the chief cells in the stomach and are activated by HCl to form pepsin Pancreas ◦ Zymogens are secreted by the pancreas (___________,trypsinogen ___________,proelastase ________________, chymotrypsinogen _____________________________)procarboxypeptidases A and B Small intestine ◦ ________________Cholecystokinin released by the mucosal cells of small intestine stimulates secretion of zymogens ◦ Enteropeptidase activates trypsinogen to trypsin, which activates the other zymogens Enterocytes: ◦ Aminopeptidases ◦ Carboxypeptidase ◦ present in the brush border and in the cytosol What enzymes are involved in protein digestion? Pancreas Zymogens Trypsinogen inactive Proelastase Pancreas Chymotrypsinogen Procarboxypeptidases A and B HCO3 Secreted into Small Intestine Why is zymogen secreted in an inactive form? 6 What enzymes are involved in protein digestion? Small Intestine Enteropeptidase Trypsinogen Trypsin HCO3 Proelastase Elastase Chymotrypsinogen Chymotrypsin Procarboxypeptidases Carboxypeptidases Inactive Form Active Form What is the role of trypsin? What is the role of HCO3? Protein Absorption Transport of Free Amino Acids Na+-dependent transport systems: ◦ Ion-coupled transport – Na+ ◦ Electrochemical potential generated from ion gradients energizes transport activity System B: neutral amino acids (brush border) ASC: alanine, serine, threonine, cysteine, glutamine (basolateral) IMINO: proline and imino acids (brush border) - XAG : aspartate and glutamate Protein Absorption Transport of Free Amino Acids Na+-independent transport systems: ◦ System L: large hydrophobic or branched amino acids leucine, isoleucine, valine, phenylalanine, tyrosine (basolateral) ◦ y+L: arginine, lysine (basolateral) ◦ Xc-: cystine/glutamate exchange 7 Protein Absorption Transport of Di- and Tripeptides Pept1 transporter ◦ It is H+-activated carrier protein on the apical membrane ◦ It is driven by proton (H+) electrochemical potentials Once inside the enterocyte, these Di- and Tripeptides are hydrolyzed by cytosolic aminopeptidases and carboxypeptidase into free amino acids Protein Absorption Fate of Free Amino Acids Free aa exit the enterocyte via the basolateral membrane into the portal vein Some aa, such as glutamine, glutamate, and aspartate are further metabolized by the enterocyte Absorption is greatest in _________the jejunum Questions Protein Digestion and Absorption Which enzymes are involved in digestion? Where are they produced/excreted? Where in the GI tract does digestion occur? What are the starting nutrients/components, intermediates, and the final products of digestion? How and where are the final products of digestion absorbed? How are they transported, and to where? 8 Lipid Digestion and Absorption Dietary lipid in the great quantity is ___________ Cholesterol/cholesterol esters Phospholipids position sn-1 position sn-2 position sn-3 Triacylglycerols sn: stereospecific numbering (sn) system Structure of Lipids R triacylglycerol phosphatidylcholine Free and Esterified Lipids Esterified Lipids Free Lipids esterification H H + glycerol H triacylglycerol Free fatty acids or non-esterified fatty acids esterification + FFA 9 Lipid Digestion Where does lipid digestion occur? ◦ Digestion of triglycerides Begins in stomach (10-25%) by gastric lipase Small intestine (duodenum) by pancreatic lipase ◦ Gastric (acid) lipase preferentially cleaves sn-3 positions of TAGs, yielding 1,2-diacylglycerols and fatty acids ◦ Pancreatic lipase cleaves sn-1 and sn-3 positions, yielding 2-monoacylglycerol and free fatty acids. Lipid Digestion Digestion of phospholipids and cholesterol ◦ Small intestine ◦ Phospholipase A2 (pancreas) and Phospholipase B (brush border): hydrolyze phospholipids (phosphatidylcholine) into fatty acid and lysophosphatidylcholine ◦ Cholesterol esterase (pancreas): hydrolyzes cholesteryl esters to free cholesterol, but also TAGs, phosphoglycerides, esters of vit. A and D, and monoglycerols Lipid Absorption Transport to Enterocyte Intestinal lumen Unstirred water layer Enterocyte 1 2 Bile salt Bile salt micelles or vesicles Lipid digestion products:__________________________________MAG, FFA, lysophosphatidylcholine & Cholesterol 10 Lipid Absorption Transport to Enterocyte Lipids are transported to the enterocyte via micelles and liposomes The concentration gradient facilitates simple diffusion from the brush-border membrane into the enterocyts In the enterocyte, re-esterification occurs immediately to keep the intracellular concentration low and maintain the gradient Intracellular Transport of Absorbed Lipids in the Enterocyte Fatty acid-binding proteins (FABPs): facilitate fatty acids transport to the ER Sterol carrier protein 1 &2 (SCP-1 & SCP-2): intracellular transport of cholesterol to the ER Re-esterification in the ER in Enterocyte 2-monoacylglycerols and fatty acids are reconstituted to form TAGs Lysophosphatidylcholine is reacylated to form phosphatidylcholine, or hydrolyzed to form 3- phosphorylcholine (transported via portal vein) Cholesterol is esterified to cholesterol ester by cholesterol esterase and acyl CoA: cholesterol acyltransferase 11 Export of Lipids from the Enterocytes Export of lipids from the enterocyte is mediated by LIPOPROTEINS - chylomicrons and VLDL ◦ chylomicrons and VLDL are triacylglycerol- rich Chylomicrons and VLDL leave the enterocyte via exocytosis, and enter the LYMPHATIC system The liver and the adipose tissue are the final destination. Differences in Digestion and Absorption of the Macronutrients 12 Digestion Carbohydrates and lipids: digestion occurs primarily in the lumen of the small
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