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Unformatted text preview: The Q Cycle Funnels Electrons from a Two-Electron Carrier to a One-Electron Carrier and Pumps Protons The mechanism for the coupling of electron transfer from Q to cytochrome c to transmembrane proton transport is known as the Q cycle (Figure 18.12). Two QH 2 molecules bind to the complex consecutively, each giving up two electrons and two H+. this proton is to be pumped bec of the flow of e- These protons are released to the cytoplasmic side of the membrane. - The two electrons travel through the complex to different destinations. One electron flows first to the Rieske 2Fe-2S cluster, then to cytochrome c1, and finally to a molecule of oxidized cytochrome c, converting it into its reduced form. The reduced cytochrome c molecule is free to diffuse away from the enzyme to continue down the respiratory chain. The second electron passes through the two heme groups of cytochrome b to an oxidized ubiquinone bound in a second binding site. This quinine (Q) molecule is reduced to a semiquinone radical anion (Q . ) by the electron from the first QH2 molecule.- On the addition of the electron from the second QH, molecule, this quinone radical anion takes up two protons from the matrix side to form QH 2 . The removal of these two protons from the matrix contributes to the formation of the proton gradient. In sum, four protons are released on the cytoplasmic side, and two protons are removed from the mitochondrial matrix. In one Q cycle , two QH 2 molecules are oxidized to form two Q molecules, and then one Q molecule is reduced to QH 2 . Why this complexity? The formidable problem solved here is to efficiently funnel electrons from a two-electron carrier (QH 2 ) to a one-electron carrier (cytochrome c). The cytochrome b component of the reductase is in essence a recycling device that enables both electrons of QH 2 to be used effectively .------------------------ Figure 18.12 Q cycle. In the first half of the cycle, - two electrons of a bound QH2 are transferred, one to cytochrome c and 1- the other to a bound Q in a second binding site to form the semiquinone radical anion (Q . ). - The newly formed Q its mean the radical dissociates and enters the Q pool. In the second half of the cycle , a second QH2 also gives up its electrons, one to a second molecule of cytochrome c and the other to reduce Q to QH2. This second electron transfer results in the uptake of two protons from the matrix. The path of electron transfer is shown in red.----------------------------- Cytochrome c Oxidase Catalyzes the Reduction of Molecular Oxygen to Water The last of the three proton-pumping assemblies of the respiratory chain is cytochrome c oxidase (Complex IV )....
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This note was uploaded on 05/30/2010 for the course MEDICEN 2100 taught by Professor Saba during the Spring '10 term at Columbia State Community College.
- Spring '10