Chapter 19 Bioenergetics

1. Chapter 19 Bioenergetics How the Body Converts Food to Energy

2. Metabolism • Metabolism: the sum of all chemical reactions involved in maintaining the dynamic state of a cell or organism • pathway: a series of consecutive biochemical reactions • catabolism: the biochemical pathways that are involved in generating energy by breaking down large nutrient molecules into smaller molecules with the concurrent production of energy • anabolism: the pathways by which biomolecules are synthesized (use ATP energy to build larger molecules from smaller building blocks)

4. Metabolism • metabolism is the sum of catabolism and anabolism

5. Stages of Catabolism Catabolic reactions are organized into three stages: • In Stage 1, digestion breaks down large molecules into smaller ones that enter the bloodstream • In Stage 2, molecules enter the cells and are broken down into two- and three-carbon compounds • In Stage 3, compounds are oxidized in the citric acid cycle to provide energy (ATP) for anabolic processes

6. Stages of Catabolism

7. Cells and Mitochondria • Animal cells have many components, each with specific functions; some components along with one or more of their functions are: • nucleus: where replication of DNA takes place • lysosomes: remove damaged cellular components and some unwanted foreign materials • Golgi bodies: package and process proteins for secretion and delivery to other cellular components • mitochondria: responsible for generation of most of the energy for cells

8. Components of Eukaryotic Cells

9. A Rat Liver Cell

10. A Mitochondrion • Schematic of a mitochondrion cut to reveal its inner organization

11. Common Catabolic Pthwy • The two parts to the common catabolic pathway • citric acid cycle, also called the tricarboxylic acid or Krebs cycle • oxidative phosphorylation, also called the electron transport chain, or the respiratory chain • The four principal compounds participating in the common catabolic pathway are: • AMP, ADP, and ATP • NAD+/NADH • FAD/FADH2 • coenzyme A; abbreviated CoA or CoA-SH

12. Adenosine Triphosphate • ATP is the most important compound involved in the transfer of phosphate groups • ATP contains two phosphoric anhydride bonds and one phosphoric ester bond

13. Adenosine Triphosphate • hydrolysis of the terminal phosphate of ATP gives ADP, phosphate ion, and energy • hydrolysis of a phosphoric anhydride liberates more energy than hydrolysis of a phosphoric ester • we say that ATP and ADP contain high-energy phosphoric anhydride bonds • ATP is a universal carrier of phosphate groups • it is also a common currency for the storage and transfer of energy

14. Hydrolysis of ATP • The hydrolysis of ATP to ADP releases 7.3 kcal (31 kJ/mole) ATP  ADP + Pi + 7.3 kcal (31 kJ/mole) • The hydrolysis of ADP to AMP releases 7.3 kcal (31 kJ/mole) ADP  AMP + Pi + 7.3 kcal (31 kJ/mole)

15. Coenzymes NAD+/NADH2 • Nicotinamide adenine dinucleotide (NAD+) is a biological oxidizing agent

16. Coenzymes NAD+/NADH • NAD+ is a two-electron oxidizing agent, and is reduced to NADH • NADH is a two-electron reducing agent, and is oxidized to NAD+ • NAD+ and NADH are also hydrogen ion transporting molecules

17. Coenzymes NAD+/NADH • When a compound is oxidized by an enzyme, 2H+ and 2e- are removed by a coenzyme, which is reduced • NAD+ (nicotinamide adenine dinucleotide) participates in reactions that produce a carbon-oxygen double bond (C=O) • For example, NAD+ participates in the oxidation of ethanol:

18. Coenzymes FAD/FADH2 • Flavin adenine dinucleotide (FAD) is also a biological oxidizing agent

19. Coenzymes FAD/FADH2 • FAD is a two-electron oxidizing agent, and is reduced to FADH2 • FADH2 is a two-electron reducing agent, and is oxidized to FAD

20. Coenzymes FAD/FADH2 • FAD participates in reactions that produce a carbon-carbon double bond (C=C) Oxidation —CH2—CH2—  —CH=CH— + 2H+ + 2e- Reduction FAD + 2H+ + 2e- FADH2

