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Cellular Respiration

Cellular Respiration. Advanced Biology November 2004 – Revised November 2007. Cellular Respiration: Harvesting Chemical NRG. Cell respiration and fermentation are catabolic pathways Cells must recycle the ATP they spend for work

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Cellular Respiration

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  1. Cellular Respiration Advanced Biology November 2004 – Revised November 2007

  2. Cellular Respiration: Harvesting Chemical NRG • Cell respiration and fermentation are catabolic pathways • Cells must recycle the ATP they spend for work • Redox reactions release NRG when electrons move closer to electronegative atoms • Electrons “fall” from organic molecules to oxygen during cellular respiration • The “fall” of electrons during respiration is stepwise, via NAD+ and an electron transport chain (ETC)

  3. Respiration is a cumulative function of glycolysis, the Krebs cycle, and electron transport • There are three metabolic stages of cellular respiration: • Glycolysis is a catabolic pathway that: • Occurs in the cytosol • Partially oxidizes glucose(6C) into two pyruvate (3C) molecules • The Krebs Cycle is a catabolic pathway that: • Occurs in the mitochondrial matrix • Completes glucose oxidation by breaking down a pyruvate derivative (acetyl CoA) into carbon dioxide

  4. Glycolysis and the Krebs Cycle produce: • A small amount of ATP by substrate-level-phosphorylation • NADH by transferring electrons from substrate to NAD+ . Krebs Cycle also produces FADH2 by transferring electrons to FAD • The electron transport chain: • Is located at the inner membrane of the mitochondrion • Accepts energized electrons from reduced coenzymes (NADH and FADH2) that are harvested during glycolysis and Krebs Cycle. Oxygen pulls these electrons down the electron transport chain to a lower NRG state

  5. Respiration is a cumulative function of glycolysis, the Krebs cycle, and electron transport - continued • Oxidative phosphorylation = ATP production that is coupled with the exergonic transfer of electrons from food to oxygen • A small amount of ATP is produced directly by the reaction of glycolysis and Krebs Cycle. This mechanism of producing ATP is called substrate-level phosphorylation • -ATP production by direct enzymatic transfer of phosphate from an intermediate substrate in catabolism to ADP

  6. Glycolysis harvests chemical NRG by oxidizing glucose to pyruvate • Glycolysis = (Glyco = sweet, sugar; lysis = to split) • Catabolic pathway during which 6-carbon glucose is split into two 3-carbon sugars • These are then oxidized and rearranged by a step-wise process that produces two pyruvate molecules • -Each reaction is catalyzed by specific enzymes dissolved in the cytosol • -No CO2 is released as glucose is oxidized to pyruvate; all carbon in glucose can be accounted for in the two molecules of pyruvate • -Occurs whether or not oxygen is present! (anaerobic)

  7. Figure 7-2Page 139 1 2 3 4 Glycolysis Formation of acetyl coenzyme A Citric acid cycle Electron transport and chemiosmosis Glucose Acetyl coenzyme A Citric acid cycle Electron transport and chemiosmosis Pyruvate 2 ATP 2 ATP 2 ATP

  8. Glycolysis • The reaction of glycolysis occurs in two phases: • 1. NRG investment phase. The cell uses ATP to phosphorylate the intermediates (mainly glucose) of glycolysis • 2. NRG yielding phase. Two 3-carbon intermediates (dihydroxyacetone phosphate, glyceraldehyde phosphate) are oxidized. • -For each glucose molecule entering glycolysis: • 1- a net gain of two ATPs is produced by substrate-level-phosphorylation • 2- two molecules of NAD+ are reduced to NADH. NRG conserved in the high-NRG electrons of NADH can be used to make ATP by oxidative phosphorylation

  9. NRG investment Phase • The NRG investment phase includes five preparatory steps that split glucose in two. • 1 -glucose enters the cell, and carbon six is phosphorylated. This ATP coupled rxn: • Is catalyzed by hexokinase. Kinase is an enzyme involved with phosphate transfer • Requires an initial investment of ATP • Makes glucose more chemically reactive • Produces glucose-6-phosphate. Since the plasma membrane is relatively impermeable to ions, addition of an electrically charged phosphate group traps the sugar in the cell • 2 -an isomerase catalyzes the rearrangement of glucose-6-phosphate to its isomer, fructose-6-phosphate

