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Unformatted text preview: Lecture 16
Overview: • oxidative phosphorylation & mitochondria • electron transfer • proton pumps and link to CAC • proton gradient – ATP synthesis • regulation HW: 18.1, 18.4, 18.24 Chapter 18: Oxidative Phosphorylation Overview of oxidative energy production oxidative phosphorylation energy released from electrons passing through the respiratory chain (mitochondrial electron transport chain) is coupled to the production of ATP (from ADP and Pi) Coupling of oxidation and ATP synthesis importance of transmembrane proton ﬂuxes Structural & functional composition of a mitochondrion oxidative phosphorylation in eukaryotes takes place in mitochondria Moving electrons requires carrier molecules • electrons are initially provided to 'universal electron acceptors' – nicotinamide nucleotides (NAD+ & NADP+) – ﬂavin nucleotides (FAD & FMN) • NAD(P)-linked dehydrogenases carry 2 electrons from oxidation of organic substrates – remove 2 hydrogen atoms (H+ and H–) & 2 electrons from substrate – NADH & NADPH are water-soluble electron carriers, and bind reversibly to dehydrogenases – NADH mainly used catabolically: transports electrons to respiratory chain to eventually yield ATP – NADPH mainly used anabolically: provides electrons for biosynthesis Moving electrons requires carrier molecules • ﬂavoproteins carry 1 or 2 electrons from oxidation of organic substrates – FAD & FMN are bound tightly, either covalently or noncovalently – unlike NAD cofactors, ﬂavin cofactors vary in their standard reduction potential, which is inﬂuenced by the protein to which they are bound • electrons will then pass to a series of membrane-bound carriers Membrane-bound electron carrier molecules • the respiratory chain consists primarily of a series of membrane-bound proteins with prosthetic groups that can accept & donate electrons – nicotinamide nucleotides (NAD+ & NADP+) – ﬂavin nucleotides (FAD & FMN) • three basic types of electron transfer in oxidative phosphorylation – direct electron transfer (ex: Fe3+ ! Fe2+ + e–) – hydrogen atom transfer (H+ + e–) – hydride ion transfer (H:–) Membrane-bound electron carrier molecules • there are three classes of electron transfer agents used after the nicotinamide and ﬂavin cofactors release their electrons – ubiquinone (also known as: coenzyme Q, or Q), a lipidsoluble organic compound – cytochromes, proteins with iron-containing prosthetic groups – iron-sulfur proteins, which contain Fe-S centers • overall electron ﬂow: – NADH/succinate ! ﬂavoproteins/ubiquinone/cytochromes/Fe-S proteins ! O2 Measurement of redox potential X– + H+ -> X + 1/2 H2 X– -> X + e– H+ + e– -> 1/2 H2 !G°" = –nF !E°"
Faraday constant 96.48 kJ/molV review pp. 507-508
Note: the standard reduction potentials (E°') can differ from that of the actual reduction potentials (E): the latter depends also on the cellular concentrations of the oxidized & reduced substances Sequence of electron carriers in the respiratory chain Four complexes: Three proton pumps (transport protons across inner membrane): NADH-Q oxidoreductase, Qcytochrome C oxidoreductase, cytochrome C oxidase Link to CAC: succinate-Q reductase (from FADH2) Electron transfer from NADH to O2: electron afﬁnity of the components increases as electrons move down the chain Respirasome: Complexes 1–III Complex I Complex II Complex III Complex IV Isolating the protein components of the respiratory chain Four major complexes in the respiratory chain Special electron carriers ferry electrons from one complex to the next. the proteins involved in the respiratory chain are part of large, independent complexes that can be experimentally isolated Ubiquinone ("Q", or "coenzyme Q") Ubiquinone uptake of two protons from the mitochondrial matrix upon reduction R = isoprene Q QH2 Heme prosthetic groups of cytochromes
cyto chro m e c is co valently b o nded to its pro sthetic hem e gro up Other important coenzymes cyto chro m es a and b are no nco valently b o nded to their pro sthetic hem e gro ups] fe r r e d o x in Iron-sulfur centers
