Download digestion and absorption of carbohydrate

Digestion and absorption of
carbohydrate
Lecture no 1
Carbohydrate metabolism
We eat, we digest, we absorb, then what?
Three fates for nutrients
1) Most are used to supply energy for life
2) Some are used to synthesize structural or functional molecules
3) The rest are stored for future use
Carbohydrates in the food
• Monosaccharides
– Glucose
• Found in fruits, vegetables, honey
• “blood sugar” – used for energy
– Fructose
• “fruit sugar”
• Found in fruits, honey
– Galactose
• Found as part of lactose in milk
• Disaccharides
• Disaccharides – two linked sugar units
– Sucrose: glucose + fructose
• “table sugar”
• Made from sugar cane and sugar beets
– Lactose: glucose + galactose
• “milk sugar”
• Found in milk and dairy products
– Maltose: glucose + glucose
• Found in cereal grains
• Product of starch breakdown
fructose
sucrose
(fructose-glucose)
glucose
maltose
(glucose-glucose)
galactose
lactose
(glucosegalactose)
Galactose does not occur in foods singly but only as part of lactose.
Fig. 4-2a, p. 101
• Complex Carbohydrates
• Starch
– Long chains of glucose units
• Amylose – straight chains
• Amylopectin – branched chains
– Found in grains, vegetables, legumes
• Glycogen
-- Highly branched chains of glucose units
– Body’s storage form of carbohydrate
– Made by animals
• in muscle and liver
Amylopectin
• Dietary Fiber
– Indigestible chains of monosaccharides
– Found in fruits, vegetables, grains, ……
Carbohydrate Digestion and Absorption
• Mouth
– Salivary amylase begins digestion of starch into maltose
• Small intestine
– Pancreatic amylase completes starch digestion
– Brush border enzymes digest the disaccharides
maltase, sucrase, lactase
• End products of carbohydrate digestion
– Glucose, fructose, galactose
– Absorbed into bloodstream
• Fiber not digested, excreted in feces
Carbohydrate Metabolism
• General
– 80% of carbohydrates ingested contain glucose; remainder: fructose,
galactose, ….
– glucose is the body's preferred carbohydrate energy source
• Metabolism of carbohydrates
– Glycolysis
– Citric acid cycle
– Pentose Phosphate Pathway
– Glycogen metabolism
– Gluconeogenesis
– Control of blood glucose level
Glycolysis
------ Anaerobic metabolism of glucose
From glucose to pyruvate
• Glycolysis takes place in the cytosol of cells
• 10 steps from glucose to pyruvate
• No need for oxygen
• Produce pyruvate, ATP and NADH.
Pathway of glycolysis
Glycolysis
• Overview.
Three stages.
• Investment stage :
– Glucose to glucose-6-phosphate to
fructose-1,6-bisphosphate.
– 2 ATPs required.
• Splitting stage.
– F-1,6-BP to two triose phosphate.
• Yield stage:
– Triose phosphate to pyruvate
– 4 ATPs and 2 NADHs are produced.
– 4 ATPs – 2 ATPs = 2 ATPs net.
1. Hexokinase catalyzes:
Glucose + ATP  glucose-6-P + ADP
A phosphoanhydride bond of ATP (~P) is cleaved.
2. Phosphohexose Isomerase catalyzes:
glucose-6-P (aldose)  fructose-6-P (ketose)
3. Phosphofructokinase-1 catalyzes:
fructose-6-P + ATP  fructose-1,6-bisP + ADP
This is the rate-limiting step of Glycolysis!
4. Aldolase catalyzes:
fructose-1,6-bisphosphate  dihydroxyacetone-P + glyceraldehyde-3-P
The reaction is an aldol cleavage, the reverse of an aldol condensation.
5. Triose Phosphate Isomerase (TIM) catalyzes:
dihydroxyacetone-P  glyceraldehyde-3-P
Glycolysis continues from glyceraldehyde-3-P.
6. Glyceraldehyde-3-phosphate Dehydrogenase catalyzes:
glyceraldehyde-3-P + NAD+ + Pi 
1,3-bisphosphoglycerate + NADH + H+
This is the only step in Glycolysis in which NAD+ is reduced to
NADH.
