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Chapter 10

Cellular Respiration. Chapter 10. Chemotrophic Energy Metabolism: Aerobic Respiration. Some cells meet their energy needs through anaerobic fermentation However, fermentation yields only modest amounts of energy due to the absence of electron transfer

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Chapter 10

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  1. Cellular Respiration Chapter 10

  2. Chemotrophic Energy Metabolism: Aerobic Respiration • Some cells meet their energy needs through anaerobic fermentation • However, fermentation yields only modest amounts of energy due to the absence of electron transfer • ATP yield is much higher in cellular respiration

  3. Cellular Respiration: Maximizing ATP Yields Cellular respiration (or respiration) uses an external electron acceptor to oxidize substrates completely to CO2 External electron acceptor: one that is not a by-product of glucose catabolism

  4. Cellular respiration defined • Respiration is the flow of electrons through or within a membrane, from reduced coenzymes to an external electron acceptor usually accompanied by the generation of ATP • Coenzymes such as FAD (flavinadeninedinucleotide) and coenzymeQ (ubiquinone) are involved

  5. The terminal electron acceptor • In aerobic respiration, the terminal electron acceptor is oxygen and the reduced form is water • Other terminal electron acceptors (sulfur, protons, and ferric ions) are used by other organisms, especially bacteria and archaea • These are examples of anaerobic respiration

  6. Mitochondria Most aerobic ATP production in eukaryotic cells takes place in the mitochondrion In bacteria, the plasma membrane and cyotoplasm are analogous to the mitochondrial inner membrane and matrix with respect to energy metabolism

  7. Figure 10-1

  8. Aerobic Respiration Yields Much More Energy than Fermentation Does With O2 as the terminal electron acceptor, pyruvate can be oxidized completely to CO2 Aerobic respiration has the potential of generating up to 38 ATP molecules per glucose Oxygen provides a means of continuous reoxidation of NADH and other reduced coenzymes

  9. Respiration Includes Glycolysis, Pyruvate Oxidation, the TCA Cycle, Electron Transport, and ATP Synthesis Respiration will be considered in five stages Stage 1: the glycolytic pathway Stage 2: pyruvate is oxidized to generate acetyl CoA Stage 3: acetyl Co A enters the tricarboxylic acid cycle (TCA cycle), where it is completely oxidized to CO2

  10. The five stages, continued Respiration will be considered in five stages Stage 4: electrontransport, the transfer of electrons from reduced coenzymes to oxygen coupled to active transport of protons across a membrane Stage 5: The electrochemical proton gradient formed in step 4 is used to drive ATP synthesis (oxidative phosphorylation)

  11. The Mitochondrion: Where the Action Takes Place The mitochondrion is called the “energy powerhouse” of the eukaryotic cell These organelles are thought to have arisen from bacterial cells Mitochondria have been shown to carry out all the reactions of the TCA cycle, electron transport, and oxidative phosphorylation

  12. Mitochondria Are Often Present Where the ATP Needs Are Greatest Mitochondria are found in virtually all aerobic cells of eukaryotes They are present in both chemotrophic and phototrophic cells Mitochondria are frequently clustered in regions of cells with the greatest need for ATP, e.g., muscle cells

  13. Are Mitochondria Interconnected Networks Rather than Discrete Organelles? In electron micrographs, mitochondria usually appear as oval structures However they can take various shapes and sizes, depending on the cell type Their appearance under EM suggests that they are large, and numerous discrete entities

  14. A challenge to the accepted view The work of Hoffman and Avers suggests that the oval profiles in electron micrographs of yeast cells reflect slices through a single large branched mitochondrion This work suggests that mitochondria may be larger than previously thought, and less numerous There is additional support for this idea

  15. The Outer and Inner Membranes Define Two Separate Compartments and Three Regions A distinctive feature of mitochondria is the presence of both outer and inner membranes The outer membrane contains porins that allow passage of solutes with molecular weights up to 5000 The intermembranespace between the inner and outer membranes is thus continuous with the cytosol

  16. The inner membrane • The inner membrane of the mitochondria is impermeable to most solutes, partitioning the mitochondrion into two separate compartments • The intermembrane space • The interior of the organelle, or mitochondrial matrix

  17. The inner boundary membrane and cristae The portion of the inner membrane adjacent to the intermembrane space is called the inner boundary membrane The inner membrane is about 75% protein by weight; the proteins include those involved in solute transport, electron transport, and ATP synthesis

  18. The cristae The inner membrane of most mitochondria has many infoldings called cristae They increase surface area of the inner membrane, and provide more space for electron transport to take place The cristae provide localized regions, intracristal spaces, where protons can accumulate during electron transport

