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Chemistry 2223B

Topic 14 Overview of electron transport • Electron carriers (NADH, FADH ) 2rom citric acid cycle give up 2 electrons each to the ETC which ultimately goes to oxygen to make water • Move protons from the matrix to the intermembrane space, which creates a concentration gradient – form of potential energy Electron Carriers • Most are embedded in the four large integral membrane protein complexes of the electron transport chain • Electron carriers differ in their affinity for electrons • Electrons pass from a carrier with low affinity to a carrier with higher affinity o Energetically favourable o Each electron carrier has a higher affinity for electrons than the previous one • Higher the redox potential the higher the affinity for electrons • The electron transport chain has five types of electron carriers • In approximate order of increasing affinity for electrons: o Flavins o Iron-sulfur centres o Ubiquinone o Cytochromes o Copper centres 1. Flavins • Isoalloxazine (flavin) ring can accept two electrons (along with two protons) • Flavin ring – group that will accept 2 electrons and 2 protons and protons go on the nitrogens Flavin adenine dinucleotide (FADH ) 2 • Flavin groups exists in protein complexes o Do not tend to float around by themselves, whereas NADH floats around by itself 2. Iron-sulfur centres • Each centre can accept one electron (Fe  Fe ) – doesn’t matter how many iron atoms are in there it will still only accept one electron – it goes on the iron • Different types exist, for example: • Embedded in a protein, doesn’t float around freely 3. Ubiquinone • A lipid that freely diffuses in the membrane; also called Coenzyme Q or simply Q • Accepts two electrons, one at a time • Lipid tail – green part • Dot represents unpaired electron – looking for an electron to pair with – free radical • Diffuses around in inner mitochondrial membrane because it is very hydrophobic 4. Cytochromes • Proteins that contain heme groups, with an iron ion at the centre • Can accept one electron (Fe Fe ) 2+ • Cytochrome means protein that has a heme group in it • Iron atom accepts electron • Heme does not tend to be floating around, tends to be bound to a protein • Should be able to recognize FADH a2d the Heme Cytochrome c • Cytochrome c is a peripheral membrane protein that accepts electrons from complex III and gives electrons to complex IV • Heme is shown in blue and orange coloured part – bound to a protein 5. Copper centres • Copper ions, usually coordinated by histidine side chains 2+ + • Can accept one electron (Cu  Cu ) • Each copper ion can pick up an electron (backwards to the iron sulfur centres) o Two copper ions, each one can pick up an electron • Strong electron affinity, towards the end of the electron chain An aside: cofactors and coenzymes • Cofactor: a non-protein chemical group that enables an enzyme to catalyze a reaction o Not necessarily tightly bound to an enzyme o Some other group that is not one of the twenty amino acids that catalyzes the reaction • Coenzyme: a cofactor that is mostly an organ (carbon-based) molecule o Metal ion – sulfur centres • Prosthetic group: a cofactor that remains tightly bound to the enzyme o FAD+ • Coenzymes are chemically changed as the reaction is catalyzed, so they can be considered substrates Electron transport protein complexes • Four large complexes (I, II, III, and IV) made up of many polypeptide chains • Embedded in the inner mitochondrial membrane • Contain electron carriers as cofactors – allow the enzymes to carry out their reactions • Electron flow: Complex I: NADH dehydrogenase • Accepts two electron from matrix NADH • Pumps 4 protons across membrane (from matrix to intermembrane space) per electron pair • As this happens it
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