Biological oxidation and oxidative phosphorylation


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Biological oxidation, electron transport chain and oxidative phosphorylation- An overview

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Biological oxidation and oxidative phosphorylation

  1. 1. BIOLOGICAL OXIDATION ANDELECTRON TRANSPORT CHAIN Biochemistry for Medics Biochemistry For Medics 9/30/2012 1
  2. 2. RESPIRATIONOrganisms can be classified based on themechanism to obtain energy-Autotrophs: are able to produce their own organicmolecules through photosynthesisHeterotrophs: live on organic compounds producedby other organisms ( e.g. Starch/Glucose)All organisms use cellular respiration to extractenergy from organic molecules. Biochemistry For Medics 9/30/2012 2
  3. 3. RESPIRATIONRespiration – a process by which cells derive energy with a controlled reaction between H+ and O2; the end product being water.Aerobic organisms are able to capture a far greater proportion of the available free energy of respiratory substrates than anaerobic organisms. Biochemistry For Medics 9/30/2012 3
  4. 4. RESPIRATIONThe objective of respiration is toproduce ATP.Energy is released from oxidationreactions in the form of electronsElectrons are shuttled by electroncarriers (e.g. NAD+) to an electrontransport chainElectron energy is converted to ATP inthe electron transport chain Biochemistry For Medics 9/30/2012 4
  5. 5. METABOLISM Metabolism is the sum of the chemical reactions in an organism. Catabolism is the energy-releasing processes. Anabolism is the energy-using processes. Catabolism provides the building blocks and energy for anabolism. Biochemistry For Medics 9/30/2012 5
  6. 6. METABOLISM Biochemistry For Medics 9/30/2012 6
  7. 7. ATP COUPLES ENERGY BETWEENCATABOLISM AND ANABOLISM Energy available for work & chemical synthesis (e.g. movement, signal amplification, anabolism etc.ATP ADP + Pi Energy from food (fuel molecules) or from catabolism photosynthesis Biochemistry For Medics 9/30/2012 7
  8. 8. BIOLOGICAL OXIDATION Biochemistry For Medics 9/30/2012 8
  9. 9. CELLULAR METABOLISM fats polysaccharides proteins Part 1: Breakdown of large fatty acids andmacromolecules to simple simple sugars amino acids glycerol subunits glucose Part 2: Breakdown of simple ATP glycolysis subunits to acetyl CoA accompanied by NADH production of limitedamounts of ATP and NADH pyruvate Acetyl CoA CoA Citric Part 3: acid Complete oxidation of 2 CO2 acetyl CoA to H2O and CO2 cycleaccompanied by production of large amounts of NADH 8 e- (Reducing power as NADH) and ATP in mitochondrion oxidative O2 phosphorylation H2 O ATP Biochemistry For Medics 9/30/2012 9
  10. 10. ATP IS THE PRINCIPAL CARRIER OFCHEMICAL ENERGY IN THE CELL! High Energy compound. Go’ = -7.3 kcal/mol  Major activities promoted by ATP:  -locomotion  -membrane transport  -signal transduction  -keeping materials in the cell  -nucleotide synthesis Biochemistry For Medics 9/30/2012 10
  11. 11. ATP- AN INTERMEDIATE HIGHENERGY COMPOUNDStandard free energy of hydrolysis of somephosphorylated compounds Biochemistry For Medics 9/30/2012 11
  12. 12. BIOLOGICAL OXIDATIONInvolves the transfer of electronsoxidation being termed for the removal ofelectrons & reduction for gain of electronsOxidation is always accompanied by reduction ofan e- acceptor Biochemistry For Medics 9/30/2012 12
  13. 13. ENZYMES INVOLVED IN OXIDATIONAND REDUCTION REACTIONSAre called as Oxidoreductases which include :oxidases, dehydrogenases, hydroperoxidases andoxygenases.Oxidases use oxygen as an electron acceptorDehydrogenases can’t use O2 as an electron acceptorHydroperoxidases use H2O2 as a substrateOxygenases catalyze the direct transfer of O2 into thesubstrateOxidases & dehydrogenases are involved inrespiration; hydroperoxidases neutralize free radicals &oxygenases are involved in biotransformation reactions. Biochemistry For Medics 9/30/2012 13
