Summary

This lecture covers the electron transport chain (ETC), a crucial process in cellular respiration. The document details the four complexes involved, mobile carriers like coenzyme Q and cytochrome c, and the Q cycle. It also describes the movement of protons and how ATP is generated.

Full Transcript

Mitochondrial electron transport: 4 inner mitochondrial membrane complexes 2 mobile e- carriers (Coenzyme Q, cyt c) Mitochondrial electron transport, ATP synthesis complexes Red = mito Green = nuclear Mobile electron carriers Coenzyme Q (Ubiquinone) = m...

Mitochondrial electron transport: 4 inner mitochondrial membrane complexes 2 mobile e- carriers (Coenzyme Q, cyt c) Mitochondrial electron transport, ATP synthesis complexes Red = mito Green = nuclear Mobile electron carriers Coenzyme Q (Ubiquinone) = mobile Cytochrome c moves through the electron carrier. Transports e- from intermembrane space, soluble. Complexes I and II, to Complex III. NADH:Ubiquinone Oxidoreductase, Complex I One of the largest macro-molecular assemblies in mammalian cells, > 40 different polypeptide chains; nuclear and mitochondrial genes NADH binding site - faces matrix Noncovalently bound FMN (flavin mononucleotide) accepts 2e- from NADH. Series of 7 Fe-S centres pass 2 electrons from NADH one e- at a time toward ubiquinone binding site. Electrons jump ~10 Angstrom units between redox centers Conformational change caused by reduction of Q traps 4 protons in matrix, then releases to intermembrane space NADH:Ubiquinone Oxidoreducase Is a Proton Pump Transfer of two electrons from NADH to ubiquinone is accompanied by a transfer of protons from the matrix (N) to the intermembrane space (P). Experiments suggest that about four protons are transported per one NADH. NADH + Q + 5H+N = NAD+ + QH2 + 4 H+P Reduced coenzyme Q picks up two protons. Protons transported by “proton wires”. – a series of amino acids (glu, lys, his) that undergo protonation and deprotonation to give net transfer of a proton from one side of a membrane to another Succinate Dehydrogenase, Complex II SDH dual role: 1. Converts succinate to fumarate, CAC. 2. Captures and donates electrons in the electron transport chain Succinate Dehydrogenase, Complex II 4 subunits, 2 membrane bound FAD accepts two electrons from succinate. Electrons are passed, one at a time, via iron-sulfur centers to ubiquinone, which becomes reduced QH2. Does not transport protons across IMM Ubiquinone:Cytochrome c Oxidoreductase, Complex III “Dimer” – each with 11 subunits contains iron-sulfur clusters, cytochrome b, hemes (bH, bL), cytochrome c1 Cytochrome c (mobile) 2 distinct binding sites for Q Uses two electrons from QH2 to reduce two molecules of cytochrome c Clearance of electrons from reduced quinones (Q-cycle) > translocation of four additional protons to the intermembrane space. The Q Cycle Q-cytochrome c oxidoreductase Complex III The Q Cycle: Overall Reaction The Q Cycle Four protons are transported across the membrane per two electrons that reach cyt c. Two of the four protons come from QH2. The Q cycle explains how two additional protons are picked up from the matrix. Two molecules of QH2 become oxidized, releasing protons into the IMS. One molecule of Q becomes re-reduced, thus a net transfer of four protons per reduced coenzyme Q. Cytochrome c The second mobile electron carrier – Ubiquinone moves through the membrane. – Cytochrome c moves through the intermembrane space, soluble. Heme-containing protein Heme iron can be either ferric (Fe3+, oxidized) or ferrous (Fe2+, reduced). Carries a single electron from cytochrome bc1 complex to cytochrome oxidase. Cytochrome Oxidase, Complex IV Membrane protein with 13 subunits. Contains two heme groups: a and a3 Contains copper ions – CuA: two ions that accept electrons from cyt c – CuB: bonded to heme a3, forming a binuclear center that transfers four electrons to oxygen Cytochrome Oxidase terminal oxidase in ETC Complex IV Four electrons are used to reduce one oxygen molecule into two water molecules. Four protons are picked up from the matrix in this process. Two additional protons are passed from the matrix to the intermembrane space. Summary of Electron Flow in the Respiratory Chain Summary of Electron Transport Complex I → Complex IV 1NADH + 11H+(N) + ½O2 ——> NAD+ + 10H+(P) + H2O Complex II → Complex IV FADH2 + 6H+(N) + ½O2 ——> FAD + 6H+(P) + H2O Difference in number of protons transported reflects differences in ATP synthesized. Standard Reduction Potentials of Respiratory Chain and Related Electron TABLE 19-2 Carriers Redox reaction (half-reaction) E'˚ (V) 2H+ + 2e2 → H2 –0.414 NAD+ + H+ + 2e2 → NADH –0.320 NADP+ + H+ + 2e2 → NADPH –0.324 NADH dehydrogenase (FMN) + 2H+ + 2e2 → NADH dehydrogenase (FMNH2) –0.30 Ubiquinone + 2H+ + 2e2 → ubiquinol 0.045 Cytochrome b (Fe3+) + e2 → cytochrome b (Fe2+) 0.077 Cytochrome c1 (Fe3+) + e2 → cytochrome c1 (Fe2+) 0.22 Cytochrome c (Fe3+) + e2 → cytochrome c (Fe2+) 0.254 Cytochrome a (Fe3+) + e2 → cytochrome a (Fe2+) 0.29 Cytochrome a3 (Fe3+) + e2 → cytochrome a3 (Fe2+) 0.35 ½O2 + 2H+ + 2e2 → H2O 0.817 NADH + H+ + 1/2O2 ➔NAD+ + H2O ΔE’° = 0.817 – (-.32) = 1.14; ΔG’° = -nFΔE’° = -220 kJ/mol NADH + 11HN+ + 1/2O2 ➔NAD+ + 10HP+ + H2O Proton-Motive Force Proteins in ETC create electrochemical proton gradient by three means: – actively transporting protons across the membrane Complex I and Complex IV – chemically removing protons from the matrix reduction of CoQ and reduction of oxygen – releasing protons into the intermembrane space oxidation of QH2 Chemiosmotic Model for ATP Synthesis Electron transport sets up a proton-motive force. Energy of proton-motive force drives synthesis of ATP.

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