Digestion of CarbohydratesBCH 340 Lecture 2

• Carbohydrates are called carbohydrates because they are essentially hydrates of carbon

(i.e. they are composed of carbon and water and have a composition of (CH2O)n.

• The major nutritional role of carbohydrates is to provide energy and digestible

carbohydrates provide 4 Kcal/g

• No single carbohydrate is essential, but carbohydrates do participate in many required

functions in the body.

• Monosaccharides

o Do not need hydrolysis before absorption

o Very little in most foods

• Di- and poly-saccharides

o Relatively large molecules

o Must be hydrolyzed prior to absorption

o Hydrolyzed to monosaccharides

Only monosaccharides can be absorbed

fructose glucose Galactose**

(fructose-glucose) (glucose-glucose) (glucose-galactose)

* Galactose does not occur in foods singly but only as part of lactose

Three types of monosaccharides…

…join together to make three types of disaccharides

maltose lactosesucrose

•Mouth:

o Digestion of CHO begins in the mouth

o During mastication, salivary alpha amylase:

Breaks starches down to maltose, dextrins,

isomaltose,

Optimum pH 6.7

Requires Cl- for its activity

Plays only a small role in breakdown because of the

short time food is in the mouth

o The chemical digestion of carbohydrates, which begins in

the oral cavity, is terminated due to a decrease in pH

CHO digestion stops in the stomach because the

high acidity inactivates salivary alpha amylase

Pancreas

o Further digestion by pancreatic enzymes occurs in the small intestine: when the acidic stomach contents reach the small intestine, they are neutralized by bicarbonate secreted by the pancreas

o At alkaline pH pancreatic alpha amylase continues the starch digestion:

Hydrolyzes alpha 1-4 linkages between glucose residues

Major importance in hydrolyzing starch and glycogen to maltose

Polysaccharides Disaccharides(maltose, isomaltose)

Amylase

Small intestine

o The final digestive processes occur at the small intestine

and include the action of several disaccharidases.

o Disaccharidases are secreted through and remain

associated with the brush border of the intestinal

mucosal cells.

Disaccharides MonosaccharidesBrush Border Enzymes

Maltose Glucose + GlucoseMaltase

LactoseLactase

Glucose + Galactose

Sucrose Glucose + FructoseSucrase

Isomaltose Glucose + GlucoseIsomaltase

Small intestine

Monosaccharides, the end product of CHO digestion, enter the capillaries of the intestinal villi

Monosaccharides travel the liver via the portal vein

In liver, galactose and fructose are converted to glucose

Distributed to tissue through circulation

Extra glucose is stored as glycogen in the liver and skeletal

muscles

Insulin is not required for the uptake of glucose by the

intestinal cells

o Absorption of glucose= 100% (taken as standard)

o Absorption of galactose = 110%

o Absorption of fructose = 43%

The maximal rate of glucose absorption from intestine is 120gm/hr

Transport of glucose into cells (Active transport) • energy-requiring process that transports glucose “against” a

concentration gradient. The energy is used by the sodium-potassium pump that requires the enzyme ATPase (Adenosine triphosphatase).

• carrier-mediated process in which the movement of glucose is coupled to the concentration gradient of Na+

• Na+, is transported into the cell at the same time. Called: cotransport

• The carrier is a sodium-dependent–glucose transporter or SGLT.

• Occurs in the epithelial cells of the intestine, renal tubules, and part of the blood brain barrier.

1- Insulin independent transport system:

o Not require insulin for glucose uptake

o Mediated through carrier protein

o Present in brain, RBCs, hepatocytes,

intestinal mucosa, renal tubules and cornea

2- Insulin-dependent transport system

o Require insulin: muscles and adipose tissue

o Insulin increases the number of glucose

transporters in tissues containing insulin

receptors

Glucose enter cells by two ways:

Passive transport (facilitated diffusion):Na+-independent facilitated diffusion transport

• This system is mediated by a family of 14 glucose transporters in cell membranes. They are designated GLUT-1 to GLUT-14

• These transporters exist in the membrane in two conformational states

• Extra cellular glucose binds to the transporter, which then alters its conformation, transporting glucose across the cell membrane.

• The glucose transporters display a tissue-specific pattern of expression. For example, GLUT-3 is the primary glucose transporter in neurons.

• Requires no energy since it goes with the gradient concentration (from high-outside the cell- to low)

• Glucose binds to receptor on carrier protein; Latter changes shape then releases solute on other side of membrane

Fructose and pentoses are absorbed by this

mechanism.

Glucose and galactose can also use the same

transporter if the concentration gradient is favorable.

Active transport is much more faster than passive

transport.

There is also sodium – independent transporter

(GLUT-2) that is facilitates transport of sugars out of

the cell i.e. to circulation.

Summary of types of functions of most important glucose transporters:

SiteFunction

Intestine and renal

tubules.

Absorption of glucose by

active transport (energy is

used by Na+- K+ pump)

SGLT-1

Intestine and spermFructose transport and to a

lesser extent glucose and

galactose.

GLUT -5

- Intestine and renal tubule

- β cells of pancreas-liver

Transport glucose out of

intestinal and renal

cells circulation

GLUT - 2

Monosaccharides (glucose, galactose and fructose) resulting from carbohydrate digestion are absorbed and undergo the following:

a- Uptake by tissues (liver):

After absorption the liver takes up sugars, where galactose and fructose are converted into glucose.

b- Glucose utilization by tissues:

Glucose may undergo one of the following fate:

Fate of absorbed sugars

1. Oxidation: through

Major pathways (glycolysis and Krebs' cycle)

for production of energy.

Hexose monophosphate pathway: for

production of ribose, deoxyribose and NADPH + H+

Uronic acid pathway: for production of glucuronic

acid, which is used in detoxification and enters in

the formation of mucopolysaccharide.

2. Storage:

o As glycogen (glycogenesis) in the liver and muscles

mainly.

o As TG (lipogenesis) in adipose tissues

3. Conversion: to substances of biological importance:

o Ribose, deoxyribose RNA and DNA.

o Amino sugars

o Non essential amino acids

o Fatty acids

o Fructose

o Glucuronic acid

o Galactose essential for formation of

lactose , glycolipids, mucopolysaccharides

4. Excretion of glucose in urine

o When blood glucose level exceeds certain

limit, it will pass to urine

o This will occur when blood glucose level is

above 180mg/dl and this is known as

glucosuria