  14. 14. OXIDASESCatalyze the removal of hydrogen from a substratewith the involvement of oxygen as a H –acceptor, forming water or hydrogen peroxide.Exist in two different forms :some of them are copper containing suchas, Cytochrome oxidase , the terminal component ofETC which transfer the e - finally to O2.Other are flavoproteins such as , L – amino acidoxidase (FMN linked) and Xanthine oxidase (FADlinked) Biochemistry For Medics 9/30/2012 14
  15. 15. DEHYDROGENASESPerform 2 main functions:Transfer hydrogen from one substrate to another in acoupled Oxidation /Reduction reactionAs components of Electron transport chain such ascytochromesDehydrogenases use coenzymes – nicotinamides &riboflavin - as hydrogen carriers Nicotinamides can be in the form of NAD + or NADP+ Riboflavin can be – FMN or FAD same as oxidases Biochemistry For Medics 9/30/2012 15
  16. 16. HYDROPEROXIDASES Includes 2 sets of enzymes : catalases andperoxidases Peroxidases reduce H2O2 at the expense ofseveral other substances H2O2 + AH2  2H2O + A Catalases uses H2O2 as electron acceptor &electron donor 2H2O2  2H2OPeroxisomes are rich in oxidases and catalases Biochemistry For Medics 9/30/2012 16
  17. 17. OXYGENASESCatalyze the incorporation of O2 into substrates in 2 steps Oxygen is bound to the active site of the enzyme Bound O2 is reduced or transferred to the substrateConsist of two sets of enzymes1. Dioxygenases : incorporate both atoms of oxygen into the substrate ; A + O2  AO22. Monooxygenases : incorporates one atom of oxygen into the substrate & the other is reduced to water A – H + O2 + ZH2  A – OH + H2O + Z Biochemistry For Medics 9/30/2012 17
  18. 18. MITOCHONDRIONMitochondria, havebeen termed the"powerhouses" ofthe cell since the finalenergy release takesplace in themitochondria only.Mitochondria havean outer membranethat is permeable tomost metabolites, aninner membrane thatis selectivelypermeable, enclosinga matrix within . Biochemistry For Medics 9/30/2012 18
  19. 19. MITOCHONDRIONThe outer membrane is characterized by thepresence of various enzymes, including acyl-CoAsynthetase and glycerol phosphatedehydrogenase.Adenylyl kinase and creatine kinase are foundin the intermembrane space. The phospholipid cardiolipin is concentrated inthe inner membrane together with the enzymesof the respiratory chain, ATP synthase and variousmembrane transporters. The matrix encloses the enzymes of TCA cycle,beta oxidation and pyruvate dehydrogenasecomplex. Biochemistry For Medics 9/30/2012 19
  20. 20. MITOCHONDRION Biochemistry For Medics 9/30/2012 20
  21. 21. THE GENERATION OF ATPATP is generated by the phosphorylation of ADP Biochemistry For Medics 9/30/2012 21
  22. 22. THE GENERATION OF ATPATP can be generated either by-Substrate level phosphorylation orBy Oxidative phosphorylation Biochemistry For Medics 9/30/2012 22
  23. 23. SUBSTRATE-LEVELPHOSPHORYLATIONSubstrate-level phosphorylation is the transfer of ahigh-energy PO4- to ADP at the expense of the energy ofthe substrate without involving the electron transportchain.The substrate has higher energy level than theproduct, the surplus energy is used up for ATPformation. Biochemistry For Medics 9/30/2012 23
  24. 24. SUBSTRATE LEVELPHOSPHORYLATIONSubstrate-level phosphorylation – transferring aphosphate directly to ADP from another molecule Biochemistry For Medics 9/30/2012 24
  25. 25. OXIDATIVE PHOSPHORYLATIONOxidative phosphorylation is the process by whichthe energy stored in NADH and FADH2 is used toproduce ATP.A. Oxidation step: electron transport chain NADH + H+ + O2 NAD+ + H2O FADH2 + O2 FAD + H2OB. Phosphorylation step ADP + Pi ATP Biochemistry For Medics 9/30/2012 25
