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Glycolysis
- 1. Metabolism , Glycolysis Subject: Biochemistry II Instructor : Anam Sharif (Lecturer University of Okara)
- 2. Metabolism refers toall of the chemical reactions that take place within an organism by which complex molecules are broken down to produce energy and by which energy is used to build up complex molecules. An example of a metabolic reaction is the one that takes place when a person eats a spoonful of sugar. The word metabolism can also refer to the sum of all chemical reactions that occur in living organisms, including digestion and the transport of substances into and between different cells Metabolic reactions may be categorized as catabolic - the breaking down of compounds (for example, the breaking down of glucose to pyruvate by cellular respiration); or anabolic - the building up (synthesis) of compounds (such as proteins, carbohydrates, lipids, and nucleic acids). Usually, catabolism releases energy, and anabolism consumes energy. Metabolism
- 3. Glycolysis • Glycolysis (fromthe Greek glykys, “sweet” or“sugar,” and lysis, “splitting”) • Sequence of reactions that converts one molecule of glucose to two molecules of pyruvate with the formation of two ATP molecules • Glucose tends to exist in ring form, very stable • Glycolysis occurs in almost every living cell. • It was the first metabolic sequence to be studied. • This pathway is also called Embden-Meyerhof pathway (E.M-Pathway). • It occurs in cytosol.
- 4. • Either aerobicallyor anaerobically, depending on the availability of oxygen and intact mitochondria. • It allows tissues to survive in presence or absence of oxygen, e.g., skeletal muscle. • RBCs, which lack mitochondria, are completely reliant on glucose as their metabolic fuel, and metabolizes it by anaerobic glycolysis. • Glycolysis occurs in the absence of oxygen (anaerobic) or in presence of oxygen (aerobic). • Lactate is the end product under anaerobic condition. • In aerobic condition, pyruvate is formed, which is then oxidized to CO2 & H2O.
- 5. What are thepossible fates of glucose?
- 7. Glycolysis consists oftwo phases- • In the first phase, a series of five reactions, six-carbon glucose is broken down to two molecules of glyceraldehyde-3- phosphate. (preparatory phase) Preparatory phase contain 1. Energy investment phase or priming phase 2. Splitting phase • In the second phase, five subsequent reactions convert these two molecules of glyceraldehyde-3- phosphate into two molecules of pyruvate. (payoff phase ). Pay off phase also called energy generation phase. • Phase 1 consumes two molecules of ATP. • The later stages of glycolysis result in the production of four molecules of ATP. • The net is 4 – 2 = 2 molecules of ATP produced per molecule of glucose.
- 8. All the intermediatesin glycolysis have either 3 or 6 carbon atoms All of the reactions fall into one of 5 categories 1. Reduction – opposite of oxidation. • Loss of oxygen atoms • It is the addition of electrons to a molecule. • Addition of Hydrogen atom 2. Oxidation – • Gain of oxygen atoms • removal of electrons from a molecule. This subsequently lowers the energy content of a molecule. • Most biological oxidations involve the loss of hydrogen atoms. This type of oxidation is referred to as a dehydrogenation. The enzymes that catalyzes these reactions are called dehydrogenases.
- 9. 3. Phosphorylation -accomplished by transferring a phosphate group to ADP 4. Decarboxylation – carbon chain is shortened by the removal of a carbon atom (COO-) as CO2 5. Isomerization - is the process by which one molecule is transformed into another molecule which has exactly the same atoms 6. Aldol cleavage
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- 11. Glucose obtained from •The diet through intestinal hydrolysis of lactose, sucrose, glycogen, or starch is brought into the hexose phosphate pool through the action of hexokinase. • Free glucose is phosphorylated to glucose 6 phosphate by hexokinase. • Hexokinase splits the ATP into ADP & Pi, the Pi is added to the glucose. • Hexokinase is a key glycolytic enzyme. Step 1 : Trapping, Destabilizing
- 12. • glucose convertedto glucose-6-PO4 , ATP is needed • catalyzed by hexokinase or glucokinase • Hexokinase is inhibited by glucose 6- phosphate. • This enzyme prevents the accumulation of glucose 6-phosphate due to product inhibition. • In all tissues, the phosphorylation of glucose is catalyzed by hexokinase, one of the three regulatory enzymes of glycolysis.
