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Biochemistry Lecture No. 18.docx

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Western University
Biochemistry 2280A

Biochemistry Lecture No. 18: Electron Transport Friday October 19 , 2012 Electron Carriers: -Most electron carriers are embedded in the four large integral membrane protein complexes of the electron transport chain. Electron carriers differ in their affinity for electrons (from strongest reducing agent to strongest oxidant). Electrons pass from a carrier with low affinity (strong reductants like NADH) to a carrier with higher affinity (strong oxidants like O )2 The electron transport chain has five types of electron carriers (in approximate order of increasing affinity for electrons): Flavins, iron-sulfur centres, ubiquinone, cytochromes, and copper centres. Flavins: -Flavins consists of a flavin (isoalloxazine) ring that can accept two electrons (along with two protons). Examples of flavins include FADH and2riboflavin (vitamin B ). 2 Iron-Sulphur Centres: 3+ 2+ -Each iron-sulphur centre can accept one electron (per centre), in which it goes from a Fe to a Fe . Two forms of iron-sulphur centres are present in nature: Fe S (sq2 2e planar centre that accepts 2electrons) and Fe S4 4attice structure centre that accepts 4 electrons). Ubiquinone: -Ubiquinone is a lipid molecule that freely diffuses in the membrane (also called Coenzyme Q or simply Q). This electron carrier accepts two electrons, one at a time. The intermediate between oxidized Q (no electrons added) and reduced QH (2 el2ctrons added) is known as ubisemiquinone QH (1 electron added) and can act as a free radical if not immediately reduced. Cytochromes: -Cytochromes are proteins that contain heme groups, with an iron ion at the centre and can accept one 3+ 2+ electron (Fe to Fe ). A notable example of this electron carrier is cytochrome c, which is a peripheral membrane protein that accepts electrons from Complex III and gives electrons to Complex IV. Copper Centres: -Copper centres consist of copper ions, usually coordinated by histidine side chains, and can accept one electron (Cu to Cu ). + Electron Transport Complexes: -The electron transport chains is comprised of four large complexes (I, II, III and IV) each made up of many polypeptide chains, embedded in the inner mitochondrial membrane and contain electron carriers as cofactors. -The flow of electrons between complexes is as follows: NADH I Q III cytochrome c IV O 2 FADH 2 II Q III cytochrome c IV O 2 NADH Dehydrogenase (Complex I): -Also known as Complex I, NADH dehydrogenase accepts two electrons from the mitochondrial matrix. It pumps 4 protons across the IM membrane (from the matrix to the intermembrane space) per electron pair.
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