  19. The cristae (continued) The cristae are thought to be tubular structures that associate in layers They have limited connections to the inner boundary membrane through small openings, crista junctions Cells with high metabolic activity seem to have more cristae in their mitochondria

  20. The mitochondrial matrix The interior of the mitochondrion is filled with a semi-fluid matrix The matrix contains many enzymes involved in mitochondrial function as well as DNA molecules and ribosomes Mitochondria contain proteins encoded by their own DNA as well as some that are encoded by nuclear genes

  21. Mitochondrial Functions Occur in or on Specific Membranes and Compartments Specific functions and pathways have been localized within mitochondria by fractionation studies Most of the enzymes involved in pyruvate oxidation, the TCA cycle, and catabolism of fatty acids and amino acids are found in the matrix Most electron transport intermediates are integral inner membrane components

  22. Table 10-1

  23. Localization of Specific Mitochondrial Functions Knoblike spheres called F1 complexes protrude from the inner membrane into the matrix These are involved in ATP synthesis Each complex is an assembly of several different polypeptides, and can be seen in an electron micrograph using negative staining

  24. F1 complexes Each F1 complex is attached by a short protein stalk to an Fo complex This is an assembly of hydrophobic polypeptides embedded in the mitochondrial inner membrane This FoF1 complex is an ATP synthase that is responsible for most of the ATP generation in the mitochondria (and in bacterial cells as well)

  25. In Bacteria, Respiratory Functions Are Localized to the Plasma Membrane and the Cytoplasm Bacteria do not have mitochondria but are capable of aerobic respiration Their plasma membrane and cytoplasm perform the same functions as the inner membrane and matrix of mitochondria

  26. The Tricarboxylic Acid Cycle: Oxidation in the Round In the presence of oxygen pyruvate is oxidized fully to carbon dioxide with the released energy used to drive ATP synthesis This involves the TCA (tricarboxylicacid) cycle, in which citrate is an important intermediate The TCA cycle is also called the Krebs cycle after Hans Krebs, whose lab played a key role in elucidating the cycle

  27. The Tricarboxylic Acid Cycle The TCA cycle metabolized acetyl CoA, produced from pyruvate decarboxylation Acetyl CoA can also arise from fatty acid oxidation Acetyl CoA transfers its acetate group to a four-carbon acceptor called oxaloacetate, generating citrate

  28. The fate of citrate After its formation, citrate undergoes two successive decarboxylations It also goes through several oxidation steps Eventually oxaloacetate is regenerated, and can accept two more carbons from acetyl CoA and the cycle begins again

  29. The overall cycle Each round of the TCA cycle involves the entry of two carbons, the release of two CO2, and the regeneration of oxaloacetate Oxidation occurs at five steps, four in the cycle itself and one when pyruvate is converted to acetyl CoA In each case, electrons are accepted by coenzymes

  30. Figure 10-6

  31. Pyruvate Is Converted to Acetyl Coenzyme A by Oxidative Decarboxylation The glycolytic pathway ends with pyruvate, which is small enough to enter the intermembrane space of the mitochondrion At the inner mitochondrial membrane, a specific symporter transports pyruvate into the matrix, along with a proton Then, pyruvate is converted to acetyl CoA by pyruvate dehydrogenase complex (PDH)

  32. Conversion of pyruvate The conversion is a decarboxylation because one carbon is liberated as CO2 It is also an oxidation because two electrons (and one proton) are transferred to NAD+ to form NADH Coenzyme A contains the B vitamin pantothenic acid Coenzyme A has a SH group that makes it a good carrier of acetate (and other organic acids)

  33. Conversion of pyruvate • The conversion of pyruvate to acetyl CoA can be summarized as follows:

  34. The TCA Cycle Begins with the Entry of Acetate as Acetyl CoA With each round of the TCA cycle, two carbon atoms enter in organic form as acetate and leave in inorganic form as carbon dioxide In the first reaction, TCA-1, the two-carbon acetate group is transferred from acetyl CoA to oxaloacetate (4C) to form citrate (6C) This reaction is catalyzed by citrate synthetase

  35. TCA-2 Citrate is converted to isocitrate in the second step of the TCA cycle The enzyme aconitase catalyzes the reaction Isocitrate has a hydroxyl group that is easily oxidized (or dehydrogenated) in step TCA-3