  26. 26. EXPRESSING REDOX REACTIONS ASHALF REACTIONSE.g. Fe 2+ + Cu 2+ = Fe 3+ + Cu +which can be expressed in the form of 2 half reactions1. Fe 2+ = Fe 3+ + e- (oxidized); Fe 2+ = reducing agent2. Cu 2+ + e- = Cu + (reduced) ; Cu 2+ = oxidizing agentReducing agent = e- donating moleculeOxidizing agent = e- accepting moleculeThey together make a conjugate redox pair. Biochemistry For Medics 9/30/2012 26
  27. 27. REDOX POTENTIALAlso known as oxidation reduction potentialRedox potential of any substance is a measure ofits affinity for electrons In O/R reactions the free energy change isproportional to the tendency of reactants to donate/ accept e-s denoted by Eo’A reaction with a + ve  Eo’ has a – ve Go’(exergonic)The redox potential of a biological system isusually compared with the potential of H electrodeexpressed at pH 7.0 Biochemistry For Medics 9/30/2012 27
  28. 28. REDOX REACTIONSDuring redox reactions, electrons carry energy fromone molecule to another.1) NAD+ as an electron carrier.NAD accepts 2 electrons and 1 proton to becomeNADH The reaction is reversible. Biochemistry For Medics 9/30/2012 28
  29. 29. TRANSFER OF ELECTRONSCan take place by any of the 4 different ways:1. Directly as e – s : Transfer of an e – from Fe2+ / Fe3+ to Cu+/ Cu2+ (Fe2+ + Cu2+ = Cu+ +Fe3+ )2. As H – atom : AH2  A + 2e - + 2H+ ; where AH2 & A make a conjugate redox pair and posses the tendency to reduce a next compd. B ( B/BH2 = redox pair) AH2 + B  A + BH23. As a hydride ion (:H- which has 2 electrons) : AH + H+  A+ + :H - + H+4. Direct combination with Molecular oxygen A – H + ½O2 = A – OH A + O2 = AO2 Biochemistry For Medics 9/30/2012 29
  30. 30. NICOTINAMIDE COENZYME: NAD+Always a 2-electron reaction transferring 2 e- and 2 H+ Biochemistry For Medics 9/30/2012 30
  31. 31. THE FLAVIN COENZYMES /FLAVOPROTEINS OH OH OH O CH2 CH CH CH CH2 O P OH Always a 2-electron H3C N N O OH reaction - riboflavin monophosphate transferring 2 e N (flavin mononucleotide, FMN) 2 H+ H3C N and O NH2 N N OH OH OH O O CH2 CH CH CH CH2 O P O P O CH2 O N N H3C N N O OH OH flavin adenine dinucleotide (FAD) N H3C N OH OH O Biochemistry For Medics 9/30/2012 31
  32. 32. OXIDATION AND REDUCTION OFFLAVIN COENZYMESIt can accept/donate 1 or 2 e-. FMN has an important role inmediating e- transfer between carriers that transfer 2 e- (e.g.,NADH) and those that transfer 1 e- (e.g., Fe+++). Biochemistry For Medics 9/30/2012 32
  33. 33. ELECTRON TRANSPORT CHAIN The electron transport chain (ETC) is a series ofmembrane-bound electron carriers. Embedded in the mitochondrial inner membrane Electrons from NADH and FADH2 are transferredto complexes of the ETC Each complex transfers the electrons to the nextcomplex in the chain Biochemistry For Medics 9/30/2012 33
  34. 34. ELECTRON TRANSPORT CHAIN -SERIES OF REDOX REACTIONS Biochemistry For Medics 9/30/2012 34
  35. 35. COMPLEXES OF ELECTRONTRANSPORT CHAIN Electrons flow through the respiratory chain through a redox span of 1.1 V from NAD+/NADH to O2/2H2O passing through three large protein complexes; NADH-Q oxidoreductase (Complex I), where electrons are transferred from NADH to coenzyme Q (Q) (also called ubiquinone); Q-cytochrome c oxidoreductase (Complex III), which passes the electrons on to cytochrome c; and Cytochrome c oxidase (Complex IV), which completes the chain, passing the electrons to O2 and causing it to be reduced to H2O . Biochemistry For Medics 9/30/2012 35
  36. 36. COMPLEXES OF ELECTRONTRANSPORT CHAIN Some substrates with more positive redox potentials than NAD+/NADH (eg, succinate) pass electrons to Q via, succinate Q reductase (Complex II), rather than Complex I. The four complexes are embedded in the inner mitochondrial membrane, but Q and cytochrome c are mobile. Q diffuses rapidly within the membrane, while Cytochrome c is a soluble protein. The flow of electrons through Complexes I, III, and IV results in the pumping of protons from the matrix across the inner mitochondrial membrane into the intermembrane space Biochemistry For Medics 9/30/2012 36