21. Coenzyme A • Coenzyme A (CoA) is an acetyl-carrying group • like NAD+ and FAD, coenzyme A contains a unit of ADP • CoA is often written CoA-SH to emphasize the fact that it contains a sulfhydryl group • the vitamin part of coenzyme A is pantothenic acid • the acetyl group of acetyl CoA is bound as a high-energy thioester

22. Coenzyme A

23. Citric Acid Cycle • overview: the two carbon acetyl group of acetyl CoA is fed into the cycle and oxidized to 2 CO2 • there are four oxidation steps in the cycle

25. Citric Acid Cycle • Step 1: condensation of acetyl CoA with oxaloacetate • the high-energy thioester of acetyl CoA is hydrolyzed • this hydrolysis provides the energy to drive Step 1 • citrate synthase is an allosteric enzyme; it is inhibited by NADH, ATP, and succinyl-CoA

26. Citric Acid Cycle • Step 2: dehydration and rehydration, catalyzed by aconitase, gives isocitrate • citrate is achiral; it has no stereocenter • aconitate is also achiral • isocitrate is chiral; it has 2 stereocenters and 4 stereoisomers are possible • only one of the 4 possible stereoisomers is formed in the cycle

27. Citric Acid Cycle • Step 2 (cont’d): Citrate isomerizes to isocitrate • The tertiary –OH group in citrate is converted to a secondary –OH that can be oxidized

28. Citric Acid Cycle • Step 3: oxidation of isocitrate followed by decarboxylation gives a-ketoglutarate • isocitrate dehydrogenase is an allosteric enzyme; it is inhibited by ATP and NADH, and activated by ADP and NAD+

29. Citric Acid Cycle • Step 4: oxidative decarboxylation of -ketoglutarate to succinyl-CoA • the two carbons of the acetyl group of acetyl CoA are still present in succinyl CoA and in succinate • this multienzyme complex is inhibited by ATP, NADH, and succinyl CoA; it is activated by ADP and NAD+

30. Citric Acid Cycle • Step 5: formation of succinate • the two CH2-COO- groups of succinate are now equivalent • this is the first energy-yielding step of the cycle; a molecule of GTP is produced

31. Citric Acid Cycle • Step 6: oxidation of succinate to fumarate • Step 7: hydration of fumarate to L-malate • L-malate is chiral and can exist as a pair of enantiomers; it is produced in the citric acid cycle as a single stereoisomer

32. Citric Acid Cycle • Step 8: oxidation of malate • oxaloacetate now can react with acetyl CoA to start another round of the cycle by repeating Step 1

33. Citric Acid Cycle In one turn of the citric acid cycle: • Two decarboxylations remove two carbons as 2CO2 • Four oxidations provide hydrogen for 3NADH and one FADH2 • A direct phosphorylation forms GTP which is used to form ATP Overall reaction of citric acid cycle:

34. Citric Acid Cycle • Control of the cycle • controlled by three feedback mechanisms • citrate synthase: inhibited by ATP, NADH, and succinyl CoA; also product inhibition by citrate • isocitrate dehydrogenase: activated by ADP and NAD+, inhibited by ATP and NADH • -ketoglutarate dehydrogenase complex: inhibited by ATP, NADH, and succinyl CoA; activated by ADP and NAD+

35. CA Cycle in Catabolism • The catabolism of proteins, carbohydrates, and fatty acids all feed into the citric acid cycle at one or more points

36. Electron Carriers • The electron transport chain consists of electron carriers that accept H+ ions and electrons from the reduced coenzymes NADH and FADH2 • The H+ ions and electrons are passed down a chain of carriers until in the last step they combine with oxygen to form H2O • Oxidative phosphorylation is the process by which the energy from transport is used to synthesize ATP

