Digestion and absorption of carbohydrate Lecture no 1 Carbohydrate metabolism

We eat, we digest, we absorb, then what? Three fates for nutrients Most are used to supply energy for life Some are used to synthesize structural or functional molecules 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

Galactose does not occur in foods singly but only as part of lactose. fructose glucose galactose sucrose maltose lactose FIGURE 4-2: HOW MONOSACCHARIDES JOIN TO FORM DISACCHARIDES. (fructose-glucose) (glucose-glucose) (glucose-galactose) 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 From glucose to pyruvate ------ 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.

2-phosphoglycerate  phosphoenolpyruvate + H2O 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

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.

--a variant of hexokinase found in liver. 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-6-phosphate. 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 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.