LESSON 7- ELECTRON TRANSPORT CHAIN S.KRITTIKA
.Electrons carried by NADH are transferred to the first molecule in the electron transport chain (the flavoprotein; FMN) .The electrons continue along the chain which includes several Cytochrome proteins and one lipid carrier. . The electrons carried by FADH2 have lower free energy and are added to a later point in the chain. Electrons from NADH or FADH2 ultimately pass to oxygen
Energy of NADH and FADH2 give a maximum yield of 34 ATP is produced by oxidative phosphorylation.
Introduction V The primary function of the citric acid cycle was identified as the generation of NADH and FADH2 by the oxidation of acetyl CoA. In oxidative phosphorylation, NADH and FADH2 are used to reduce molecular oxygen to water. VThe highly exergonic reduction of molecular oxygen by NADH and FADH2 occurs in a number of electrontransfer reactions, taking place in a set of membrane proteins known as the electron-transport chain.
Overview of the Electron Transport Chain > The components of the electron transport chain are organized into 4 complexes. Each complex contains several 1. Complex I also known as the NADH-coenzyme Q reductase 2. Complex II also known as succinate-coenzyme Q reductase 3. Complex III also known as coenzyme Q reductase different electron carriers. or NADH dehydrogenase. or succinate dehydrogenase. 4. Complex IV also known as cytochrome c reductase. Each of these complexes are large multisubunit complexes embedded in the inner mitochondrial membrane. 5
Complex I: Also called NADH-Coenzyme Q reductase because this large protein complex transfers 2 electrons from NADH to coenzyme Q. Complex I was known as NADH dehydrogenase. . Complex I (850,000 kD) contains a FMN prosthetic group which is absolutely required for activity and seven or more Fe-S clusters. This complex binds NADH, transfers two electrons in the form of a hydride to FMN to produce NAD+ and FMNH2- The subsequent steps involve the transfer of electrons one at a time to a series of iron-sulfer complexes.
The importance of FMN:- First it functions as a 2 electron acceptor in the hydride transfer from NADH. Second it functions as a1 electron donor to the series of iron sulfur clusters. The process of transferring electrons from NADH to CoQ by complex I results in the net transport of protons from the matrix side of the inner mitochondrial membrane to the inter membrane space where the H+ ions accumulate generating a proton motive force.
Complex II: . It is none other than succinate dehydrogenase, the only enzyme of the citric acid cycle that is an integral membrane protein, so its the only membrane-bound enzyme in the citric acid cycle This complex is composed of four subunits. Two of which are iron-sulfur proteins and the other two subunits together bind FAD through a covalent link to a histidine residue.
. In the first step of this complex, succinate is bound and a hydride is transferred to FAD to generate FADH2 and fumarate. FADH2 then transfers its electrons one at a time to the Fe-S centers. Thus once again FAD functions as 2 electron acceptor and a 1 electron donor. The final step of this complex is the transfer of 2 electrons one at a time to coenzyme Q to produce CoQH2.
. For complex II the standard free energy change of the overall reaction is too small to drive the transport of protons across the inner mitochondrial membrane. This accounts for the 1.5 ATP's generated per FADH.2 compared with the 2.5 ATP's generated per NADH 10
Q-cycle: The Q-cycle is initiated when CoQH2 diffuses through the bilipid layer to the CoQH2 binding site which is near the intermembrane face. This CoQH2 binding site is called the QP site. > The electron transfer occurs in two steps. First one electron from CoQH2 is transferred to the Fe-S protein which transfers the electron to cytochrome cl. This process releases 2 protons to the intermembrane space Op site space iP phave COH Cytg -FeS First half o1 Q-cycle Pool tQ Matrix (N phaeQa site 12
The second half of the Q-cycle is similar to the first half. A second molecule of CoQH2 binds to the QP site. In the next step, one electron from CoQH2 (bound at QP) is transferred to the Rieske protein which transfers it to cytochrome c1. This process releases another 2 protons to the intermembrane space.
Cytochromes These are iron- containing electron transferring proteins. They are heme proteins. 3 classes have been identified a,b and c Each cytochrome molecule in its ferric (Fe 3+) form accepts one electron and reduced to the ferrous state (Fe2+). .In addition to iron, Cyt a3 also contain 2 bound copper atom which undergo cupric (Cu 2+) to cuprous (Cu+) redox changes during electron transfer. 19
Uncouplers . Electron transport and phosphorylation can be uncoupled by compounds that increase the permeability of the innermitochondrial membrane to protons in any place. i.e Uncouplers causes electron transport to be proceed at a rapid rate without the establishing of proton gradient e.g: 2,4 dinitrophenol
Electron transport inhibitors These compounds prevent the passage of electrons by binding to chain components, blocking the oxidation/reduction reaction * Inhibition of electron transport also inhibits ATP synthesis. e g: Amytal and Rotenone block e- transport between FMN and Co Q Antimycin A blocks between Cyt b and Cyt c Sodium azide blocks between Cyt a a3 and oxygen 21
M.Sc Advanced Biochemistry- University of Madras.Awaiting INSPIRE fellowship. Believe in sharing is caring,as knowledge is not materialistic