  37. 37. ELECTRON CARRIERS IN ETC Biochemistry For Medics 9/30/2012 37
  38. 38. FLAVOPROTEINSFlavoproteins are important components of ComplexesI and II. The oxidized flavin nucleotide (FMN or FAD) canbe reduced in reactions involving the transfer of twoelectrons (to form FMNH2 or FADH2), but they can alsoaccept one electron to form the semiquinone Biochemistry For Medics 9/30/2012 38
  39. 39. IRON-SULFUR CENTERS (CLUSTERS)Iron-sulfur centers (Fe-S) are prosthetic groups containing 1-4 iron atomsIron-sulfur centers transfer only one electron, even if they contain two ormore iron atoms. Biochemistry For Medics 9/30/2012 39
  40. 40. UBIQUINONE Coenzyme Q (CoQ, Q or ubiquinone) is lipid-soluble. It dissolves in the hydrocarbon core of a membrane. •the only electron carrier not bound to a protein. it can accept/donate 1 or 2 e-. Q can mediate e- transfer between 2 e- that transfer and 1 e- carriersFree CoQ can undergo a 2 e- oxidation/reduction:Q + 2 e- + 2 H+  QH2.When bound to special sites in respiratory complexes, CoQcan accept 1 e- to form a semiquinone radical (Q•-). Biochemistry For Medics 9/30/2012 40
  41. 41. CYTOCHROMES Biochemistry For Medics 9/30/2012 41
  42. 42. CYTOCHROMESCytochromes are electron carrierscontaining heme .Heme in the 3 classes of cytochromes (a, b,c) differ in substituents on the porphyrin ring.Some cytochromes(b,c1,a,a3) are part oflarge integral membrane protein complexes.Cytochrome c is a small, water-solubleprotein. Biochemistry For Medics 9/30/2012 42
  43. 43. STRUCTURE OF CYTOCHROMEHeme is a prosthetic group of cytochromes.Heme contains an iron atom in a porphyrin ringsystem.The heme iron can undergo 1 e- transitionbetween ferric and ferrous states: Fe3+ + e- Fe2+Copper ions besides two heme A groups (a and a3)act as electron carriers in Cyta,a3Cu2++e-  Cu+ Biochemistry For Medics 9/30/2012 43
  44. 44. ELECTRON CARRIERSNAD+, flavins and Co Q carry electrons and H+ Cytochromes and non-haem iron proteins carryonly electrons.NAD+ FAD undergoes only a 2 e- reaction;Cytochromes undergo only 1e- reactionsFMN Q undergoes 1e- and 2 e- reaction Biochemistry For Medics 9/30/2012 44
  45. 45. ORDER AND REDOX POTENTIALS -0.32 -0.3 +0.045 +0.03 +0.077 +0. 55 +0. 22 +0. 25 +0. 29 +0.82The electron carriers are arranged in terms of risingredox potential in the electron transport chain Biochemistry For Medics 9/30/2012 45
  46. 46. Q CYCLE :THE MECHANISM OF H+TRANSPORT IN COMPLEX III Biochemistry For Medics 9/30/2012 46
  47. 47. SITE SPECIFIC INHIBITORS OF ETC Amobarbital, Rotenone, Piericidin AMalonate, GuanethidineCarboxin, TTFA Antimycin A, BAL, Hypoglycemi c agents CN, CO, H2S, Azide Biochemistry For Medics 9/30/2012 47
  48. 48. INHIBITORS OF ETC Barbiturates such as Amobarbital inhibit electron transport via Complex I by blocking the transfer from Fe-S to Q. At sufficient dosage, they are fatal in vivo. Antimycin A and dimercaprol inhibit the respiratory chain at Complex III. The classic poisons H2S, carbon monoxide, and cyanide inhibit Complex IV and can therefore totally arrest respiration. Malonate is a competitive inhibitor of Complex II. Atractyloside inhibits oxidative phosphorylation by inhibiting the transporter of ADP into and ATP out of the mitochondrion . Biochemistry For Medics 9/30/2012 48
  49. 49. H+ /PROTON TRANSPORTComplexes I, III, and IV act as proton pumps. Since the innermitochondrial membrane is impermeable to ions in general andparticularly to protons, these accumulate in the intermembranespace, creating the proton motive force predicted by thechemiosmotic theory. Biochemistry For Medics 9/30/2012 49
  50. 50. PROTON TRANSPORT 4H+ are pumped per 2e- passing through complex III. The H+/e- ratio is less certain for the other complexes: probably 4H+/2e- for complex I; 2H+/2e- for complex IV. Biochemistry For Medics 9/30/2012 50