  10. NRG investment Phase Cont. • 3 -carbon 1 of fructose-6-phosphate is phosphorylated. This rxn: • Requires an investment of another ATP • Is catalyzed by phosphofructokinase, an allosteric enzyme that controls the rate of glycolysis • Forms fructose-1,6-bisphosphate • 4 -an aldolase cleaves (splits) the six carbon sugar into two isometric three carbon sugars • This is the rxn for which glycolysis is named • For each glucose molecule that begins glycolysis, there are two product molecules for this and each succeeding step

  11. NRG investment Phase Cont. • 5 -an isomerase catalyzes the reversible conversion between the two three carbon sugars. This rxn: • Never reaches equilibrium because only one isomer,glyceraldehyde phosphate, is used in the next step of glycolysis • It is thus pulled towards the direction of glyceraldehyde phosphate, which is removed as fast as it forms • Results in the net effect that, for each glucose molecule, two molecules of glyceraldehyde phosphate progress through glycolysis

  12. Figure 7-3(2)Page 140 Energy investment phase and splitting of glucose Two ATPs invested per glucose Glucose 2 ATP 3 steps 2 ADP Fructose-1,6-bisphosphate P P Glyceraldehyde phosphate (G3P) Glyceraldehyde phosphate (G3P) P P (see next slide)

  13. Figure 7-3(3)Page 140 Energy capture phase Four ATPs and two NADH produced per glucose P P (G3P) (G3P) NAD+ NAD+ NADH NADH 5 steps 2 ADP 2 ADP 2 ATP 2 ATP Pyruvate Pyruvate Net yield per glucose:Two ATPs and two NADH

  14. NRG Yielding Phase The NRG yielding phase occurs in 5 additional steps following glycolysis. This phase occurs after glucose is split into two three-carbon sugars. Sugars are oxidized and ATP and NADH are produced • 1 - an enzyme catalyzes two sequential reactions : • 1. Glyceraldehyde phosphate is oxidized and NAD+ is reduced to NADH. • This rxn is very exergonic and is coupled to the endergonic phosphorylation phase • For every glucose molecule, 2 NADH are produced • 2. Glyceraldehyde phosphate is phosphorylated on carbon one • The phosphate source is inorganic phosphate, which is always present in the cytosol • The new phosphate bond is a high NRG bond with even more potential to transfer a phosphate group than ATP

  15. 2 -ATP is produced by substrate-level phosphorylation • In a very exergonic rxn, the phosphate group with the high NRG bond is transferred from 1,3-diphosphoglycerate to ADP • For each glucose molecule, two ATP molecules are produced. The ATP ledger now stands at zero as the initial debt of two ATP from steps one and three is repaid • 3 - In preparation for the next rxn, a phosphate group on carbon 3 is enzymatically transferred to carbon two • 4 - Enzymatic removal of a water molecule • Creates a double bond between carbons one and two of the substrate • Rearranges the substrate’s electrons, which transforms the remaining phosphate bond into an unstable bond • Phosphoenol pyruvate (PEP) • 5 - ATP is produced by substrate-level phosphorylation (pyruvate kinase) • In a highly exergonic rxn, a phosphate group is transferred from PEP to ADP • For each glucose molecule, this step produces 2 ATP

  16. Summary Equation for Glycolysis C6H12O6 2 C3H4O3 (Pyruvate) + 2 NAD+ + 2 NADH + 2 ADP + 2 P + 2 ATP + 2 H2O Glucose has been oxidized into two pyruvate molecules The process is exergonic; most of the NRG harnessed is conserved in the high NRG electrons of NADH and in the phosphate bonds of ATP

  17. Most of the chem NRG originally stored in glucose still resides in the two pyruvate molecules produced by glycolysis. • The fate of pyruvate depends upon the presence or absence of oxygen. • If oxygen is present, pyruvate enters the mitochondrion where it is completely oxidized by a series of enzyme-controlled rxns • If oxygen is NOT present then the Krebs cycle cannot be completed. An alternative path is followed. • Fermentation enables some cells to produce ATP without the help of oxygen (anaerobic)