7. Phosphoglycerate Kinase catalyzes:
1,3-bisphosphoglycerate + ADP  3-phosphoglycerate + ATP
One ATP is synthesized in this step
8. Phosphoglycerate Mutase catalyzes:
3-phosphoglycerate  2-phosphoglycerate
Phosphate is shifted from the OH on C3 to the OH on C2.
9. Enolase catalyzes:
2-phosphoglycerate  phosphoenolpyruvate + H2O
NaF can inhibits activity of enolase
10. Pyruvate Kinase catalyzes:
phosphoenolpyruvate + ADP  pyruvate + ATP
One ATP is synthesized in this step
Removal of Pi from PEP yields an unstable enol, which spontaneously
converts to the keto form of pyruvate.
X2
X2
X2
X2
Glycolysis
Balance sheet for ATP:
 How many ATP expended? ________
2
 How many ATP produced? (Remember there
are two 3C fragments from glucose.) ________
4
 Net production of ATP per glucose: ________
2
What happened for 2 pyruvates?
Basically three options depending on the environmental conditions
In animal tissues under anaerobic conditions
• Reoxidize NADH to NAD+ that is needed for glycolysis;
• Lactate, end-product of fermentation, serves as a form of nutrient
energy
• Cell membranes contain carrier proteins that facilitate transport of
lactate.
Skeletal muscles ferment glucose to lactate during exercise, when the
exercise is brief and intense.
Lactate released to the blood may be taken up by other tissues, or by
skeletal muscle after exercise, and converted via Lactate Dehydrogenase
back to pyruvate, which may be oxidized in Citric Acid Cycle or (in
liver) converted to back to glucose via gluconeogenesis
Lactate serves as a fuel source for cardiac muscle as well as brain
neurons.
Fermentation in yeast
Some anaerobic organisms metabolize pyruvate to ethanol.
NADH is converted to NAD+ in the reaction catalyzed by Alcohol
Dehydrogenase.
Regulation of Glycolysis
• In 3 irreversible steps
• In 3 important enzymes
hexokinase/glucokinase; phosphofructokinase-1; pyruvate kinase
• PFK-1 is rate limiting enzyme and primary site of regulation.
Hexokinase
 Inhibited by product glucose-6-phosphate:
by competition at the active site
by allosteric interaction at a separate enzyme site.
 Cells trap glucose by phosphorylating it, preventing exit
through glucose carriers.
Glucokinase
--a variant of hexokinase found in liver.
 Glucokinase has a higher KM for glucose. It is active only at high
[glucose].
 Glucokinase is not subject to product inhibition by glucose-6phosphate. Liver will take up & phosphorylate glucose even when
liver [glucose-6-phosphate] is high.
Phosphofructokinase-1 (PFK-1)
• The rate-limiting step of the Glycolysis pathway
• Inducible enzyme
– Induced in feeding by insulin
– Repressed in starvation by glucagon
• Allosteric regulation
– Activated by AMP
– Inhibited by ATP and Citrate
– Activated by Fructose-2,6-bisphosphate
Pyruvate Kinase
• Activated by fructose-1,6-biphosphate
• Inhibited by ATP
Glycolysis is important for erythrocytes
Erythrocyte:
•
•
•
•
•
Simplest cell in body.
No subcellular organelles.
No DNA-RNA-protein synthesis.
No mitochondria—no oxidative phosphorylation
Relies exclusively on glucose as fuel.
– Glucose derived from blood.
– Yields ATP from glycolysis.
– End product is lactate.
2,3-bisphosphoglycerate pathway in RBC
• 2,3-Bisphosphoglycerate (2,3-BPG) is present in human red
blood cells at approximately 5 mmol/L.
• It binds with greater affinity to deoxygenated hemoglobin
(e.g. when the red cell is near respiring tissue) than it does to
oxygenated hemoglobin (e.g. in the lungs).
• In bonding to partially deoxygenated hemoglobin it
upregulates the release of the remaining oxygen molecules
bound to the hemoglobin, thus enhancing the ability of RBCs to
release oxygen near tissues that need it most.