Lecture 7 Review of CHO Digestion & Absorption Flashcards
What are the two major steps of CHO metabolism?
- intraluminal hydrolysis (amylases)
- Membrane digestion (brush border glycohydrases)
Basic functions that occur in intraluminal hydrolysis
- intial digestion - salivary 𝝰-amylase & continues with pancreatic 𝝰-amylases (endosaccharidases)
- Salivary 𝝰-amylase is deactivated by stomach acid
- Pancreatic juices in SI neutralize acid, 𝝰-amylase continues hydrolysis
Basic function of membrane digestion
Maltose, maltotriose (isomaltose), trisaccharides, oligosaccharides, and 𝝰-limit dextrins (ex. starches) require further breakdown
Basic steps of complex CHO digestion
- Intraluminal hydrolysis of poly- & oligosaccharides to oligosaccharides
- Brush border membrane digestion of oligosaccharides to tri-, di and monosaccharides (absorbed)
- Brush border membrane digestion of remaining disaccharides, and other disaccharides (sucrose, lactose, trehalose) to monosaccharides (absorbed)
Digestion of amylose and amylopectin in the mouth
𝝰-1,4 glycosidic bonds only breaking down linear structure but not branches
* Amylose: salivary glands release salivary 𝝰-amylase, which hydrolyzes 𝝰-1,4 glycosidic bonds in amylose, forming dextrins
* Amylopectin: salivary glands release salivary 𝝰-amylase, which hydrolyzes 𝝰-1,4 glycosidic bonds in amylose, forming dextrins
Digestion of amylose and amylopectin in the stomach
- Amylose: Acidity of gastric juice destroys the enzymatic activity of salivary 𝝰-amylase. The dextrins pass unchanged into the SI.
- Amylopectin: Acidity of gastric juice destroys the enzymatic activity of salivary 𝝰-amylase. The dextrins pass unchanged into the SI.
Digestion of amylose and amylopection in the SI
𝝰-1,4 glycosidic bonds only breaking down linear structure but not branches
* Amylose: The pancreas releases pancreatic 𝝰-amylase in the SI, which hydrolyzes 𝝰-1,4 glycosidic bonds. Dextrins are broken down into maltose.
* Amylopectin: The pancreas releases pancreatic 𝝰-amylase in the SI, which hydrolyzes 𝝰-1,4 glycosidic bonds. Dextrins are broken down into maltose and limit dextrins
Digestion of Amylose and Amylopection on the brush border of the SI
𝝰-1,4 glycosidic bonds only
* Amylose: Maltose is hydrolyzed by maltase, a brush border enzyme, forming free glucose.
* Amylopectin: Maltose is hydrolyzed by maltase, a brush border enzyme, forming free glucose. The 𝝰-1,6 glycosidic bonds in limit dextrins are hydrolyzed by 𝝰-dextrinase, forming glucose.
What are the products of amylose and amylopection after hydrolysis by 𝝰-amylase (salivary & pancreatic)?
- Maltotriose & maltoase result from amylose hydrolysis
- α-limit dextrins (or α-dextrins), along with maltose and maltotriose, are final products of amylopectin hydrolysis
amylase levels throughout growth
Both salivary and pancreatic amylase levels are low at birth but typically reach adult levels before 1 year of age.
What is the action of intestinal surface membrane oligosaccharides & disaccharides on a remaining 𝝰-dextrin from amylopectin?
- Remove glucose units from the nonreducing end (exoglucosidases) whereby Glc is readily absorbed since transporters are most concentrated near the enzyme complexes
Fructose
- typical source
- bonds
- brush-border membrane enzymes
- monosaccharide products
- typical source: fruit & honey
- bonds: non
- brush-border membrane enzymes: none
- monosaccharide products: fructose
Glucose
- typical source
- bonds
- brush-border membrane enzymes
- monosaccharide products
- typical source: fruit, honey, & grapes
- bonds: none
- brush-border membrane enzymes: none
- monosaccharide products: glucose
Amylopectin
- typical source
- bonds
- brush-border membrane enzymes
- monosaccharide products
- typical source: potatoes, rice, corn, & bread
- bonds: 𝝰-1,4 linear & 𝝰-1,6 branches
- brush-border membrane enzymes: maltase, glucoamylase, isomaltase
- monosaccharide products: glucose
Amylose
- typical source
- bonds
- brush-border membrane enzymes
- monosaccharide products
- typical source: potatoes, rice, corn, & bread
- bonds: 𝝰-1,4 linear
- brush-border membrane enzymes: maltase, glucoamylase
- monosaccharide products: glucose
Sucrose
- typical source
- bonds
- brush-border membrane enzymes
- monosaccharide products
- typical source: table sugar and dessert
- bonds: 𝝰-1,2
- brush-border membrane enzymes: sucrase
- monosaccharide products: glucose & fructose
non-reducing
Trehalose
- typical source
- bonds
- brush-border membrane enzymes
- monosaccharide products
- typical source: young mushrooms
- bonds: 𝝰-1,1
- brush-border membrane enzymes: trehalase
- monosaccharide products: Glucose
Lactose
- typical source
- bonds
- brush-border membrane enzymes
- monosaccharide products
- typical source: milk & milk products
- bonds: β-1,4
- brush-border membrane enzymes: lactase
- monosaccharide products: glucose and galactose
Lactose
- typical source
- bonds
- brush-border membrane enzymes
- monosaccharide products
- typical source: milk & milk products
- bonds: β-1,4
- brush-border membrane enzymes: lactase
- monosaccharide products: glucose and galactose
Maltose
- typical source
- bonds
- brush-border membrane enzymes
- monosaccharide products
- typical source: grains, sweet potato
- bonds: 𝝰-1,4 glucose
- brush-border membrane enzymes: maltase
- monosaccharide products: glucose
Galactose
- typical source
- bonds
- brush-border membrane enzymes
- monosaccharide products
- typical source: milk products as lactose
- bonds: none
- brush-border membrane enzymes: none
- monosaccharide products: galactose
Absorption of glucose and galactose
glucose and galactose are transported into enterocytes via energy requiring, active transport mechanisms
* transported from lower to higher concentration.
* cotransport with Na (renewed via Na-K ATPase)
Absorption of fructose
fructose is transported into enterocytes via facilitated diffusion
* transported from high to lower concentration
What monosaccharide has a higher glycemic index?
fructose because it is more readily absorbed since it takes less energy
Where do monosaccharides go once absorbed?
Hepatic portal vein transports the absorbed nutrients to the liver
* enter capillary from entercyte via transport pathways located in the apical membrane of the SI villous epithelial cells
Absorption of monosaccharides in low-sugar meal
hexoses enter enterocytes via SGLT1 and GLUT5, and then exit the cell via GLUT2 to enter bloodstream
Absorption of monosaccharides in sugar-rich meals
Excess monosaccharides saturates SGLT1 and GLUT5 which can result in the recruitment of GLUT2 into the apical membrane from its intracellular pool increasing the uptake by enterocytes to faciliate absorption. GLUT2 apical translocation is maximal within 10 min after consumption, then is internalized back and becomes similar to low-sugar meal.
What does the rate of CHO assimilation depend on?
- enzymatic hydrolysis at the brush border membrane - how quickly we can get it into monosaccharide form
- How quickly the monosaccharide can be transported
How CHO is presented dictates how well this occurs
What is the RLS of CHO digestion?
Transport across the epithelium
What happens to undigested lactose?
fermented by bacteria (hypolactasia)
* diagnosed by hydrogen breath test or stoll acidity test
* hydrolysis defect not absorption defect
* experience bloating, watery diarrhea and malasborption