  36. Two Oxidative Decarboxylations Then Form NADH and Release CO2 Four of the eight steps in the TCA cycle are oxidations (TCA-3, TCA-4, TCA-6, TCA-8) These all involve coenzymes that enter in the oxidized form and leave in the reduced form The first two of these are decarboxylation steps, releasing one CO2 in each step

  37. TCA-3, TCA-4 Isocitrate is oxidized by isocitrate dehydrogenase to oxalosuccinate, with NAD+ as the electron acceptor Oxalosuccinate immediately undergoes decarboxylation to form a-ketoglutarate (5C)(TCA-3) a-ketoglutarate is oxidized to succinyl CoA, by a-ketoglutarate dehydrogenase (TCA-4)

  38. Direct Generation of GTP (or ATP) Occurs at One Step in the TCA Cycle So far in the cycle, two carbon atoms have entered and two have left (but not the same two carbons), and two molecules of NADH have been generated Succinyl CoA has been generated; like acetyl CoA it has a high-energy thioester bond The energy from hydrolysis of this bond is used to generate one ATP (bacteria, plants) or GTP(animals) (TCA-5)

  39. The Final Oxidative Reactions of the TCA Cycle Generate FADH2 and NADH Of the remaining three steps in the cycle, two are oxidations Succinate is oxidized to fumarate (TCA-6); this transfers electrons to FAD, a lower-energy coenzyme than NAD+ In the next step, fumarate is hydrated to produce malate (TCA-7) by fumarate hydratase

  40. TCA-8 In the final step of the TCA cycle, the hydroxyl group of malate becomes the target of the final oxidation in the cycle Electrons are transferred to NAD+, producing NADH as malate is converted to oxaloacetate (TCA-8)

  41. Summing Up: The Products of the TCA Cycle Are CO2, ATP, NADH, and FADH2 • The TCA cycle accomplishes the following: 1. Two carbons enter the cycle as acetyl CoA, which joins oxaloacetate to form the six-carbon citrate 2. Decarboxylation occurs at two steps to balance the input of two carbons by releasing two CO2 3. Oxidation occurs at four steps, with NAD+ the electron acceptor in three steps and FAD in one

  42. The Products of the TCA Cycle Are CO2, ATP, NADH, and FADH2 (continued) • The TCA cycle accomplishes the following: 4. ATP is generated at one point, with GTP as an intermediate in the case of animal cells 5. One turn of the cycle is completed as oxaloacetate, the original 4C acceptor, is regenerated

  43. Summing up the TCA cycle • The TCA cycle can be summarized as follows: • Including glycolysis, pyruvate decarboxylation, and the TCA cycle

  44. Several TCA Cycle Enzymes Are Subject to Allosteric Regulation Like all metabolic pathways, the TCA cycle must be carefully regulated to meet cellular needs Most of the control of the cycle involves allosteric regulation of four key enzymes by specific effector molecules Effector molecules may be activators or inhibitors

  45. Regulation of PDH PDH is reversibly inactivated by phosphorylation and activated by dephosphorylation of one of its protein components PDH is inhibited by ATP, which is abundant where energy is plentiful PDH and isocitrate dehydrogenase are activated by AMP and ADP, which are more abundant when energy is needed

  46. Regulation of acetyl CoA Overall availability of acetyl CoA is determined mainly by the activity of the PDH complex PDH is allosterically inhibited by ATP, NADH, and acetyl CoA and high [ATP/ADP] (enzyme: PDH kinase) It is activated by AMP, NAD+, and free CoA and low [ATP/ADP] (enzyme: PDH phosphatase)

  47. The TCA Cycle Also Plays a Central Role in the Catabolism of Fats and Proteins The TCA cycle represents the main conduit of aerobic energy metabolism for a variety of substrates besides sugar, in particular, fats and proteins

  48. Fat as a Source of Energy Fats are highly reduced compounds that liberate more energy per gram upon oxidation than do carbohydrates They are a long-term energy storage form for many organisms Most fat is stored as deposits of triacylglycerols, neutral triesters of glycerol and long-chain fatty acids

  49. Catabolism of triacylglycerols Triacylglycerol catabolism begins with their hydrolysis to glycerol and free fatty acids The glycerol is channeled into the glycolytic pathway by oxidative conversion to dihydroxyacetone phosphate Fatty acids are linked to coenzyme A, to form fatty acyl CoAs, then degraded by b-oxidation

  50. b-oxidation b-oxidation is a catabolic process that generates acetyl CoA and the reduced coenzymes NADH and FADH2 b-oxidation occurs in different compartments in different organisms Here, we will focus on the mitochondrion of animals using saturated fatty acids with an even number of carbons as an energy source

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