  18. Lactic Acid Fermentation • 2 pyruvate gets turned into lactic acid as 2 NAD+ get reduced to NADH • This goes to an ETC to yield 2 ATP • Recycles the NAD+ for glycolysis • This occurs in the cytosol • Anaerobic

  19. Formation of Acetyl CoA • The junction btwn glycolysis and the Krebs Cycle is the oxidation of pyruvate to acetyl CoA: (oxygen must be present) • Pyruvate molecules are translocated (moved) from the cytosol into the mitochondrion by a carrier protein in the mitochondrial membrane • This step is catalyzed by a multienzyme complex which: • 1. Removes CO2 from the carboxyl group of pyruvate, changing it from a three carbon to a two carbon compound. ( This is the first step where CO2 is released) • 2. Oxidizes the two carbon fragment to acetate, while reducing NAD+ to NADH. Since glycolysis produces two pyruvate molecules per glucose, there are two NADH molecules produced • 3. Attaches coenzyme A to the acetyl group, forming acetyl CoA. This bond is unstable making the acetyl group very reactive

  20. Figure 7-5(2)Page 144 Carbon dioxide Pyruvate NAD+ Coenzyme A NADH Acetyl Coenzyme A

  21. Krebs Cycle (Citric Acid Cycle) • The Krebs Cycle rxns oxidize the remaining acetyl fragments of acetyl-CoA to CO2. NRG released from this exergonic process is used to reduce coenzyme (NAD+ and FAD) and to phosphorylate ATP (substrate-level phosphorylation) • The Krebs cycle has eight enzyme-controlled steps that occur in the mitochondrial matrix • For every turn of the Krebs Cycle: • Two carbons enter in the acetyl fragment of acetyl CoA • Two different carbons are oxdized and leave as CO2 • Coenzymes are reduced; three NADH and one FADH2 are produced • One ATP molecule is produced by substrate level phosphorylation • Oxaloacetate is regenerated • For every glucose molecule split during glycolysis: • Two acetyl fragments are produced • It takes two turns of the Krebs Cycle to complete the oxidation of glucose

  22. Step 1 • The unstable bond of acetyl CoA breaks, and the two-carbon acetyl group bonds to the four-carbon oxaloacetate to form six-carbon citrate • Step 2 • Citrate is isomerized to isocitrate • Step 3 • The two major events occur during this step: • Isocitrate loses CO2 leaving a five-carbon molecule • Th five-carbon compound is oxidized and NAD+ is reduced • Step 4 • Removal of CO2 • Oxidation of the remaining four-carbon compound and reduction of NAD+ • Attachment of CoA with a high NRG bond to form succinyl CoA

  23. Step 5 • Substrate-level phosphorylation occurs in a series of enzyme catalyzed reactions • The high NRG bond of succinyl CoA breaks, and some NRG is conserved as CoA and is displaced by a phosphate group • The phosphate group is transferred to GDP to form GTP and succinate • GTP donates a phosphate group to ADP to form ATP • Step 6 • Succinate is oxidized to fumarate and FAD is reduced • Two hydrogens are transferred to FAD to form FADH2 • The dehydrogenase that catalyzes this reaction is bound to the inner mitochondrial membrane

  24. Step 7 • Water is added to fumarate which rearranges its chemical bonds to form malate • Step 8 • Malate is oxidized and NAD+ is reduced • A molecule of NADH is produced • Oxaloacetate is regenerated to begin the cycle again

  25. Some FINAL Krebs Cycle Thoughts • Two turns of the Krebs Cycle produces two ATP’s by substrate-level phosphorylation. However, most ATP output of respiration results from oxidative phosphorylation • Reduced coenzymes produced by the Krebs Cycle (6 NADH and 2 FADH2 per glucose) carry high energy electrons to the electron transport chain • The ETC couples electron flow down the chain to ATP synthesis

  26. Glucose Fatty acids Aconitase Isocitrate dehydrogenase Acetyl coenzyme A H2O H2O Citrate Isocitrate CO2 Citrate synthase NAD+ Coenzyme A NADH CITRIC ACID CYCLE Part 1 NAD+ NADH a-ketoglutarate Oxaloacetate Coenzyme A a-ketoglutarate dehydrogenase (see next slide) (see next slide) CO2

