Glycolysis: 10 enzyme-catalyzed reactions that convert glucose to pyruvate

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1 Glycolysis

2 Glycolysis: 10 enzyme-catalyzed reactions that convert glucose to pyruvate Glucose: Major fuel of most organisms. A central molecule in metabolism. omplete oxidation of 1 mole of glucose to O 2 and H 2 O gives a free energy change of kj/mol. Some of this energy is stored in the form of ATP and much remains in the product pyruvate. Glycolysis occurs in the cytosol.

3 The oxidation of glucose is accompanied by the conversion of 2 molecules of ADP to ATP and two molecules of NAD+ to NADH. NAD + (and NADP) are nucleotide cofactors that participate in oxidation-reduction reactions. As substrates are oxidized NAD + accepts a hydride ion, :H -, (2e - + proton) and is reduced to NADH.

4 Glycolysis: Glucose + 2ADP + 2NAD + + 2Pi 2Pyruvate + 2ATP + 2NADH + 2H + + 2H 2 O Two fates for pyruvate: Aerobic conditions: converted to actyl oa (to TA cycle) Anaerobic conditions: converted to lactate or ethanol (fermentation)

5 Hexose Stage: 6-carbon intermediates; 2 ATP hydrolyzed Mg Hexokinase: glucose + ATP G-6-PO4 + ADP + H + ( G = kj//mol) -2 H 2 OH O 3 P-OH 2

6 Regulation of Hexokinase: Glucose-6-PO is an allosteric 4 inhibitor of hexokinase.

7 2. G-6-PO 4 Isomerase : G-6-PO 4 F-6-PO 4 (Phosphoglucoisomerase) ( G = kj//mol) -2-2 O 3 P-OH O 2 3 P-OH 2 H2 2 OH

8 3. Phosphofructokinase: Mg 2+ F-6-PO 4 + ATP F-1,6 Bisphosphate + ADP + H + ( G = kj//mol) O 3 P-OH O 2 3 P-OH 2 H 2 O-PO 3 H 2 OH

9 v Regulation of Phosphofructokinase: Inhibited by high levels of ATP, which lowers the affinity of the enzyme for F-6-PO 4.. [F-6-PO 4.] low [ATP] high [ATP] Inhibitory action of ATP reversed by AMP ADP activates the mammalian enzyme but inhibits the plant kinase

10 Glycolysis and the TA cycle are coupled via PFK because citrate is an allosteric inhibitor of PFK. When TA cycle is saturated, glycolysis slows down. TA cycle produces ATP and carbon skeletons for biosynthesis. This is an example of negative feedback inhibition.

11 In 1980 a new regulator of PFK was discovered, fructose-2,6-bisphosphate Is an allosteric activator of PFK by increasing affinity for F-6-PO 4 and diminishing inhibitory effects of ATP. F-2,6BP is formed from F-6-PO 4 by phosphorylation. An abundance of F-6-PO 4 leads to a higher conc. of F-2,6BP which stimulates glycolysis. Example of feed forward activation.

12 F-2,6BP is formed by the action of PFK-2. It is converted back to F-6-PO 4 by F-2,6 bisphosphatase. Inhibition of PFK leads to a buildup of F-6-PO 4 ; this leads to a buildup of Glucose-6-PO 4 (due to isomerase). The result is an inhibition of hexokinase, therefore, two regulatory points are shut down.

13 4. Aldolase: F -1,6 Bisphosphate G-3-P + DHAP ( G = kj//mol) O 3 P-OH 2 H 2 O-PO 3 O H H OH 2- H 2 OPO 3 G-3-P H 2 OH O 2- H 2 OPO 3 DHAP

14 5. Triose Phosphate Isomerase: DHAP G-3-P ( G = kj//mol) H 2 OH O 2- H 2 OPO 3 DHAP O H H OH 2- H 2 OPO 3 G-3-P

15 Three arbon Stage of Glycolysis

16 6. Glyceraldehyde-3-Phosphate Dehydrogenase (G-3-P d hase): G-3-P + Pi + NAD + 1,3 Bisphosphoglycerate + NADH + H + ( G = kj//mol) O H H OH 2- H 2 OPO 3 NADH O H 2- OPO 3 OH 2- H 2 OPO 3 G-3-P 1,3 BPG

17 7. Phosphoglycerate Kinase: Mg 2+ 1,3 BPG + ADP 3-Phosphoglycerate + ATP ( G = +0.1 kj//mol) O H 2- OPO 3 OH 2- H 2 OPO 3 1,3 BPG ATP O H OH OH 2- H 2 OPO 3 3-Phosphoglycerate

18 8. Phosphoglycerate Mutase: 3-Phosphoglycerate 2-Phosphoglycerate ( G = kj//mol) O H OH OH 2- H 2 OPO 3 O OH H 2- OPO 3 H 2 OH 3-Phosphoglycerate 2-Phosphoglycerate