  51. 51. ATP SYNTHASE COMPLEXATP synthase is embedded in the inner membrane,together with the respiratory chain complexes .Several subunits of the protein form a ball-likeshape arranged around an axis known as F1, whichprojects into the matrix and contains thephosphorylation mechanism .F1 is attached to a membrane protein complexknown as F0, which also consists of several proteinsubunits.F0 spans the membrane and forms a protonchannel.The flow of protons through F0 causes it to rotate,driving the production of ATP in the Medicscomplex. Biochemistry For F 1 9/30/2012 51
  52. 52. ATP SYNTHASE COMPLEX The enzyme complex consists of an F0 subcomplex which is a disk of "C" protein subunits.  Attached is a Υ subunit in the form of a "bent axle." Protons passing through the disk of "C" units cause it and the attached Υ subunit to rotate.  The Υ subunit fits inside the F1 sub complex of three α and three βsubunits, which are fixed to the membrane and do not rotate. Biochemistry For Medics 9/30/2012 52
  53. 53. PROTON MOTION AND ROTATION OFC RING Biochemistry For Medics 9/30/2012 53
  54. 54. OXIDATIVE PHOSPHORYLATION-CHEMIOSMOSIS As the electrons are transferred, some electronenergy is lost with each transfer. This energy is used to pump protons (H+) acrossthe membrane from the matrix to the innermembrane space. A proton gradient is established. Biochemistry For Medics 9/30/2012 54
  55. 55. OXIDATIVE PHOSPHORYLATION- CHEMIOSMOSISThe higher negative charge in the matrix attracts theprotons (H+) back from the intermembrane space to thematrix.The accumulation of protons in the intermembranespace drives protons into the matrix via diffusion.Most protons move back to the matrix through ATPsynthase.ATP synthase uses the energy of the proton gradientto synthesize ATP from ADP + Pi. Biochemistry For Medics 9/30/2012 55
  56. 56. CHEMIOSMOTIC HYPOTHESIS Biochemistry For Medics 9/30/2012 56
  57. 57. OXIDATIVE PHOSPHORYLATION The chemiosmotic theory, proposed by Peter Mitchell in 1961, postulates that the two processes are coupled by a proton gradient across the inner mitochondrial membrane so that the proton motive force caused by the electrochemical potential difference (negative on the matrix side) drives the mechanism of ATP Biochemistry For Medics 9/30/2012 57 synthesis.
  58. 58. OXIDATIVE PHOSPHORYLATION Biochemistry For Medics 9/30/2012 58
  59. 59. ATP/ ADP EXCHANGE ATP/ADP exchange transporter is inhibited by Atractyloside and Bongregate Biochemistry For Medics 9/30/2012 59
  60. 60. UNCOUPLERS OF OXIDATIVEPHOSPHORYLATIONUncouplers dissociate oxidation in the respiratory chainfrom phosphorylation.These compounds are toxic in vivo, causing respiration tobecome uncontrolled, since the rate is no longer limited bythe concentration of ADP or Pi.2,4-dinitrophenol2, 4- dinitrocresolCCCPTCCPValinomycinHigh dose of AspirinThe antibiotic oligomycin completely blocks oxidation andphosphorylation by blocking the flow of protons through ATPsynthase Biochemistry For Medics 9/30/2012 60
  61. 61. UNCOUPLERS OF OXIDATIVEPHOSPHORYLATIONPhysiological UncouplersLong chain fatty acidsThyroxinBrown Adipose tissue-Thermogenin (orthe uncoupling protein) is a physiologicaluncoupler found in brown adipose tissuethat functions to generate bodyheat, particularly for the newborn andduring hibernation in animalsCalcium ions Biochemistry For Medics 9/30/2012 61
  62. 62. MECHNISM OF ACTION OFUNCOUPLING PROTEIN-THERMOGENIN Proton gradient is dissipated, no ATP formation Biochemistry For Medics 9/30/2012 62
