Carbohydrate Metabolism

I. Introduction: A.More than 60% of our foods are carbohydrates. Starch, glycogen, sucrose, lactose and cellulose are the chief carbohydrates in our food. Before intestinal absorption, they are hydrolysed to hexose sugars (glucose, galactose and fructose).

Glucose Oxidation major Pathway

Food (Fuel of energy)Food RespirationRespiration O2O2 O2O2EnergyEnergy H2OH2OH2OH2O CO 2 + Cellular Activities Cellular Respiration Organic compounds + O 2 Energy + CO 2 + H 2 O

I. Glycolysis (Embden Meyerhof Pathway): A. Definition: 1. Glycolysis means oxidation of glucose to give pyruvate (in the presence of oxygen) or lactate (in the absence of oxygen). B. Site: cytoplasm of all tissue cells, but it is of physiological importance in: 1. Tissues with no mitochondria: mature RBCs, cornea and lens. 2. Tissues with few mitochondria: Testis, leucocytes, medulla of the kidney, retina, skin and gastrointestinal tract. 3. Tissues undergo frequent oxygen lack: skeletal muscles especially during exercise.

C. Steps: Stages of glycolysis 1. Stage one (the energy requiring stage): a) One molecule of glucose is converted into two molecules of glycerosldhyde-3-phosphate. b) These steps requires 2 molecules of ATP (energy loss) 2. Stage two (the energy producing stage(: a) The 2 molecules of glyceroaldehyde-3-phosphate are converted into pyruvate (aerobic glycolysis) or lactate (anaerobic glycolysis(. b) These steps produce ATP molecules (energy production). D. Energy (ATP) production of glycolysis: ATP production = ATP produced - ATP utilized

In the energy investment phase, ATP provides activation energy by phosphorylating glucose. –This requires 2 ATP per glucose. In the energy payoff phase, ATP is produced by substrate-level phosphorylation and NAD + is reduced to NADH. 2 ATP (net) and 2 NADH are produced per glucose. Fig. 9.8

Fig. 9.9a Energy Investment Phase (steps 1- 5)

Fig. 9.9b Energy-Payoff Phase (Steps 6- 10)

Steps of Glycolysis

Contd…

Energy production of glycolysis: Net energyATP utilizedATP produced 2 ATP From glucose to glucose -6-p. From fructose -6-p to fructose 1,6 p. 4 ATP (Substrate level phosphorylation) 2ATP from 1,3 DPG. 2ATP from phosphoenol pyruvate In absence of oxygen (anaerobic glycolysis) 6 ATP Or 8 ATP 2ATP -From glucose to glucose -6-p. From fructose -6-p to fructose 1,6 p. 4 ATP (substrate level phosphorylation) 2ATP from 1,3 BPG. 2ATP from phosphoenol pyruvate. In presence of oxygen (aerobic glycolysis) + 4ATP or 6ATP (from oxidation of 2 NADH + H in mitochondria).

E. oxidation of extramitochondrial NADH+H+: 1. cytoplasmic NADH+H+ cannot penetrate mitochondrial membrane, however, it can be used to produce energy (4 or 6 ATP) by respiratory chain phosphorylation in the mitochondria. 2. This can be done by using special carriers for hydrogen of NADH+H+ These carriers are either dihydroxyacetone phosphate (Glycerophosphate shuttle) or oxaloacetate (aspartate malate shuttle). a) Glycerophosphate shuttle: 1) It is important in certain muscle and nerve cells. 2) The final energy produced is 4 ATP. 3) Mechanism: - The coenzyme of cytoplasmic glycerol-3- phosphate dehydrogenase is NAD+. - The coenzyme of mitochodrial glycerol-3-phosphate dehydogenase is FAD. - Oxidation of FADH, in respiratory chain gives 2 ATP. As glycolysis gives 2 cytoplasmic NADH + H+  2 mitochondrial FADH, 2 x 2 ATP  = 4 ATP. b) Malate – aspartate shuttle: 1) It is important in other tissues patriculary liver and heart. 2) The final energy produced is 6 ATP.