- 13. • Glucose 6P is a central molecule with a variety of metabolic fates- glycolysis, glycogenesis, gluconeogenesis • The isomerization of Glucose 6-P (an aldose sugar) to Fructose 6-P (a ketose sugar) is catalyzed by phosphohexose isomerase .it requires Mg+2 ions. • The reaction is readily reversible, is NOT a rate limiting or regulated step. Step 2: Isomerization of Glucose 6-P
- 14. Step :3 Phosphorylationof Fructose 6-P • Fructose 6- phosphate is phosphorylated to Fructose 1, 6- bisphosphate by Phosphofructokinase (PFK) • The PFK reaction is the rate-limiting step. • It is controlled by the concentrations of the substrates ATP & Fructose 6-P
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- 17. • The sixcarbon Fructose 1, 6- bisphosphate is split to 2 three carbon compounds. Glyceraldehyde 3- phosphate & Dihydroxy acetone phosphate by the enzyme aldolase (Fructose 1, 6- bisphosphate aldolase). • six carbon molecule split into 2 and 3 carbon molecules • Aldolase cleaves a bond in the open-chain form of fructose 1,6-bisphosphate to glyceraldehyde 3-phopshate GAP and dihydroxyacetone phosphate (DHAP) Step 4: Lysis
- 18. Step 5: Isomerizationof DHAP • Phosphotriose isomerase catalyzes the reversible interconversion of dihydroxyacetone phosphate & glyceraldehyde 3-phosphate. • Two molecules of glyceraldehyde 3-phosphate are obtained from one molecule of glucose. • An enzyme called triose phosphate isomerase (TPI) catalyzes the conversion of the ketose to the aldose via an intramolecular oxidation-reduction reaction in which a hydrogen atom is transferred from the first carbon to the second carbon.
- 19. Pay off phase EnergyGeneration Phase
- 20. • Glyceraldehyde 3-phosphatedehydrogenase converts Glyceraldehyde 3-phosphate to 1,3- bisphosphoglycerate. This step is important as it is involved in the formation of NADH +H+ & a high energy compound 1,3- bisphosphoglycerate. • Conversion of GAP into 1,3-BPG by the action of the enzyme Gap-dehydrogenase. • This enzyme involves the coenzyme NAD+ (oxidized nicotinamide adenine dinucleotide), which plays the role of accepting a hydride group and ultimately form the 1,3-BGP. Step 6: Oxidation of glyceraldehyde 3P Iodoacetae inhibit glycolysis and enzyme glyceraldehyde 3 P dehydrogenase. Arsenate replace inorganic P
- 22. • Substrate-level phosphorylationis a metabolic reaction that results in the formation of ATP or GTP by the direct transfer of a phosphoryl (PO3) group to ADP or GDP from another phosphorylated compound. • The enzyme phosphoglycerate kinase acts on1,3- bisphosphoglycerate resulting in the formation of ATP and 3- phosphoglycerate. Reaction 7: Substrate level Phosphorylation (Formation of ATP from 1,3- bisphosphoglycerate & ADP)
- 24. • Unlike mostother kinases, this reaction is reversible. • 3- Phosphoglycerate is converted to 2- Phosphoglycerate by phosphoglycerate Mutase. This is isomerization reaction. • In a reaction catalyzed by phosphoglycerate mutase, a phosphoryl group is moved from the third carbon of 3-phosphoglycerate and onto the second carbon to form 2- phosphoglycerate. Reaction 8: Shifting of Phosphoryl Group
- 25. • The highenergy compound PEP is generated from 2- Phosphoglycerate by the enzyme enolase. • This enzyme requires Mg+2 or Mn+2 and is inhibited by fluoride. • This reaction is catalyzed by enolase. • PEP is a high energy molecule and contains a high phosphoryl transfer potential. • Less stable enol state. Reaction 9: dehydration
- 26. • The enzymepyruvate kinase catalyses the transfer of high energy phosphate from PEP to ADP, leading to the formation of ATP. • This step is also a substrate level phosphorylation. • Pyruvate kinase catalyzes the transfer of a phosphoryl group from the phosphoenol pyrvate. • Now the enol can convert into the more stable ketone • form of pyruvate. Reaction 10: substrate level phosphorylation
