The Electron Transport and Oxidative Phosphorylation Pathways Extracting energy from reduced coenzymes In general reactions that extract all of the energy from a molecule in a single step are inefficient they waste much of the available energy The method used for taking energy from NADH involves several steps which allows a more efficient recovery of the energy from the molecule The process has two phases electron transport and oxidative phosphorylation The mechanism for extracting the energy from the reduced cofactors was a matter of considerable debate The Chemiosmotic hypothesis proposed by Peter Mitchell in 1961 has the most experimental support and is probably correct in its essential points In essence Mitchell proposed that the electron transport pathway conserves the energy from the electrons being transported by creating a proton gradient across the mitochondrial membrane and that this proton gradient is then used to provide the energy required for ATP synthesis How these processes work has been the subject of considerable research As mentioned in the section on Bioenergetics the G for the transfer of electrons from 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 energy stored in the molecule The task of the electron transport pathway is to conserve this energy in a form that can be used for the synthesis of more than one ATP Mitochondrial Structure In order to understand how the pathways for electron transport and oxidative phosphorylation work we need to look at the general structure of a mitochondrion A mitochondrion contains two membranes an outer membrane which appears to largely be responsible for maintaining the shape of the organelle and a much less permeable inner membrane The outer membrane contains porin a protein that forms pores large enough allow molecules less than 10 kDa to diffuse freely across the membrane The region between the membranes is called the intermembrane space The intermembrane space is occupied by soluble proteins large enough that they cannot pass through porin For small molecules the cytoplasm and intermembrane space are essentially contiguous regions The inner membrane acts as a barrier to prevent the movement of most molecules A few molecules have specific transporters that allow them to enter or exit the mitochondrion The inner membrane contains cristae which are involutions in inner membrane The function of the cristae is to increase the surface Copyright 2000 2003 Mark Brandt Ph D 74 area of the inner membrane The mitochondrial inner membrane may have a larger surface area than the cell plasma membrane due to the involutions in the membrane Finally within the inner membrane is the matrix The matrix is a very dense protein solution 50 protein by weight The TCA cycle enzymes are located in the matrix as are the enzymes for several other metabolic pathways Mitochondria contain a small genome 16 500 bp The genome contains 22 transfer RNA genes 2 ribosomal RNA genes and 13 polypeptide genes the polypeptides are all involved in the electron transport pathway or oxidative phosphorylation pathway Note that the TCA cycle enzymes including succinate dehydrogenase are all produced from nuclear genes the multisubunit complexes of the electron transport pathway and ATP synthase with the exception of succinate dehydrogenase are made up of proteins derived from both nuclear and mitochondrial genes Electron transport chain NADH and FADH2 can donate electron pairs to a specialized set of proteins that act as an electron conduit to oxygen the electron transport chain As the electrons are passed down the chain they lose much of their free energy Some of this energy can be captured and stored in the form of a proton gradient that can be used to synthesize 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 the intermembrane space side of the mitochondrial inner membrane has a higher concentration of protons that does the other side Concentration gradients of any kind contain some energy gradients of charged entities such as protons usually involve electrical potential gradients also which also contain energy The proton gradient generated by the electron transport chain has both concentration and electrical potential terms Extensive research has located a total of five protein complexes in the mitochondrial inner membrane involved in the electron transport and oxidative phosphorylation pathways Complexes I II III and IV are part of the electron transport chain Complex V is the enzyme complex that carries out the oxidative phosphorylation reaction the actual conversion of ADP to ATP All of these are large multiprotein complexes In addition to the membrane bound complexes the electron transport chain requires mobile electron carriers cytochrome c and Coenzyme Q Coenzyme Q is not a protein but is a membrane bound cofactor Cytochrome c is a small soluble protein located in the intermembrane space The overall reaction involves the oxidation of NADH or FADH2 cofactors and results in the reduction of oxygen to water This process is the major reason for the requirement for oxygen in most organisms The electron transport pathway is often called the respiratory chain because this pathway is the major reason for respiration breathing in animals Copyright 2000 2003 Mark Brandt Ph D 75 Electron transport complexes The Complexes are proteins Complexes I IV have a variety of prosthetic groups including 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 accepts electrons from NADH to regenerate NAD Complex I also pumps protons each pair of electrons results in the movement of about 4 H from the matrix to the intermembrane space Complex I donates electrons to Coenzyme Q Coenzyme Q Coenzyme Q is a non protein electron carrier located in the inner mitochondrial membrane Mammals use Q10 note the side chain in the structures below in mammals the compound has ten isoprene units while some other species use versions with 6 or 8 isoprene units Note that Coenzyme Q can transfer one or two electrons the structures below correspond to the fully oxidized form the singly reduced form and the fully reduced form of the molecule Coenzyme Q can accept electrons from Complex I and II and from other proteins it donates the