Biochemistry 2280 Topic 14.docx

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Western University
Biochemistry 2280A
Christopher Brandl

Topic 14Electron TransportReadings p388389 453461 466467 470475By the end of this topic you should be able toDescribe the five types of electron carriers found in the mitochondrial electron transport chainExplain how electrons flow between complexes of the electron transport chain and ultimately to O2Explain the purpose of electron transportState how many protons are pumped out of the matrix for each reduced electron carrier In the citric acid cycle two carbons are oxidized to CO to reduce the electron carriers 2NAD and FAD to NADH and FADH respectively The electrons carried by NADH and 2FADH are delivered to O through many intermediates in the electron transport chain 22the components of which are embedded in or associated with the inner mitochondrial membrane The energy released by these oxidationreduction reactions is used to pump protons H ions out of the matrix and into the intermembrane space Fig 146 p495 The electrochemical gradient thus established is used to synthesize ATP by a mechanism described in Topic 15 This process of transferring electrons from one carrier to another to form ATP is called oxidative phosphorylation We will first consider electron transport in the mitochondrionMitochondrial electron transport systemThe mitochondrial electron transport system contains four large proteinbased complexes that work in sequence to deliver electrons to oxygen Fig 149 p461 Movie 142 Each complex contains an enzyme that catalyzes electron transfer and several nonprotein cofactors that accept and release electrons In addition three of the complexes contain protein subunits that enable movement of protons across the membraneThere are many electron carriers at work in the electron transport chain Electrons pass from one carrier to the next in a strictly defined order from a carrier with relatively low electron affinity to one with higher affinity Fig 1420 p472 A carriers electron affinity is determined by its chemical structure and by its surrounding environment The five classes of electron carriers found in the electron transport chain are described below in approximate order of increasing electron affinity1 Flavins Fig 1312B p439 The isoalloxazine or flavin ring of riboflavin the coloured ring structure at the top of Fig 1312B can accept two hydrogen atoms ie two protons and two electrons to become reduced Compounds containing flavin groups are found associated with protein complexes A protein that contains a flavin is called a flavoprotein2 Ironsulfur centresClusters of iron and sulfur atoms can be covalently linked to proteins via cysteine residues The number of atoms in an ironsulfur centre can vary from protein to protein
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