ATP Synthesis. Lecture 13. Dr. Neil Docherty

Transcription

1 PG1005 The Electron Transport Chain and ATP Synthesis Lecture 13 Dr. Neil Docherty

2 My Teaching Objectives Define and describe the electron transport chain Explain how electron transfer couples to proton gradient generation in the mitochondrial intermembrane space Ilustrate how the chemiosmotic model of ATP synthesis proceeds via rotational catalysis

3 ATP Requirements in the Body Daily requirement for around 83kg of ATP Body reserve is only 250g Solution: Recycling of ATP from ADP (300 times per day per molecule) Recycling in a quantitative sense in most cells depends upon the process of oxidative phosphorylation in the mitochondria

4 Oxidative Phosphorylation Summary KEY COMPONENTS 1) Electron pair transfer chain and reduction of oxygen 2) Proton pumping and development of proton gradient 3) Proton motive force down electrochemical gradient 4) The above drives enzymatic synthesis of ATP e - e - e - I II III IV OMM IMS + H+ H+ H + H + H +Pi ATP IMM O 2 H 2 OADP

5 Electron Transport Chain:Key Components 3 electron driven proton pumps NADH-Q-oxidoreductase-Complex I Q-cytochrome c oxidoreductase-complex d III Cytochrome c oxidase-complex IV 1 electron transfer only complex Succinate Q reductase-complex II (linked to SDH of KCAC) 2 Electron carrier molecules Ubiquinone inone (Q)-2 electron carrier Cytochrome c- Single electron carrier Final Electron Acceptor Oxygen

6 Arrangement of Electron Transport Chain H + to IMS tron af ffinity NADH 4 Complex I Inc creasin ng elec Q 2 Complex III 4 Cytochrome c Complex IV FADH 2 Complex II O 2

7 NADH-Q-oxidoreductase (Complex I) >900kDa L-shaped enzyme complex consisting of 46 polypeptide chains Electron transport t from NADH-FMN-Fe-S F centres-q Horizontal arm in membrane, long arm in matrix REACTION NADH + Q + 5H + (matrix) NAD + +QH 2 + 4H + (IMS) Reduction of Q involves Reduction of Q involves proton binding

8 Succinate-Q-reductase (Complex II) Succinate to Fumarate oxidation transfers electrons to FAD in KCAC FADH 2 Transfer to Fe-S centres Reduction of Q N.B. This is not a proton pump and therefore final contribution of FADH 2 oxidation to ATP generation is less than that for NADH

9 Q-Cytochrome c oxidase (Complex III) Homodimeric protein of 11 polypeptide p chains Haeme and Fe-S prosthetic groups for electron transfer (Q in membrane to Cytochrome c in IMS) Uses Q cycle to sequentially pass single electrons to Cytochrome c REACTION 2QH 2 + 2Cyt c (ox) 2Q + 2 Cyt c (red) + 4H + (IMS) THE Q CYCLE CONSECUTIVE BINDING OF 2QH 2

10 Cytochrome c oxidase (Complex IV) Large enzyme consisting of 13 polypeptide chains Haeme and copper prosthetic groups Binds four cytochrome c molecules consecutively for 4 electron transfer to molelcular oxygen (final acceptor) REACTION 4 Cyt c (red) + 8H + (matrix) + O 2 4 Cyt c (ox) +2H 2 O + 4H + (IMS) First two electrons reduce Fe and Cu centres Promoting oxygen binding and Fe-Cu peroxide bridge formation Subsequent addition of two remaining electrons and two proton cleaves the peroxide bridge leaving Fe-OH and Cu-OH Reaction with a further two protons leads to Release of 2 molecules of water

11 What about ATP Generation? The Chemiosmotic Model (Mitchell P 1961) The proton pumps establish -a ph (proton concentration gradient) 1.4 units -a voltage gradient (matrix negative) 140mV Flow of protons back into matrix provides the Flow of protons back into matrix provides the energy to run ATP synthesis

12 ATP Synthase (Complex V) Two major subunits F 0 and F 1 F 0 stick embedded in IMM F 1 protrudes into matrix (catalytic) Rotor (F 0 and gamma) and stator components (F 1) Protons enter through half channels in c and protonate aspartate residue. This allows c to rotate into hydrophobic environment and release proton to matrix side half channel Prevents F 1 rotation Linker coupled to rotation of F 0 Causes distinction in beta subunits by virtue of interaction with different face of gamma catalysis

13 Binding Change Mechanism of ATP synthesis ADP 3- +HPO 2- +H 4 + ATP 4- +HO 2 alpha gamma ADP + Pi -ATP synth beta Tight (O) TL T, and dc conformations are a consequence of gamma interaction Movement of protons through F 0 couples F 0 rotation to gamma rotation As gamma rotates it switches betas through T, L and C conformations Gamma rotation -120-degree counterclockwise per step -3 protons for one full turn (360-degree) =Output of 1 ATP

14 Complete Oxidation of Glucose:ATP Yield A net 2 ATP are generated from glycolysis l A further 2 ATP are directly generated in KCAC =Outwith oxidative phosphorylation h 4 ATP Less than anticipated 3:1 H + /ATP-results from H + usage for ATP export Oxidative Phosphorylation 1 NADH from glycolysis=6 protons=1.5 ATP X2=3 ATP 1 NADH from pyruvate oxidation=10 protons=2.5 ATP X2=5 ATP 1 NADH from KCAC =10 protons=2.5 ATP X6=15 ATP 1FADH 2 from KCAC = 6 protons=1.5 5ATP X2=3 ATP TOTAL= =30 ATP

15 Control of Electron Transport Electron transport does not proceed in the absence of ATP synthesis 1) ATP>ADP ADP reduced d flux 2) ADP>ATP increased flux

16 Your Learning Objectives Your learning from today should focus on being able to; Detail the sequence and mechanics of electron transport chain Explain how electron transfer couples to the generation of a proton gradient generation in the mitochondrial inter-membrane space Detail how the passive flow of protons at complex V drives ATP synthesis via rotational catalysis