8/9/2019 Chok Biochem 2nd Shift Reviewer Carbohydrate Digestion

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DIGESTION, ABSORPTION AND TRANSPORT OF CARBOHYDRATES

Carbohydrates / Carbs / CHO = 60-70% of dietSources:

• Starch - polysaccharide

• Cellulose – dietary fiber 

• Disaccharides – sucrose (fructose (F) + glucose(G)),

lactose

• Monosaccharides – fructose, glucose

Sugar  Maltose Maltotriose Isomaltose Limit Dextrin Trehalose

Linkage 2G α1,4 3G α1,4 2G α1,6 3-8G α1,4 & α1,6 branching 2G α1,1

I. DIGESTION

• Glycosidases – cleave α / β linkages between sugar units via hydrolysis

• Specific for type of sugar , linkage type (α or β), and # of units

Phase Cleaves Produces Location Notes

ORAL PHASEmaltose, maltotriose, dextrin, glucose(by random endoglycosidic αamylase)

mouthSaliva = 1L/day; mucin (dispersespolysacchs) + α amylase; Ca&Cl cofactors,optimum pH = 6.6-6.8 (mouth pH = 6)

GASTRIC PHASE Minimal acid hydrolysis stomachstomach pH = 2 (very acidic) = α amylasedenatured

LUMINAL I(Pancreatic Exocrine)

maltose, isomaltose, limit dextrins,glucose

duodenum bicarbs reduce acidity, contains α amylase

   L   U   M   I   N   A   L   I

   I

   (   i  n 

   I  n   t .

   b  r  u  s   h 

   b  o  r   d  e  r   )

   G   L   Y   C   O   S   I   D   I   C

   C   O   M   P   L   E   X   E   S

Sucrase-Isomaltase

sucrose, isomaltose, maltase(80% total digestion), maltotriose

glucose,fructose

 jejunum

Glucoamylase

oligosaccharides (dextrinase),maltase (20% total digestion)

glucose ileumexoglycosidases (cleaves non-reducingunits)

Lactase/β-galactosidase

lactoseglucose,galactose

 jejunum27-32 wks = ↑ activity,>7 yrs = ↓ activity

Trehalasetrehalose (mushrooms, fungi, insects;2G α1,1)

glucose

• DIETARY FIBERS

• vegetables, fruits as cellulose, hemicellulose, pectins, mucilages, gums, lignin

• not enzymatically digested, no β1,4 glycosidase

• intestinal bacteria – metabolized in large intestine = CO2, methane (CH4), H2O ≈ flatulence

• bacterial fermentation acetate, proprionate, butyrate

II. ABSORPTION and TRANSPORT

A. Simple Passive Diffusion – pentoses (xylose, arabinose), very little for glucose

B. Facilitated Diffusion – fructose, mannose, glucose; forms intermediate complex with specific carrier protein

C. Active Transport – glucose and galactose; coupled with Na-K ATPase pump

D. Inhibitors

1. ouabain – cardiac glycoside, inhibits Na-K ATPase pump

2. phlorizin – plant glycoside, blocks luminal transport

3. cytochalazin – inhibits serosal glucose carrier at contraluminal (GLUT2) side

E. Glucose Transporters

• Insulin dependent – skeletal muscle, adipose tissue

• Insulin independent – intestinal mucosa, liver, RBC, brain

GLUT Mechanism Function Location

1 & 3 Na-glucose symport Basal glucose uptake (Km = 1mM) All tissues, intestinal lumen

5 With Na-glucose transport intestinal lumen

2↑ glucose Km (15-20mM), allows entry to liver at high glucose

levels only,

  pancreas adjusts insulin secretion

liver and pancreas,

intestinal serosa4 ↑ transporters due to insulin Muscle and adipose

III. DISEASES

A. β-galactosidase (lactase) deficiency – lactose intolerance, different from lactoglobulin intolerance (milk protein)

B. Sucrase (Sucrase-Isomaltase Complex) deficiency – inherited lack of sucrase and isomaltase, chronic diarrhea, abdominal

pain, low stool pH (lactic acid)

C. Disacchariduria - ↑ urinary excretion of disaccharides ≈ 300 mg

D. Monosaccharide malabsorption – autosomal recessive, congenital, defect in glucose/galactose carrier,

severe diarrhea, abdominal distention

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