Fe-S centers have several possible structural forms these proteins are involved in 1-electron transfers in which an iron atom in the center is oxidized or reduced (Fe2+, Fe3+) The respiratory chain: electron and proton passage through the four complexes 2Fe–2S 4Fe–4S Electron paths initiated by reduced cofactors Coupled electron-proton transfer reaction through NADH-Q oxidoreductase (Electron-Transferring Flavoprotein) from !-oxidation Complex I: NADH-Q (ubiquinone) oxidoreductase
electrons from NADH to ubiquinone
energy from electron passage is used to pump H+ into the intermembrane space to build up a proton gradient Complex II: succinate-Q reductase (succinate dehydrogenase)
recall: this is the 3rd oxidation in the citric acid cycle electrons from succinate to ubiquinone endergonic exergonic e le ctr o ch e m ica l p o te n tia l e sta b lish e d a cro ss m em brane only membranebound enzyme in the citric acid cycle complex I is a proton pump complex II is not a proton pump Citric acid cycle Complex III: cytochrome bc1 complex (Q-cytochrome c oxidoreductase) electrons from ubiquinone to cytochrome c The structure of cytochrome c is highly conserved Structure of Qcytochrome c oxidoreductase evolutionary tree constructed from cytochrome c sequences Homodimer with 11 distinct polypeptide chains; three hemes and a 2Fe–2S cluster; well positioned for mediating e– transfer between quinones in the membrane and cytochrome C in the intermembrane space Complex III: cytochrome bc1 complex The Q cycle The "Q" cycle Complex IV: cytochrome c oxidase
electrons from cytochrome c to O2 Structure of cytochrome c oxidase 13 polypeptide chains; two hemes Cytochrome oxidase mechanism Cycle begins and ends with prosthetic groups in their oxidized forms; reduced forms are red; four cyt c molecules donate 4 e–; import of H+; note role of O2 Complex IV: electron path The electron-transport chain The electron-transport chain; another view intermembrane space
(easily accessed from cytosol)
g ly c e r o l 3 - P
glycerol 3phosphate dehydrogenase FA D ETF:Q ETF
citric acid cycle reaction acyl-CoA dehydrogrenase fatty acyl-derived reducing equivalents enter here FA D matrix Distance dependence of electron-transfer rates Energy changes along the respiratory chain • The rate of electron transfer decreases as the electron donor and acceptor move apart. In a vacuum, the rate decreases by a factor of 10 for every increase of 0.8 Å. In proteins, the rate decreases more gradually, by a factor of 10 for every increase of 1.7 Å. This rate is only an approximation and depends on protein sequence, protein structure, medium, and other factors. • The rate of an electron-transfer reaction at ﬁrst increase as the driving force for the reaction increases. The rate reaches a maximum and then decreases at very large driving forces. Part B: A proton gradient powers the synthesis of ATP; Proton-motive force & the chemiosmotic model The chemiosmotic model & mitochondria
refers to the coupling of electron transfer by way of a build-up of an electrochemical H+ gradient across the inner membrane to ATP synthesis ATP synthase: F1 & Fo complexes Structure of ATP synthase
F1 comprised of 9 subunits of 5 types (!3"3#$%) F1 the 3 " sites are the location for ATP/ADP binding F1 (side view) F1 (from above) ATP synthase: F1 & Fo complexes ATP synthesis mechanism ADP3– + HPO42– + H+ ATP4– + H2O Binding-change mechanism for ATP synthase
F1 h a s 3 n o n e q u i v a l e n t A D P / A T P b in d in g sit e s (o n e fo r e a ch ! / " su b u n it p a ir ) a t e a c h m o m e n t in t im e , t h e sit e s h a v e t h e fo llo w in g o c c u p a n c y : 1 ) " - A T P - t ig h t -b in d in g t o A T P 2 ) " - A D P - lo o se -b in d in g t o A T P 3 ) " - e m p t y - v e r y lo o se b in d in g t h e ﬂ o w o f H + t h r o u g h Fo c a u s e s t h e !" s u b u n it s ( c y l in d e r ) a n d # ( s h a f t ) to rotate E a c h 1 2 0 ° r o t a t io n le a d s t o a n in t e r a ct io n b e t w e e n # a n d a n e w " su b u n it , a n d t h e in t e r a c t io n fo r c e s " in t o a n e m p t y co n fo r m a t io n fo r o n e c o m p le t e r o t a t io n , 3 A T P m o le cu le s a r e p r o d u ce d ATP-driven rotation of ATP synthase Overview of oxidative phosphorylation Sites of action of some inhibitors of electron transport Three stages of cellular respiration
1) glycolysis 2) citric acid cycle 3) oxidative phosphorylation ...
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