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Home , Digestion

Digestion and Absorption of Macronutrients

Nutrition and FScN4621 Xiaoli Chen, Ph.D. Science and University of Minnesota

Study Questions

 Which 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

Intestinal lumen

Unstired water Layer

Brush Border

Basolateral

Caplillary

1 Digestion and Absorption Forms of Carbohydrate in Food • : - , , and

• : - , , and trehalose

• : - Naturally present in : -galactosides and β-fructans - Produced during food processing: maltodextrins (glucose/corn syrups)

• : - – a form of CHO stored in plant

• Sugar alcohols: - and mannitol

Structure of Starches

Amylose -1,4 linkages

Amylopectin -1,6 linkages

Carbohydrate Digestion

 Where does digestion of carbs occur? ◦ Mostly in the ◦ Some in the

 What enzymes are involved in digestion of carbs? ◦ What digests ?  ______produced by the salivary glands and the

◦ What enzymes digest oligosaccharides and disaccharides?  ______produced by of

, -dextrinase, glucoamylase, trehalase, -galactosidase,

2 Carbohydrate Digestion

 Starches Amylose (-1,4 linkages)

- 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 /transporters

Transport of

Secondary

Glucose Enterocyte

Galactose SGLT1

Na+ 2 Na+ Portal Blood GLUT2

Fructose GLUT5

Na+, K +-ATPase Facilitated 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 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 , di- or tripeptides

5 Where does protein digestion occur?

Step I in ______Stomach - of linkages in the protein

Step II in ______The lumen of the - digestion of polypeptides to smaller

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?

◦ ______are secreted by the chief cells in the stomach and are activated by HCl to form  Pancreas ◦ are secreted by the pancreas (______, ______,proelastase ______, chymotrypsinogen ______)procarboxypeptidases A and B  Small intestine ◦ ______Cholecystokinin released by the mucosal cells of small intestine stimulates of zymogens ◦ activates trypsinogen to , which activates the other zymogens  Enterocytes: ◦ Aminopeptidases ◦ ◦ 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 secreted in an inactive form?

6 What enzymes are involved in protein digestion?

Small Intestine

Enteropeptidase Trypsinogen Trypsin

HCO3

Proelastase Elastase Chymotrypsinogen Procarboxypeptidases 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 + H triacylglycerol Free fatty acids or non-esterified fatty acids

esterification

+ FFA

9 Lipid Digestion

 Where does lipid digestion occur? ◦ Digestion of  Begins in stomach (10-25%) by gastric  Small intestine (duodenum) by pancreatic lipase

◦ Gastric () 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

◦ (pancreas) and Phospholipase B (brush border): hydrolyze phospholipids (phosphatidylcholine) into 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 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

 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 - and VLDL

◦ chylomicrons and VLDL are triacylglycerol- rich

 Chylomicrons and VLDL leave the enterocyte via exocytosis, and enter the LYMPHATIC system

 The and the adipose 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 intestine

 Proteins: digestion continues inside the enterocyte by cytosolic aminopeptidases and carboxylpeptidases

 Lipids: digestion is complete in the lumen of the small intestine, intracellular transport, re- esterification

Transport into the Enterocyte

 Carbohydrates and proteins: secondary active transport and facilitated diffusion

 Lipids: simple diffusion via bile salt micelles

Transport Out of the Enterocyte

 Carbohydrates and proteins are transported via the portal vein

 Lipids are transported via the lymphatic system, with the exception of medium- and short-chain fatty acids

 Lipids are exported from enterocytes by lipoproteins

13 Assigned Reading

 Biochemical, Physiological, Molecular Aspects of Nutrition by Martha H, Stipanuk, 3nd edition

◦ Unit III: Digestion and Absorption of the Macronutrients ◦ Chapter 8: Digestion and absorption of Carb ◦ Chapter 9: Digestion and absorption of protein ◦ Chapter 10:Digestion and absorption of lipids

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