19 9. Enolase: 2-Phosphoglycerate PEP + H 2 O ( G = +1.1 kj//mol) O OH H 2- OPO 3 H 2 OH O H 2 OH 2- OPO 3 2-Phosphoglycerate Phosphoenolpyruvate (PEP)

20 10. Pyruvate Kinase: PEP + ADP + H + Mg 2+ Pyruvate + ATP ( G = -23 kj//mol) O OH 2- OPO 3 O OH O H 2 ATP H 3 Phosphoenolpyruvate (PEP) Pyruvate

21 Regulation of Pyruvate Kinase: There are several isozymes of pyruvate kinase. The liver, kidney and red blood cell isozymes are allosterically activated by Fructose-1,6Bisphosphate. V + F-1,6BP - F-1,6BP This is an example of feed forward activation. [PEP]

22 Pyruvate kinase from intestine and liver is subject to regulation by covalent modification. camp-dependent protein kinase phosphorylates pyruvate kinase, making it less active. Removal of the phosphate by a phosphatase restores activity. V no glucagon after glucagon treatment camp kinase is activated by the hormone glucagon. [PEP]

23 Pyruvate kinase is additionally allosterically activated by AMP and inhibited by ATP, acetyl oa and alanine.

24 The product of glycolysis, pyruvate, has several possible fates. Under aerobic conditions pyruvate is oxidized to O 2 and H 2 0 by the TA cycle. Under anaerobic conditions pyruvate is converted to either lactate or ethanol.

25 Alcoholic Fermentation: Glucose + 2Pi + 2ADP + 2H+ + 2ATP + 2O 2 + 2H 2 O 2 ethanol O OH O H 3 Pyruvate H+ O 2 pyruvate decarboxylase H O H 3 Acetaldehyde Pyruvate decarboxylase contains Thiamine Pyrophosphate as a coenzyme (derivative of vitamin B1; deficiency causes beriberi).

26 Alcohol dehydrogenase catalyzes the reduction of acetaldehyde to ethanol by transferring electrons from NADH. H O H 3 Acetaldehyde H+, NADH NAD+ alcohol dehydrogenase H H OH H 3 Ethanol Regeneration of NAD+ allows glycolysis to continue under anaerobic conditions.

27 Homolactic Fermentation: Glucose + 2Pi + 2ADP 2 lactate + 2ATP + 2H 2 O O OH O H 3 Pyruvate H+, NADH NAD+ lactate dehydrogenase O H OH OH H 3 Lactate

28 Lactate has no other fate and is considered a metabolic dead end. Lactate formation regenerates NAD+ from NADH, this ensures that glycolysis continues to run since G-3-P d hase requires NAD+. Thus, there is no net oxidation or reduction in either type of fermentation.

29 The ori ycle: Named for arl and Gerti ori whose work in the 30 s and 40 s contributed to its eluidation. Skeletal muscle: Lactate and pyruvate are transported out of muscle to the liver where lactate is converted to pyruvate by liver lactate d hase. Liver is the major organ regulating the supply of glucose to other cells of the body.

30 In liver pyruvate 1) can be oxidized by the TA cycle; 2) converted to alanine for protein synthesis; 3) converted to glucose by gluconeogenesis. ori cycle is the interorgan metabolism whereby liver receives pyruvate and lactate from muscle, then furnishes glucose back to muscle to be metabolized to produce ATP for muscle contraction.

31 atabolism of Sucrose and Lactose Sucrose is hydrolyzed in the intestine to glucose and fructose by sucrase. Most of the fructose is metabolized by the liver. Fructose Fructokinase ATP ADP Fructose-1-P

32 F-1-P Aldolase DHAP Triose-P Isomerase G-3-P (GAP) Fructose-1-P Glyceraldehyde ATP Triose kinase G-3-P (GAP) ADP G-3-P (GAP) enters the glycolytic pathway at the glyceraldehyde 3-P d hase step.

33 Why not convert fructose directly to F-6-P? Fructose is a poor substrate for liver hexokinase. Metabolism of 1 molecule of fructose to pyruvate yields 2 ATP and 2 NADH, the same as for the conversion of glucose to pyruvate. Fructose catabolism bypasses the PFK step and its associated regulation. Diets high in fructose lead to overproduction of pyruvate, some of which is converted to fats.

34 Lactose Metabolism via Glycolysis Lactose is the major source of energy for nursing mammals. It is converted to glucose and galactose by lactase. Galactose Galactokinase ATP ADP Galactose-1-P

35 Gal-1-P UDP-Glucose Glu-6-P Phosphoglucomutase Glu-1-P UDP-Glucose:Gal-1-P Uryidylyltransferase UDP-Gal UDP-Glucose 4 Epimerase

36 onversion of one molecule of galactose to 2 pyruvate produces 2 ATP and 2 NADH, the same yields as for glucose and fructose. Genetic defects of sugar metabolism: Galactosemia-defective uridylyltransferase; causes permanent neurological disorders; Lactose intolerance-disappearance of lactase activity and inability to digest diary products.