Post on 17-Jan-2016
Digestion and absorption
Carbohydrates
DIETCarbohydrates
Dietary carbohydrates
• Largest source of calories – 40-45%
• Plant– starch ( grains, tubers, veggies )
sucroseglucose fructosedietary fiber
• Animal– glycogen
glycolipidslactose
Carbohydrates in our diet
α-D-glucose
OH
OH
OH
CH₂OH
OH
O OH
OH
OH
CH₂OH
OH
β-D-glucose
O
Carbohydrates in our diet
α-D-glucose
OH
OH
OH
CH₂OH
OH
OH
OH
OH
CH₂OH
OH
β-D-galactose
O O
Carbohydrates in our diet
β-D-fructose
OH
OH
CH₂OH
OHOCH₂
OH
α-D-glucose
OH
OH
OH
CH₂OH
OH
O
Carbohydrates in our diet
α-D-glucoseβ-D-glucose
OH
OH
CH₂OH
OH
OH
CH₂OH
O O
Carbohydrates in our diet
α-D-glucoseβ-D-galactose
O
OH
OH
CH₂OH
OH
OH
CH₂OH
O
Carbohydrates in our diet
β-D-fructose α-D-glucose
OH
CH₂OH
OHOCH₂
OH
O
OH
OH
CH₂OH
OH
Carbohydrates in our diet
α-D-glucose
O
OH
OH
CH₂OH
α-D-glucose
O
OH
OH
CH₂
Carbohydrates in our diet
O
O
O
β-D-fructose
α-D-glucose
Sucrose
So what’s our relationship?
Its called α-1—2 glycosidic bond
Carbohydrates in our diet
Lactose
O OO
So what’s our relationship?
Its called β-1—4 glycosidic bond
Carbohydrates in our diet
Maltose
OO
O
So what’s our relationship?
Its called α-1—4 glycosidic bond
Carbohydrates in our diet
Maltotriose
OO
O
O
O
So what’s our relationship?
Of course! It’s the α-1—4 glycosidic
linkages
Carbohydrates in our diet
Iso- maltose
O
O
O
So what’s our relationship?
Its called α-1—6 glycosidic bond
Carbohydrates in our diet
Trehalose
OO
O
So what’s our relationship?
Its called α-1—1 glycosidic bond ,
is’nt it cool?
Carbohydrates in our diet
Lactulose
OO
O
O OO
So what’s our relationship?
Its called β-1—2 glycosidic bond
I don’t understand!
Carbohydrates in our diet
OO
O
O
OO O
Amylose
We’ve got a strong
relashionship, the α-1—4 glycosidic
linkage
Carbohydrates in our diet
OO
O
O
OO O
Amylo-pectin
O
O
O
OWe are stronger, we
have got α-1—4 glycosidic linkages
as well α-1—6 branch points
Carbohydrates in our diet
Cellulose
O O
OO
O
O
OO
We are a team, we have got the β-1—4 glycosidic linkages!
