Digestion and absorption

Dr Jariya Wajahat Biochemistry of gastrointestinal tract

Digestion and absorption

Carbohydrates


Whenever Zachariah went into the sanctuary where she was, he found that she had food. He said: O Mary! Whence cometh unto thee this? She answered: it is from Allah. Allah gives without stint to whom he will.

37 the family of ImranAl-Qur’an


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



α-D-glucose

OH

OH

OH

CH₂OH

OH

OH

OH

OH

CH₂OH

OH

β-D-galactose

O O



β-D-fructose

OH

OH

CH₂OH

OHOCH₂

OH

α-D-glucose

OH

OH

OH

CH₂OH

OH

O



α-D-glucoseβ-D-glucose

OH

OH

CH₂OH

OH

OH

CH₂OH

O O



α-D-glucoseβ-D-galactose

O

OH

OH

CH₂OH

OH

OH

CH₂OH

O



β-D-fructose α-D-glucose

OH

CH₂OH

OHOCH₂

OH

O

OH

OH

CH₂OH

OH



α-D-glucose

O

OH

OH

CH₂OH

α-D-glucose

O

OH

OH

CH₂



O

O

O

β-D-fructose

α-D-glucose

Sucrose

So what’s our relationship?

Its called α-1—2 glycosidic bond



Lactose

O OO


Its called β-1—4 glycosidic bond



Maltose

OO

O





Maltotriose

OO

O

O

O


Of course! It’s the α-1—4 glycosidic

linkages



Iso- maltose

O

O

O





Trehalose

OO

O


Its called α-1—1 glycosidic bond ,

is’nt it cool?



Lactulose

OO

O

O OO


Its called β-1—2 glycosidic bond

I don’t understand!



OO

O

O

OO O

Amylose

We’ve got a strong

relashionship, the α-1—4 glycosidic

linkage



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



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



O

O

O

Sucrose



Maltose

OO

O



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

Digestion and absorption

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