AEROBIC METABOLISM II: ELECTRON TRANSPORT CHAIN Khadijah Hanim Abdul Rahman School of Bioprocess Eng, UniMAP Week 15: 17/12/2012

Introduction  The electron transport chain (ETC) is a series of electron carriers in the inner membrane of the mitochondria of eukaryotes and the plasma membrane of aerobic prokaryotes.  To increase electron affinity that transfer the electrons derived from reduced coenzymes to O2.  During this transfer, a decrease in oxidation- reduction potential occurs.

 The process of which O2 is used to generate energy from food molecules is referred as aerobic respiration.  Energy released during electron transfer is coupled to several endergonic processes- ATP synthesis.  Reduced coenzymes from glycolysis, the citric acid cycle and fatty acid oxidation- principle sources of electrons.

 The cells of all eukaryotes (all animals, plants, fungi, algae – in other words, all living things except bacteria and archaea) contain intracellular organelles called mitochondria that produce ATP.eukaryotes bacteriaarchaea organellesmitochondria  Energy sources such as glucose are initially metabolized in the cytoplasm. The products are imported into mitochondria.  Mitochondria continue the process of catabolism using metabolic pathways including the Krebs cycle, fatty acid oxidation and amino acid oxidation.catabolismKrebs cyclefatty acidamino acid  The end result of these pathways is the production of two energy-rich electron donors, NADH and FADH2.NADHFADH2

 Electrons from these donors are passed through an electron transport chain to oxygen, which is reduced to water. This is a multi-step redox process that occurs on the mitochondrial inner membrane.  The enzymes that catalyze these reactions have the remarkable ability to simultaneously create a proton gradient across the membrane, producing a thermodynamically unlikely high- energy state with the potential to do workproton gradient

Electron transport and its components  The components of the ETC in eukaryotes are located in the inner mitochondrial membrane.  Most ETC components are organized into 4 complexes.  The 2 other molecules, coenzyme Q (ubiquinone, UQ) and cytochrome C (cyt c).

OUTLINE OF ETC NADH → Complex I → UQ → Complex III → cytochrome c → Complex IV → O 2 Complex II FADH2

Complex I  Referred as NADH dehydrogenase complex- catalyzes the transfer of 2 electrons from NADH to UQ.  The reduced product, ubiquinol (QH 2 ) freely diffuses within the membrane, and Complex I translocates four protons (H + ) across the membrane, thus producing a proton gradient.ubiquinol  Major sources of NADH include several reactions of TCA cycle and fatty acid oxidation.  Composed of at least 25 different polypeptides, Complex 1 is the largest protein component in the inner membrane.

Transfer of electrons thru Complex I of the mitochondrial ETC  NADH reduces FMN to FMNH2  Electrons are then transferred from FMNH2 to an iron-sulfur centre (Fe-S), 1 electron at a time.  After transfer from 1 Fe-S centre to another, the electrons are eventually donated to UQ.  Electron transport accompanied by the movement of protons from the matrix across the inner- membrane and into the intermembrane space.

Complex II  Complex II- succinate dehydrogenase complex consists of the TCA cycle enzyme succinate dehydrogenase and 2 Fe-S proteins.  Complex II mediates the transfer of electrons from succinate to UQ.  In Complex II additional electrons are delivered into the quinone pool (UQ) originating from succinate and transferred (via FAD) to UQ. FAD

Complex III  Complex III transfers electrons from reduced coenzyme Q (UQH2) to cytochrome c.  Complex III contains 2 b-type cytochromes, 1 cytochrome c 1 (cyt c 1 ) and 1 Fe-S center- referred as cytochrome bc 1 complex.

 Electron transfer begins with the oxidation of UQH 2 by the iron-sulfur protein in complex III, which generates ubisemiquinone (UQH. )  The reduced Fe-S protein transfers an electron to cyt c 1, which transfers it to cyt c.  4 protons are released on the cytoplasmic side of the inner membrane.

Complex IV  In Complex IV (cytochrome c oxidase) sometimes called cytochrome A3, four electrons are removed from four molecules of cytochrome c (protein that loosely attached to the inner membrane, transfers electrons 1 at a time to cyt a)- and transferred to molecular oxygen (O 2 ), producing two molecules of water.cytochrome c oxidasecytochrome c  At the same time, four protons are translocated across the membrane, contributing to the proton gradient.  O 2 + 4H + + 4e -  2H 2 O

 During each sequential redox reaction in the ETC, an electron loses energy.  During the oxidation of NADH there are 3 steps in which the change in reduction potential is sufficient for ATP synthesis.  This steps occurs in complexes I, III and IV.  The resulting transmembrane proton gradient is used to make ATP via ATP synthase.  Recent experimental evidence indicates that approximately 2.5 molecules of ATP are synthesized for each pair of electrons transferred between NADH and O2 in the ETC.  ~1.5 molecules of ATP result from transfer of each pair donated by FADH 2 produced by succinate oxidation.

Oxidative Phosphorylation  The process whereby the energy generated by the ETC is conserved by the phosphorylation of ADP to yield ATP. The Chemiosmotic Theory  Peter Mitchell, a Britiah biochemist, in 1961, proposed a mechanism by which the free energy generated during electron transport drives ATP synthesis

 As electrons pass through the ETC, protons ae transported from the matrix and released into the inter membrane space  As a result, an electrical potential and proton gradient (pH) arise across the inner membrane and this elecrochemical proton gradient is often referred as protonmotive force

 Protons, present in the intermembrane in excess can pass through the inner membrane and back into the matrix down their concentration gradient only through special channels as the inner membrane is impermeable to ions (protons)  As the themodynamically favorable flow of protons occur through a channel, each of which contains an ATP synthase activity, an ATP synthesis occurs.

SUMMARY  The mitochondrial electron transport chain removes electrons from an electron donor (NADH or FADH2) and passes them to a terminal electron acceptor (O2) via a series of redox reactions.  These reactions are coupled to the creation of a proton gradient across the mitochondrial inner membrane.  There are three proton pumps: I, III and IV. The resulting transmembrane proton gradient is used to make ATP via ATP synthase.