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Chapter 18 Glycolysis

Chapter 18 Glycolysis. Outline. What are the essential features of glycolysis ? Why are coupled reactions important in glycolysis ? What are the chemical principles and features of the first phase of glycolysis ?

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Chapter 18 Glycolysis

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  1. Chapter 18Glycolysis

  2. Outline • What are the essential features of glycolysis ? • Why are coupled reactions important in glycolysis ? • What are the chemical principles and features of the first phase of glycolysis ? • What are the chemical principles and features of the second phase of glycolysis ? • What are the metabolic fates of NADH and pyruvate produced in glycolysis ? • How is glycolysis regulated ? • Is glucose the only substrates used in glycolysis ?

  3. Terms Used in Metabolism Pathway Steps Metabolites Glycolysis 10 glucose to pyruvate Anaerobic glycolysis 11 glucose to lactate Alcoholic Fermentation 12 glucose to ethanol/CO2 Respiration (Aerobic) - glucose to CO2 and HOH, (includes glycolysis, PDH, Krebs, ETS/OP) Substrate level phosphorylation: Synthesis of of ATP using energy from a metabolic reaction. Oxidative phosphorylation: Synthesis of ATP using energy from electron transport.

  4. 18.1 What are the Essential Features of Glycolysis ? Glycolysis is also called the Embden-Meyerhof Pathway • Essentially all cells carry out glycolysis. • Glycolysis consists of ten reactions which are essentially the same in all cells. These are divided into two phases. • The first phase converts glucose to two G-3-P and is the energy requiring phase. • The second phase generates two pyruvates and is the energy producing phase. • Products of glycolysis are pyruvate, ATP and NADH. • There are three major fates for pyruvate.

  5. Figure 18.1 The Glycolytic Pathway Energy requiring Energy producing

  6. Figure 18.1 The Glycolytic Pathway

  7. The Fates of Pyruvate From Glycolysis Figure 18.2 Pyruvate produced in glycolysis can be utilized by cells in several ways. In animals, pyruvate is normally converted to acetyl-coenzyme A, which is then oxidized in the TCA cycle to produce CO2. When oxygen is limited, pyruvate can be converted to lactate. Alcoholic fermentation in yeast converts pyruvate to ethanol and CO2.

  8. 18.2 Why Are Coupled Reactions Important in Glycolysis? • Coupled reactions involving ATP hydrolysis are used to drive the glycolytic pathway. • Coupled reactions convert some, but not all of the metabolic energy of glucose into ATP. • Under cellular conditions, approximately 5% of the energy of glucose is released in glycolysis.

  9. Reactions and Thermodynamics of Glycolysis

  10. Steady-State Concentrations of Glycolytic Intermediates These steady-state concentrations are used to obtain the cellular values of ΔG found in Table 18.1 and Figure 18.22.

  11. Rxn 1: Hexokinase The first reaction - phosphorylation of glucose • Catalyzed by hexokinase or glucokinase. • This is a priming reaction - ATP is consumed here in order to get more later. • ATP makes the phosphorylation of glucose thermodynamically spontaneous. • Be sure you can interconvert Keq and standard state free energy change. • Be sure you can use Eq. 3.13 to generate far right column of Table 18.1.

  12. Calculation of the overall ΔG for Rx 1 Glucose + ATP <==> Glucose-6-P + ADP G = Go’ + RT ln (G-6-P)(ADP)/ (Glucose)(ATP) (0.083x10-3)(0.83x10-3) G = -16700 + RT ln ------------------------------ (5x10-3)(1.85x10-3) G= -16700 + 2577 ln (1.256x10-3) = -16700 + 2577 (-6.68) = -16700 – 17213 = -33910 or -33.91 kJ/mol

  13. Rxn 1: Hexokinase • G for the 1st step is large and negative. • Phosphorylation of glucose by hexokinase is done to keep glucose in the cell. • Glucose-6-P has no transporter and is trapped. • Km for glucose is 0.1 mM; cell has 4 mM glucose. • So hexokinase is normally active! • Glucokinase (Kmglucose = 10 mM) only turns on when cell is rich in glucose. • Hexokinase is regulated - allosterically inhibited by (product) glucose-6-P - but is not the most important site of regulation of glycolysis - Why?

  14. Glucose is kept in most cells by phosphorylation to glucose-6-phosphate Figure 18.4 Glucose-6-P cannot cross the plasma membrane.

  15. Glucose-6-P, intermediate forming an important branch point in metabolism Figure 18.5 Glucose-6-phosphate is the branch point for several metabolic pathways.

  16. Rxn 1: Hexokinase Figure 18.6 The (a) open and (b) closed states of yeast hexokinase. Binding of glucose (green) induces a conformation change that closes the active site, as predicted by Daniel Koshland. The induced fit model for enzymes is discussed on page 409 of the text.

  17. Rxn 2: Phosphoglucoisomerase Glucose-6-P to Fructose-6-P • Why does this reaction occur ? • next step (phosphorylation at C-1) would be tough for hemiacetal -OH, but easy for primary –OH. • isomerization activates C-3 for cleavage in aldolase reaction. • An ene-diol is an intermediate in this reaction . • Be able to write a mechanism for this interesting enzyme reaction (see Figure 18.8).

  18. A mechanism for phosphoglucoisomerase Figure 18.8 The phosphoglucoisomerase mechanism involves opening of the pyranose ring (step 1), proton abstraction leading to enediol formation (step 2), and proton addition to the double bond, followed by ring closure (step 3)

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