37. Oxidative Phosphorylation • Carried out by four closely related multisubunit membrane-bound complexes and two electron carriers, coenzyme Q and cytochrome c • in a series of oxidation-reduction reactions, electrons from FADH2 and NADH are transferred from one complex to the next until they reach O2 • O2 is reduced to H2O • as a result of electron transport, protons are pumped across the inner membrane to the intermembrane space

38. Electron Transport System • The electron carriers in the electron transport system are attached to the inner membrane of the mitochondrion • They are organized into four protein complexes: Complex I NADH dehydrogenase Complex II Succinate dehydrogenase Complex III CoQ-Cytochrome c reductase Complex IV Cytochrome c Oxidase

39. Electron Transport Chain

40. Complex I • The sequence starts with complex I • this large complex contains some 40 subunits, among them are a flavoprotein, several iron-sulfur (FeS) clusters, and coenzyme Q (CoQ, ubiquinone) • complex I oxidizes NADH to NAD+ • the oxidizing agent is CoQ, which is reduced to CoQH2 • some of the energy released in this reaction is used to move 2H+ from the matrix into the intermembrane space

41. Complex II • complex II oxidizes FADH2 to FAD • the oxidizing agent is CoQ, which is reduced to CoQH2 • the energy released in this reaction is not sufficient to pump protons across the membrane

42. Complex III • complex III delivers electrons from CoQH2 to cytochrome c (Cyt c) • this integral membrane complex contains 11 subunits, including cytochrome b, cytochrome c1, and FeS clusters • complex III has two channels through which the two H+ from CoQH2 are pumped from the matrix into the intermembrane space

43. Complex IV • complex IV is also known as cytochrome oxidase • it contains 13 subunits, one of which is cytochrome a3 • electrons flow from Cyt c (oxidized) in complex III to Cyt a3 in complex IV • from Cyt a3 electrons are transferred to O2 • during this redox reaction, H+ are pumped from the matrix into the intermembrane space • Summing the reactions of complexes I - IV, six H+ are pumped out per NADH and four H+ per FADH2

44. Coupling of Ox and Phos • To explain how electron and H+ transport produce the chemical energy of ATP, Peter Mitchell proposed the chemiosmotic theory • the energy-releasing oxidations give rise to proton pumping and a pH gradient across the inner mitochondrial membrane • there is a higher concentration of H+ in the intermembrane space than inside the mitochondrion • this proton gradient provides the driving force to propel protons back into the mitochondrion through the enzyme complex called proton translocating ATPase

45. Coupling of Ox and Phos • protons flow back into the matrix through channels in the F0 unit of ATP synthase • the flow of protons is accompanied by formation of ATP in the F1 unit of ATP synthase • The functions of oxygen are: • to oxidize NADH to NAD+ and FADH2 to FAD so that these molecules can return to participate in the citric acid cycle • provide energy for the conversion of ADP to ATP

46. Coupling of Ox and Phos • The overall reactions of oxidative phosphorylation are:

47. The Energy Yield • A portion of the energy released during electron transport is now built into ATP • for each two-carbon acetyl unit entering the citric acid cycle, we get three NADH and one FADH2 • for each NADH oxidized to NAD+, we get three ATP • for each FADH2 oxidized to FAD, we get two ATP • thus, the yield of ATP per two-carbon acetyl group oxidized to CO2 is

48. Other Energy Forms • The chemical energy of ATP is converted by the body to several other forms of energy • Electrical energy • the body maintains a K+ concentration gradient across cell membranes; higher inside and lower outside • it also maintains a Na+ concentration gradient across cell membranes; lower inside, higher outside • Special transport proteins in cell membranes constantly pump K+ into and Na+ out of the cells • this pumping requires energy, which is supplied by the hydrolysis of ATP to ADP • thus, the chemical energy of ATP is transformed into electrical energy, which operates in neurotransmission

49. Other Forms of Energy • Mechanical energy • ATP drives the alternating association and dissociation of actin and myosin and, consequently, the contraction and relaxation of muscle tissue • Heat energy • hydrolysis of ATP to ADP yields 7.3 kcal/mol • some of this energy is released as heat to maintain body temperature

50. Bioenergetics End Chapter 19