  63. 63. FREE ENERGY RELEASE AND ATP FORMATIONΔ Go = -n F Δ EoΔ Go‘= Free energy changen= Number of electrons transferred in the redox reaction (2)F = Faraday’s constant (23.06 Kcal/mol)Δ E o‘ = E o(acceptor) – E o(donor)- 1.14 VΔGo = -2x23.06x1.14 = - 52.6 K cal/molThe free energy change between NAD + and Water is equalto 52.6 Kcal/mol, it is release in small increments, so that thisenergy can be used as chemical bond energy.The synthesis of one molecule of ATP requires an input of 8Kcal/mol.The total energy trapped is 24 kcal/mol,About 40 % of energy is trapped , the rest is lost in the formof heat for the maintenance of body temperature. Biochemistry For Medics 9/30/2012 63
  64. 64. P: O RATIODefined as the number of inorganicphosphate molecules incorporated in to ATPfor every atom of oxygen consumed.Oxidation of NADH yields 3 ATPmolecules(P: O ratio 3, Latest concept 2.5)Oxidation of FADH2 yields 2 ATP molecules(P: O ratio 2, Latest concept 1.5) Biochemistry For Medics 9/30/2012 64
  65. 65. REGULATION OF ATP SYNTHESISThe rate of respiration of mitochondria can becontrolled by the availability of ADP.This is because oxidation and phosphorylation aretightly coupled; ie, oxidation cannot proceed via therespiratory chain without concomitantphosphorylation of ADP.This is called respiratory control or acceptorcontrol. Biochemistry For Medics 9/30/2012 65
  66. 66. SUBSTRATE SHUTTLESOxidation of Extra mitochondrial NADH Is Mediated bySubstrate Shuttles.NADH cannot penetrate the mitochondrial membrane, butit is produced continuously in the cytosol by 3-phosphoglyceraldehyde dehydrogenase, an enzyme in theglycolysis sequence.However, under aerobic conditions, extra mitochondrialNADH does not accumulate and is presumed to be oxidizedby the respiratory chain in mitochondria.The transfer of reducing equivalents through themitochondrial membrane requires substrate pairs, linked bysuitable dehydrogenases on each side of the mitochondrialmembrane Biochemistry For Medics 9/30/2012 66
  67. 67. SUBSTRATE SHUTTLES Two main shuttle systems are of importance Glycerophosphate shuttle – present in skeletal muscle and brain Malate shuttle – present in liver, kidney and of more universal utility Biochemistry For Medics 9/30/2012 67
  68. 68. GLYCEROL-3-PHOSPHATE SHUTTLE Biochemistry For Medics 9/30/2012 68
  69. 69. MALATE ASPARTATE SHUTTLE Biochemistry For Medics 9/30/2012 69
  70. 70. MITOCHONDRIAL INNER MEMBRANETRANSPORTERS Mitochondrial inner membrane is selectively permeable. The movement of biomolecules is mediated through specific transporters Biochemistry For Medics 9/30/2012 70
  71. 71. CLINICAL ASPECTThe condition known as fatal infantile mitochondrialmyopathy and renal dysfunction involves severe diminutionor absence of most oxidoreductases of the respiratory chain. MELAS (mitochondrial encephalopathy, lactic acidosis, andstroke) is an inherited condition due to NADH:Qoxidoreductase (Complex I) or cytochrome oxidase (ComplexIV) deficiency. It is caused by a mutation in mitochondrial DNA and maybe involved in Alzheimers disease and diabetes mellitus.A number of drugs and poisons act by inhibition ofoxidative phosphorylation. Biochemistry For Medics 9/30/2012 71
  72. 72. SUMMARY Virtually all energy released from the oxidation of carbohydrate, fat, and protein is made available in mitochondria as reducing equivalents (—H or e–). These are funneled into the respiratory chain, where they are passed down a redox gradient of carriers to their final reaction with oxygen to form water. The redox carriers are grouped into four respiratory chain complexes in the inner mitochondrial membrane. Three of the four complexes are able to use the energy released in the redox gradient to pump protons to the outside of the membrane, creating an electrochemical potential between the matrix and the inner membrane space. ATP synthase spans the membrane and acts like a rotary motor using the potential energy of the proton gradient or proton motive force to synthesize ATP from ADP and Pi. In this way, oxidation is closely coupled to phosphorylation to meet the energy needs of the cell. Biochemistry For Medics 9/30/2012 72