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Rose-Hulman CHEM 330 - The Electron Transport and Oxidative Phosphorylation Pathways

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Copyright © 2000-2003 Mark Brandt, Ph.D.74The Electron Transport and Oxidative Phosphorylation PathwaysExtracting energy from reduced coenzymesIn general, reactions that extract all of the energy from a molecule in a single stepare inefficient: they waste much of the available energy. The method used for takingenergy from NADH involves several steps, which allows a more efficient recovery ofthe energy from the molecule. The process has two phases: electron transport andoxidative phosphorylation.The mechanism for extracting the energy from the reduced cofactors was a matterof considerable debate. The Chemiosmotic hypothesis proposed by Peter Mitchell in1961 has the most experimental support, and is probably correct in its essentialpoints. In essence, Mitchell proposed that the electron transport pathway conservesthe energy from the electrons being transported by creating a proton gradientacross the mitochondrial membrane, and that this proton gradient is then used toprovide the energy required for ATP synthesis. How these processes work has beenthe subject of considerable research.As mentioned in the section on Bioenergetics, the ∆G´° for the transfer of electronsfrom NADH to oxygen is –219.2 kJ/mol. This is considerably larger than the ∆G°´for ATP hydrolysis (–30.5 kJ/mol). Clearly NADH has a large amount of energystored in the molecule. The task of the electron transport pathway is to conservethis energy in a form that can be used for the synthesis of more than one ATP.Mitochondrial StructureIn order to understand how the pathways for electron transport and oxidativephosphorylation work, we need to look at the general structure of a mitochondrion.A mitochondrion contains two membranes: an outer membrane, which appears tolargely be responsible for maintaining the shape of the organelle, and a much lesspermeable inner membrane. The outer membrane contains porin, a protein thatforms pores large enough allow molecules less than ~10 kDa to diffuse freely acrossthe membrane.The region between the membranes is called the intermembrane space. Theintermembrane space is occupied by soluble proteins large enough that they cannotpass through porin. For small molecules, the cytoplasm and intermembrane spaceare essentially contiguous regions.The inner membrane acts as a barrier to prevent the movement of mostmolecules. A few molecules have specific transporters that allow them to enter orexit the mitochondrion. The inner membrane contains cristae, which areinvolutions in inner membrane. The function of the cristae is to increase the surfaceCopyright © 2000-2003 Mark Brandt, Ph.D.75area of the inner membrane. The mitochondrial inner membrane may have a largersurface area than the cell plasma membrane, due to the involutions in themembrane.Finally, within the inner membrane is the matrix. The matrix is a very denseprotein solution (~50% protein by weight). The TCA cycle enzymes are located inthe matrix, as are the enzymes for several other metabolic pathways.Mitochondria contain a small genome (~16,500 bp). The genome contains 22transfer RNA genes, 2 ribosomal RNA genes, and 13 polypeptide genes; thepolypeptides are all involved in the electron transport pathway or oxidativephosphorylation pathway. Note that the TCA cycle enzymes (including succinatedehydrogenase) are all produced from nuclear genes; the multisubunit complexes ofthe electron transport pathway and ATP synthase (with the exception of succinatedehydrogenase) are made up of proteins derived from both nuclear andmitochondrial genes.Electron transport chainNADH and FADH2 can donate electron pairs to a specialized set of proteins that actas an electron conduit to oxygen: the electron transport chain. As the electrons arepassed down the chain, they lose much of their free energy. Some of this energy canbe captured and stored in the form of a proton gradient that can be used tosynthesize ATP from ADP; the remainder of the energy is lost as heat.The term “proton gradient” means that one side of the membrane (in this case, theintermembrane space side of the mitochondrial inner membrane) has a higherconcentration of protons that does the other side. Concentration gradients of anykind contain some energy; gradients of charged entities (such as protons) usuallyinvolve electrical potential gradients also, which also contain energy. The protongradient generated by the electron transport chain has both concentration andelectrical potential terms.Extensive research has located a total of five protein complexes in the mitochondrialinner membrane involved in the electron transport and oxidative phosphorylationpathways. Complexes I, II, III, and IV are part of the electron transport chain.Complex V is the enzyme complex that carries out the oxidative phosphorylationreaction: the actual conversion of ADP to ATP. All of these are large multiproteincomplexes.In addition to the membrane-bound complexes, the electron transport chainrequires mobile electron carriers: cytochrome c and Coenzyme Q. Coenzyme Q isnot a protein, but is a membrane bound cofactor. Cytochrome c is a small solubleprotein located in the intermembrane space.The overall reaction involves the oxidation of NADH or FADH2 cofactors, andresults in the reduction of oxygen to water. This process is the major reason for therequirement for oxygen in most organisms. The electron transport pathway is oftencalled the “respiratory chain”, because this pathway is the major reason forrespiration (= breathing in animals).Copyright © 2000-2003 Mark Brandt, Ph.D.76Electron transport complexesThe Complexes are proteins. Complexes I-IV have a variety of prosthetic groupsincluding metal ions, iron-sulfur centers, hemes, and flavins.NADH dehydrogenase (Complex I)The first complex contains a iron-sulfurs center and an FMN. Complex I acceptselectrons from NADH to regenerate NAD. Complex I also pumps protons: eachpair of electrons results in the movement of about 4 H+ from the matrix to theintermembrane space. Complex I donates electrons to Coenzyme Q.Coenzyme QCoenzyme Q is a non-protein electron carrier located in the inner mitochondrialmembrane. Mammals use Q10 (note the side chain in the structures below; inmammals, the compound has


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Rose-Hulman CHEM 330 - The Electron Transport and Oxidative Phosphorylation Pathways

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