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BIOC13H3 (52)
Daman Bawa (12)
Lecture 8


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Biological Sciences
Daman Bawa

LECTURE 8 Cytochromes  Heme containing proteins  A, b or c based on absorption spectrum  ETC has a, a3, L (b566), bH(b562), c1are all integral proteins and cyt c is a peripheral membrane protein on the outer surface of IMM  Each carry 1 e per heme iron  Different types of heme The Chemiosmotic Theory  ETC links redox energy to ATP synthesis  Involves pumping out protons from the matrix to IMM and protons flowing back through the ATP synthase in response to the chemical and electrical differential  Unequal distribution of [H ] is energy rich called proton motive force  Proton circuit is established where protons respond to the chemical and electrical gradient and flow back to matrix to equalize distribution though ATP synthase to catalyze ATP synthesis Experimental Evidence for Chemiosmosis  Mitochondria oxidizes substrates and consume oxygen only when ADP is present  Mitchell’s proposal: synthetic compounds uncouple oxidation from phosphorylation (using synthetic molecules disrupts the ATP synthesis)  ATPase activity in damaged mitochondria  In Bacteriorhodopsin, reconsitituion in membrane vesicles with ATP synthase and a proton pump o Inside-out mitochondrial membrane vesicle that can pump protons into interior of vesicle when oxidizable substrate was made available o Artificial vesicle containing bacterial rhodopsin protein were exposed to light, resulting in inward proton pumping o ATP synthesis on the vesicle surface Complex I: NADH-ubiquinone oxidoreductase  46 subunits, ~900kDa, possible largest protein in IMM  L-shaped, one arm in IMM and other into the matrix  Contains one FMN and 8 or 9 iron sulfur clusters  2 e from NADH are passed through to CoQ using coupled reaction resulting in the net + movement of 4 H across the membrane  FMN, covalently bound, accepts the two electrons (FMNH·,FMNH ), and the Fe-S clusters carry _ 2 one e from one end of complex to the other  Final transfer occurs to CoQ (ubiquinone), and is reduced to ubiquionol (CoQH ) 2  Proton transfer is possibly through conformational changes in the protein by alteration of pK a values of functional groups located on the inner and outer faces of the membrane  Proton wire: series of H-bonded protein groups plus water that form a chain which protons can be relayed  Rotenone is a poison ta blocks electron transfer within complex I by preventing redox reaction between two Fe-S centers Complex II: Succinate CoQ Oxidoreductase  Aka succinate dehydrogenase  2e from succinate are transferred to FAD to form FADH 2  FADH t2ansfers electrons to CoQ via Fe-S clusters  Low delta G, no protons transferred Ubiquinone  3 states CoQ, CoQH· (ubisemiquinone) and CoQH 2  Can diffuse through lipid bilayer thus serves as electron shuttle  Has isoprenoid unit that allows it to be soluble in IMM bilayer  Hydrophobic tail consists of 10 C 5soprenoid units (Q )10  Tissues with higher energy requirement have highest concentrations of CoQ Complex III: Ubiquinone-Cytochrome c oxidoreductase  Aka Cytochrome b comc1ex  Docking site for CoQH 2  Consists of 11 polypeptides in each of the two monomer subunits  Q pnear positive inter-membrane space) and Q (nearNnegative matrix) are binding sites that play roles in diverting one electron at a time to cytochrome c via the Q cycle  Translocation of 4 protons  Each monomer has 2 active sites for quinone to undergo redox reactions  Three prosthetic groups that serve as redox centers (respiratory subunits) o Cytochrome b: two heme groups b and L H o Cytochrome c c1ntains one heme group c 1 o Rieske Fe-S protein (R-ISP) contains 2Fe-2S cluster Q cycle  Mobile electron carrier and transformer converting the 2 e transport system into a 1 e - transport system required by cytochrome C  One CoQH reduces 2 cytochrome c (one electron carrier) 2  Cycle requires 2QH m2lecules get oxidized by complex III, one of QH bein2 re-formed by reduction (net oxidation of 1 QH m2lecule) First Round of Q-cycle  CoQH f2om complex I binds to Q siteo  One electron is transferred to Fe-S protein (R-ISP) which releases two protons in to the IM space - and generates CoQ·  Electrons from R-ISP reduced cytochrome C 1  CoQ· transfers the remaining electron to cytochrome b and generates CoQ L  BLtransfers electron to b H  CoQ from Q sote is released and binds to Q i -  Cytochrome b trHnsfers electron to CoQ to form CoQ· Second Round of Q-cycle  Same as round one but the second electrons from cytochrome B transferH the electron to CoQ · - at Qisite and reduces it to CoQH 2  Protons for the regeneration of CoQH com2 from the mitochondrial matrix  CoQH i2 released and then replaces in the Q bindong site Cytochrome C  Peripheral membrane protein acting as a mobile electron carrier  Hydrophilic associated with the cytosolic side of IMM  Transports one electron at a time from complex III to IV using heme prosthetic group 3+ 2+  Oxidized cyt c contains Fe and reduced contains Fe  Accepts electrons from cytochrome c of c1mplex III and moves along the membrane surface to interact with subunit II of cytochrome c oxidase and donates electron to the Cu site A Complex IV: Cytochrome c oxidase  Accepts electrons one at a time from cyt c and donates them to oxygen to form water  2 protons are pump across using similar mechanism to complex I  Cyt c docks on the intermembrane space side near the Cu site Ahich accepts electrons leading to oxidation of heme group in cyt c
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