- 29. The flow ofcarbon through the glycolytic pathway is regulated in response to metabolic conditions, both inside and outside the cell, essentially to meet two needs: the production of ATP and the supply of precursors for biosynthetic reactions. And in the liver, to avoid wasting energy, glycolysis and gluconeogenesis are reciprocally regulated so that when one pathway is active, the other slows down. different enzymes to catalyze the essentially irreversible reactions of the two pathways, whose activity are regulated separately. Three regulatory enzymes: • Hexokinase & glucokinase • Phosphofructokinase • Pyruvate kinase • Catalysing the irreversible reactions regulate glycolysis. Regulation of glycolysis
- 30. • Hexokinase isinhibited by glucose 6- phosphate. This enzyme prevents the accumulation of glucose 6-phosphate due to product inhibition. • In humans, hexokinase has four tissue specific isozymes, designated as hexokinase I, II, III, and IV, encoded by as many genes. • Hexokinase I is the predominant isozyme in the brain, • whereas in skeletal muscle hexokinase I and II are present, accounting for 70-75% and 25-30% of the isozymes, respectively. • Hexokinase IV, also known as glucokinase , is mainly present in hepatocytes and β cells of the pancreas, where it is the predominant isozyme. • Glucokinase, which specifically phosphorylates glucose, is an inducible enzyme. Hexokinase and Glucokinase
- 31. • Phosphofructokinase 1is the key control point of carbon flow through the glycolytic pathway. • Phosphofructo kinase (PFK) is the most important regulatory enzyme in glycolysis. • PFK is an allosteric enzyme regulated by allosteric effectors . ATP, citrate, and hydrogen ions are allosteric inhibitors of the enzyme, whereas Fructose 2 ,6- bisphosphate, ADP, AMP & Pi are the allosteric activators. • Fructose-2,6-bisphosphate (F2,6-BP) is considered to be the most important regulatory factor (activator) for controlling PFK & ultimately glycolysis in the liver. Phosphofructokinase (PFK)
- 32. • Pyruvate KinaseInhibited by ATP & activated by Fructose 1,6-Bi P. • Pyruvate kinase is active (a) in dephosphorylated state & inactive (b) in phosphorylated state. • Inactivation of pyruvate kinase is brought about by cAMP-dependent protein kinase. • The hormone glucagon inhibits hepatic glycolysis by this mechanism. Pyruvate kinase
- 33. Energy yield fromglycolysis Glycolysis is the major pathway of glucose metabolism and occurs in the cytosol of all cells. ... This is clinically significant because oxidation of glucose under aerobic conditions results in 32 mol of ATP per mol of glucose. However, under anaerobic conditions, only 2 mol of ATP can be produced. During anaerobic: • One molecule of glucose is converted to 2 molecules of lactate, there is a net yield of 2 molecules of ATP. • 4 molecules of ATP are synthesized by 2 substrate level phosphorylation. • 2 ATP molecules are used in steps 1 & 3, Hence, net yield is 2 ATP. During Aerobic condition 2 NADH molecules, generated in the glyceraldehyde 3P-dehydrogenase reaction & enter ETC. NADH provides 3 ATP, this reaction generates 3x2=6 ATP Total ATP is 6+2=8 ATP.
- 34. Conversion of pyruvateto lactate In anaerobic condition, pyruvate is reduced to lactate by lactate dehydrogenase (LDH). LDH has 5 iso-enzymes. The cardiac iso-enzyme of LDH will be increased in myocardial infarcts. Glycolysis is very essential in skeletal muscle during exercise where oxygen supply is very limited. In RBCs, there are no mitochondria. Glycolysis in the erythrocytes leads to lactate production. RBCs derive energy only through glycolysis, where the end product is lactic acid.