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Digestion, Absorption and Transport of Carbohydrates

Lippincot Chapter 7 page 86-88 + 97OR

Marks Chapter 27 pages 502-514

Dietary Carbohydates

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Digestion of carbohydrates• Polysaccharides and disaccharides→

monosacchrides• Hydrolysis of glycosidic bonds• Catalyzed by glycosidases• Differ in specificity with regard to

type of sugar and type of glycosidic bondα- amylase (Salivary and pancratic)Disaccharidases

• Not all carbohydrates are digested

1’4’

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Salivary and Pancreatic amylase

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Disaccharidases of the intestinal brush border• Attached to the membrane of the brush border• Glycoproteins

• Glucoamylase• sucrase isomalats• β Glycosidase• Trehalase

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Glucoamylase(Oligosaccharidase)Heavily glycosylated

Similar structure to sucrase isomaltaseTwo domains with similar activities

Exoglucosidase (non reducing ends)Limit dextrins→ isomaltose

Glucoamylase Activity

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Sucrase Isomaltase complexStructure similar to glucoamylaseEach subunit has maltase activityIsomaltase-

maltase

Sucrase-maltase

β Glycosidase complex Large glycoprotein attached to the brush borderTwo catalytic sites

lactase

glucosylceramidaseGlycolipids

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Lactose Intolerance– Ingestion of lactose in food

↓

Pain, nausea, flatulence– Lactase level start to decrease starting from

the first month of age– Nonpersistant lactase (in most population)

Adult level (10% of infant level) at age 5-7

– Persistant lactase(in some populations)infant level remains throughout adulthood

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Absorption of Sugars

• Sugars are polar molecules. Bind to H2O by H bonds

• Transport proteins are needed.• Transport across membrane by

– active transport: Na+ dependent glucose– facilitated: Facilitative glucose transporter

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Na+ -dependant and facilitative transporters in the intestinal epithelial cells

Facilitative Transport of Glucose

From high to low concentration

Family of similar proteinsAll contain 12 membrane spanning domains

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Transporter Tissue Km Comments

GLUT 1 Most tissues Erythrocytes

1 mM In cells with barrier functions

GLUT 2 LiverPancreaticβ cells

15 mM Low affinity, high capacityGlucose sensor

GLUT 3 Most tissuesNeurons

1mM

GLUT 4 Adipose tissueSkeletal muscle

5mM Number increases with insulin

GLUT 5 Intestinal epitheliumspermatozoa

Mainly fructose transporter

Properties of the glucose transporters family

Stimulation of Glucose Transport into Muscle and Adipose Cells by Insulin

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Glucose transport through the capillary endothelium