DIGESTIONCarbohydrates
Digestion of carbohydrates
• Endoglycosidases / exoglycosidases– specific for sugar, type of bond, number of
saccharide units• Glycosidases– Enzymes that hydrolyze glycosidic bonds b/w
sugars
Mouth
• Salivary alpha amylase– Endoglycosidase• internal alpha 1,4 bonds• random intervals
– No activity against• Alpha 1,6 bonds• Little or no activity against Alpha 1,4 bond at non
reducing ends
Salivary α-amylase
O
O
OO
OO
OO
OO
OO
OO
OO
OO
OO
α-limit dextrins
Shortened polysaccharides
Stomach
• Salivary alpha amylase– Inactivated by gastric HCl
Small intestine
• Pancreatic alpha amylase• Bicarbonate• Intestinal disaccharidases
Pancreatic α-amylase
• Endoglycosidase– Continues digestion of starch and glycogen• Limit dextrins • Maltotriose • Maltose
• Duodenum O
OO
OO
OO
O
4-9 glycosyl residues and 1 or more 1,6 branches
Intestinal disaccharidases
• Glucoamylase• Sucrase isomaltase complex• Lactase glucosylceramidase/ β-glycosidase
complex • Trehalase
Glucoamylase
• Really an oligosaccharidase– Hydrolyses 1,4 bonds of dextrins– Exoglucosidase– Begins at nonreducing end and sequentially
cleaves glycosyl units– It cleaves dextrins down to isomaltose
• Highest activity in ileum
OO
OO
OO
OO
OO
O OO
OO
OO O
Sucrase-isomaltase complex
• Sucrase-maltase– Splits sucrose, maltose and maltotriose
• Isomaltase-maltase– Splits alpha 1,6 bonds in isomaltose and limit
dextrins– alpha 1,4 bonds in maltose and maltotriose
• Jejunum
Sucrase-isomaltase complex
O
O
O
Sucrose
Sucrase-isomaltase complex
Maltose
OO
O
Sucrase-isomaltase complex
Iso- maltose
O
O
O
Lactase glucosylceramidase/ β-glycosidase complex
• Glucosyl Ceramidase– Splits β-glycosidic bonds b/w glucose/galactose
and hydrophobic residues• Lactase– Splits β1,4 bonds b/w glucose and galactose
• Jejunum
β-glycosidase complex
O OO
Glycolipids
• Cerebrosides – Ceramide-monosaccharides
• Globosides – Ceramide-oligosaccharides
GLU CERAMIDE
GAL CERAMIDEGLU
Glycolipids
• Forssman antigen – Ceramide-oligosaccharides
GAL CERAMIDEGLUGALGALNAcGALNAc
Trehalase
• Splits α1,1 bond in trehalose• Found in mushrooms,insects and seafood
Trehalose
OO
O
ABSORPTION Carbohydrates
Absorption of monosaccharides
• Glucose • Fructose • Galactose
• Site – Duodenum– Upper jejunum
O
O
O
Hi!
GLUT 5
GLUT 2
3Na+
2K+
O
Na+
O
Na+
Na+
Na+
Na+
Na+
O
SGLT-1
O
ONa+
O
GLUT 5
GLUT 2
O
Na+
O
Na+
Na+
Na+
Na+
Na+
O
SGLT-1O
Na+
O
GLUT 2O
INDIGESTIBLE Carbohydrates
Indigestible carbohydrates
• Cellulose • Hemicelluloses • Gums • Mucilages • Pectin • Raffinose • Lignin
Colon
• Dietary fibre and nondigested carbohydrates– Bacterial action• Gases
– H2, CO2, methane
• Lactate• Short chain FA
– Acetic acid– Propionic acid– Butyric acid
• What is the cause of excessive flatulence after having a meal containing beans?
DISORDERS Carbohydrates
Abnormal degradation of disaccharides
• Disorders – Lactose intolerance– Disaccharide intolerance– Isomaltase sucrase deficiency– Defect in absorption of fructose
• Symptoms – Osmotic diarrhea – Bacterial fermentation • 2 and 3 carbon compounds • Gases
Lactose intolerance
• Etiology – Deficiency of enzyme lactase
• Clinical features – Abdominal cramps– GI bloating– Intermittent diarrhea– 45min-1hr after eating dairy products
Lactose intolerance
• Treatment – Reduce consumption of dairy products– Consume • Green veggies• Yogurts• Cheese • Lactase treated products
– Lactase pills
• Yogurt is a dairy product, why is it recommended in lactose intolerance?
Disaccharide intolerance
• Etiology – Hereditary deficiency– Injured Mucosa• Disease, drugs, malnutrition
• Clinical features – Osmotic diarrhea – Abdominal cramps– Flatulence
Sucrase-isomaltase complex deficiency
• intolerance of ingested sucrose
Defect in absorption of fructose
• Clinical features– Gas and distented abdomen after eating fruit,
sweets or juices
DIAGNOSISCarbohydrate disorders
Disorders of carbohydrate digestion and absorption
• Diagnosis – Oral tolerance tests– H2 breath test
TREATMENT Carbohydrate disorders
Treatment
• Avoid foods containing specific disaccharide• Tablets and capsules containing enzymes