Differences between aerobic and anaerobic glycolysis: AnaerobicAerobic LactatePyruvate1. End product 2 ATP6 or 8 ATP2.energy Through Lactate formation Through respiration chain in mitochondria 3. Regeneration of NAD + Not available as lactate is cytoplasmic substrate Available and 2 Pyruvate can oxidize to give 30 ATP 4. Availability to TCA in mitochondria

Substrate level phosphorylation: This means phosphorylation of ADP to ATP at the reaction itself.in glycolysis there are 2 examples: Bisphosphoglycerate + ADP give 3 Phosphoglycerate + ATP - Phospho-enol pyruvate + ADP give Enolpyruvate + ATP I. Special features of glycolysis in RBCs: 1. Mature RBCs contain no mitochondria, thus: a) They depend only upon glycolysis for energy production (=2 ATP). b) Lactate is always the end product. 2. Glucose uptake by RBCs is independent on insulin hormone. 3. Reduction of met-hemoglobin: Glycolysis produces NADH+H+, which used for reduction of met-hemoglobin in red cells.

–Oxidative phosphorylation produces almost 90% of the ATP generated by respiration. Some ATP is also generated in glycolysis and the citric acid cycle by substrate-level phosphorylation. –Here an enzyme transfers a phosphate group from an organic molecule (the substrate) to ADP, forming ATP. Ultimately 38 ATP are produced per mole of glucose that is degraded to carbon CO 2 and H 2 O by respiration.Ultimately 38 ATP are produced per mole of glucose that is degraded to carbon CO 2 and H 2 O by respiration. Fig. 9.7

Biological importance (functions) of glycolysis: 1. Energy production: a) anaerobic glycolysis gives 2 ATP. b) aerobic glycolysis gives 8 ATP. 2. Oxygenation of tissues: 3. Provides important intermediates: a) Dihydroxyacetone phosphate: can give glycerol-3phosphate, which is used for synthesis of triacylglycerols and phospholipids (lipogenesis). b) 3 Phosphoglycerate: which can be used for synthesis of amino acid serine. c) Pyruvate: which can be used in synthesis of amino acid alanine. 4. Aerobic glycolysis provides the mitochondria with pyruvate, which gives acetyl CoA Krebs' cycle.

Reversibility of glycolysis (Gluconeogenesis): 1. Reversible reaction means that the same enzyme can catalyzes the reaction in both directions. 2. all reactions of glycolysis -except 3- are reversible. 3. The 3 irreversible reactions (those catalyzed by kinase enzymes) can be reversed by using other enzymes. Glucose-6-p  Glucose F1, 6 Bisphosphate  Fructose-6-p Pyruvate  Phosphoenol pyruvate 4. During fasting, glycolysis is reversed for synthesis of glucose from non- carbohydrate sources e.g. lactate. This mechanism is called: gluconeogenesis.

Importance of lactate production in anerobic glycolysis: 1. In absence of oxygen, lactate is the end product of glycolysis: 2. In absence of oxygen, NADH + H + is not oxidized by the respiratory chain. 3. The conversion of pyruvate to lactate is the mechanism for regeneration of NAD This helps continuity of glycolysis, as the generated NAD+ will be used once more for oxidation of another glucose molecule. Glucose  Pyruvate  Lactate

As pyruvate enters the mitochondrion, a multienzyme complex modifies pyruvate to acetyl CoA which enters the Krebs cycle in the matrix. –A carboxyl group is removed as CO 2. –A pair of electrons is transferred from the remaining two-carbon fragment to NAD + to form NADH. –The oxidized fragment, acetate, combines with coenzyme A to form acetyl CoA.

Overview of metabolism of carbohydrates All Monosaccharides Blood glucose Glucose in tissues Metabolised in the following pathways Gluconeogenesis Glycogenolysis Glycogenesis Glycolysis Non-carbohydrates Pentose phosphate pathway TCA cycle Electron transport Pyruvate + ATP Anaerobic Lactate ATP + H 2 O Riboses + NADPH Glycogen Oxidation CO 2 Aerobic Pyruvate + ATP Glycogen Riboses + NADPH Non-carbohydrates