We eat, we digest, we absorb, then what? Three fates for nutrients

1) Most are used to supply energy for life2) Some are used to synthesize structural or functional molecules3) 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

Fig. 4-2a, p. 101

fructose glucose galactose

sucrose maltose lactose

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

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

• 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? ________

How many ATP produced? (Remember there are two 3C fragments from glucose.) ________

Net production of ATP per glucose: ________

2

4

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 has a higher KM for glucose. It is active only at high

[glucose].

Glucokinase is not subject to product inhibition by glucose-6-

phosphate. Liver will take up & phosphorylate glucose even when

liver [glucose-6-phosphate] is high.

Glucokinase

--a variant of hexokinase found in liver.

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

• Activated by fructose-1,6-biphosphate

• Inhibited by ATP

Pyruvate Kinase

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.