  27. (see previous slide) (see previous slide) Malate dehydrogenase CITRIC ACID CYCLE Part 2 NADH NAD+ Succinyl coenzyme A Malate GTP ADP GDP Succinyl CoA synthetase H2O ATP Fumarase Coenzyme A Succinate dehydrogenase FAD Fumarate Succinate FADH2

  28. The Pathway of Electron Transport • The electron transport chain is made of electron carrier molecules embedded in the inner mitochondrial membrane • Each successive carrier in the chain has a higher electronegativity than the carrier before it, so the electrons are pulled downhill towards oxygen, the final electron acceptor and the molecule with the highest electronegativity • Except for ubiquinone(Q), most of the carrier molecules are proteins with tightly bound to prosthetic groups (nonprotein cofactors)

  29. Fig. 7-10a(2)Page 149 Cytosol Outer mitochondrial membrane Intermembrane space Complex V:ATP synthase Complex III Complex IV Inner mitochondrial membrane Complex I Complex II Matrix of mitochondrion (a)

  30. Prosthetic groups alternate btwn reduced and oxidized states as they accept and donate electrons Protein electron carriersProsthetic group Flavoproteins flavin mononucleotide Iron-sulfur proteins iron and sulfur Cytochromes heme group Heme group - prosthetic group composed of four organic rings surrounding a single iron atom Cytochrome - type of protein molecule that contains a heme prosthetic group and that functions as an electron carrier in the ETCs of mitochondria and chloroplasts. It is the iron of cytochromes that transfers electrons

  31. Electron Transfers along the ETC • The ETC does not make ATP directly. It generates a proton gradient across the inner mitochondrial membrane, which stores potential NRG that can be used to phosphorylate ADP • Chemiosmosis: the coupling of exergonic electron flow down an ETC to an endergonic ATP production by the creation of a proton gradient across a membrane • ETC pumps protons across mitochondrial membrane from the matrix to the intermembrane space • The membrane is impermeable to H+ so they cannot diffuse back across • ATP synthase uses the potential energy to add a third phosphate onto ADP to make ATP

  32. In mitochondrion, the production of ATP by chemiosmosis is via oxidative phosphorylation • In chloroplasts, it is via photophosphorylation

  33. Oxidative phosphorylation Substrate-level phosphorylation Figure 7-11Page 150 Glycolysis Glucose 2 NADH 4 – 6 ATP 2 ATP Pyruvate Acetyl coenzyme A 2 NADH 6 ATP 6 NADH 18 ATP Citric acid cycle 2 ATP 2 FADH2 4 ATP Electron transport and chemiosmosis 32 - 34 ATP Total ATP from oxidative phosphorylation

  34. Figure 7-12Page 151 FATS PROTEINS CARBOHYDRATES Glycerol Amino acids Fatty acids Glycolysis Glucose G3P Pyruvate CO2 Acetyl Coenzyme A Citric acid cycle Electron transport and chemiosmosis End products: NH3 H2O CO2

  35. Feedback mechanisms control cell respiration • Cells respond to changing metabolic needs by controlling reaction rates • Anabolic pathways are switched off when ther products are in ample supply. The most common mechanism of control is feedback inhibition • Catabolic pathways, such as glycolysis and Krebs cycle, are controlled by regulation enzyme activity at strategic points • A key control point of catabolism is the third step of glycolysis, which is catalyzed by an allosteric enzyme, phosphofructokinase • The ration of ATP to ADP and AMP reflects the NRG status of the cell, and phosphofructokinase is sensitive to this ratio • Citrate (produced in Krebs cycle) and ATP are allosteric inhibitors of phosphofructokinase, so when their conc. Rise, the enzyme slows glycolysis. As the rate of glycolysis slows, Krebs cycle also slows since the supply of acetyl CoA is reduced. This synchronizes the rates of glycolysis and Krebs cycle • ADP and AMP are allosteric activators for phosphofructokinase, so when their conc. Relative to ATP rise, the enzyme speeds up glycolysis and the Krebs cycle • There are other allosteric enzymes that control glycolysis and Krebs

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