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Electron Transport Chain

This file is a lecture in Biochemistry about Electron Transport Chain.
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bs nursing

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ELECTRON TRANSPORT CHAIN

 The NADH and FADH 2 produced in the citric cycle pass to the electron transport chain, it is a biochemical reaction in which electrons and hydrogen ions from NADH and FADH 2 are passed to the intermediate carriers and then ultimately react with molecular oxygen to produce water. NADH and FADH 2 are in the process of:

NADH + H+ NAD + 2H+ + 2e

FADH 2 FAD+ 2H+2e

 The enzymes and electron carriers for electron transport are located along the inner membrane of the mitochondria.

 The reduced coenzymes NADH and FADH 2 produced from glycolysis, oxidation of pyruvate, and the citric acid cycle are oxidized to provide the energy for the synthesis of ATP.

In the electron transport system,

 There are five protein complexes, which are numbered I, II, III, IV, and V.

 Two electron carriers, coenzyme Q and cytochrome c, attached to the inner membrane of the mitochondrion, carry electrons between these protein complexes bound to the inner membrane.  The oxidation of NADH and FADH 2 provides hydrogen ions and electrons that eventually react with oxygen to form water.

Complex I: NADH-coenzyme Q reductase

In complex I,

  • Largest of the four protein complexes.

  • Contains more than 40 subunits, including the vitamin Bvitamin containing flavin mononucleotide and several iron-sulfur proteins.

  • Electron transport begins when hydrogen ions and electrons are transferred from NADH (from the citric acid cycle) to complex I.

  • It has two types of electron-carrying structures: FMN and several iron- sulfur centers.

  • The first electron transfer that occurs in complex I involve the interaction of NADH with FMN. The NADH is oxidized to NAD (which can again participate in the citric acid cycle) as it passes 2 hydrogen ions and two electrons to FMN, which is reduced to FMNH 2.

NADH + H+ oxidation NAD+ + 2H+ + 2e-

2H++ 2e- +FMN reduction FMNH 2

 The next steps involve transfer of electrons from the reduced FMNH 2 through a series of FeSPs. The iron present in the FeSP is Fe3+, which is reduced to Fe2+, 2 H+ atoms of FMNH 2 are released to solution as 2 H+ ions. Two FeSP molecules are needed to accommodate the two electrons released by FMNH 2 because Fe3+/ Fe2+ reduction involves only one electron.

FMNH 2 oxidation FMN + 2H++ 2e-

2e- + 2Fe(III)SP reduction 2Fe(II)SP

 In the final complex I reaction, Fe(II)SP is reconverted into Fe(III)SP as each of 2Fe(II)SP units passes an electron to CoQ, changing it from its oxidized from (CoQ) to its reduced form (CoQH 2 )

 CoQ is associated with the operations in Complex II in a manner similar to its actions in Complex I. It is the final recipient of the electrons from FADH2, with iron-sulfur proteins serving as intermediaries. Thus, complexes I and II produce a common product, the reduced form of coenzyme Q (CoQH2). As was the case with complex I, the reduced CoQH2 shuttles electrons to Complex III.

Complex III: Q-cytochrome c reductase

 Contains 11 different subunits. Electron carriers present include several iron-sulfur proteins as well as several cytochromes. A cytochrome is a heme-containing protein in which reversible oxidation and reduction of an iron atom occur. Heme, a compound also present in hemoglobin and myoglobin has the structure  All H+ ions required for the reactions of NADH, CoQ, and O 2 in the ETC comes from the matrix side of the inner mitochondrial membrane.  In cytochromes the iron of the heme is involved in redox reactions in which the iron changes back and forth between the +2 and + oxidation states.  In cytochromes the heme present is bound to protein in such a way as to prevent the heme from combining with oxygen as it does when it is present in hemoglobin  Various cytochromes, abbreviated cyt a, cyt b, cyt c, and so on, differ from each other in (1) their protein constituents, (2) the manner in which the heme is bound to the protein and (3) attachments to the heme ring. Again, because the Fe 3+/Fe 2+ system involves only a one- electron change, two cytochrome molecules are needed to move two electrons along the chain.  Iron/sulfur protein (FeSP) is a nonheme iron protein. Most protein of this type contains sulfur, as is the case with FeSP. Often the iron is bound to the sulfur atom in the amino acid cysteine.  The initial subtrate for complex III is CoQH2 molecules carrying the electrons that have been processed through complex I (from NADH) and also those processed through complex II (FADH2),The electron transfer process proceed form CoQH2 to a FeSP, then to cyt b, then to another FeSP, then to cyt1, and finally to cyt c.  Cyt c can move laterally into the intermembrane space, it delivers its electrons to complex IV. Cyt c is the only cytochrome that is water soluble. A feature that all steps in the ETC share is that as each electron carrier passes electrons along the chain, it becomes reoidied and thus able to accept more electrons.  The initial oxidation, reduction at complex III is between CoQH2 and an iron-sulfur protein (FeSP).

 The H ions produced from the oxidation of CoQH 2 go into cellular solution. All further redo reactions at complex III involve only electrons, which are conveyed further down enzyme complex chain.

Complex IV: Cytochrome c oxidase

At complex IV,

  • Four electrons from four cytochrome c are passed to other electron carriers.

  • It is composed of cytochrome proteins c, a and a 3. This complex contains two heme groups (one in each of the two cytochromes, a and a3) and three copper ions (a pair of CuA and one CuB in cytochrome a 3.

  • The cytochrome holds the oxygen molecule very tightly between iron and copper ions until oxygen is completely reduced.

  • When oxygen is already reduced, the reduced oxygen then picks up two hydrogen ions from the surrounding medium to make water.

  • Electrons combine with hydrogen ions and oxygen (O 2 ) to form two molecules of water.

  • Energy is used to pump H+ from the mitochondrial matrix into the intermembrane space, further increasing the hydrogen ion gradient.

  • The interdependence (coupling) of ATP synthesis with the reactions of the ETC is related to the movement of protons(H+ ions) across the inner mitochondrial membrane. Complexes I, III, and IV to act as hydrogen ion or protons pumps, producing a hydrogen ion gradient.

  • Some of the H+ ions crossing the inner mitochondrial membrane come from the reduced electron carriers, and some come from the matrix.

  • For every 2 electrons passed through the ETC, four protons cross the inner mitochondrial membrane through complex I, four through complex III and two more through complex IV. This proton flow causes a buildup of H+ ions (protons) in the intermembrane space; this high concentration of protons becomes the basis for ATP synthesis.

  • H+ cannot move through the inner membrane but returns to the matrix by passing through a fifth protein complex in the inner membrane called ATP synthase (also called complex V).

  • The flow of H+ from the intermembrane space through the ATP synthase generates energy that is used to synthesize ATP from ADP and Pi.

Electron Transport and ATP Synthesis

  • When NADH enters electron transport at complex I, the energy transferred can be used to synthesize 2 ATP.

  • When FADH 2 enters electron transport at complex II, it provides energy for the synthesis of 1 ATP.

  • Current research indicates that the oxidation of one NADH yields 2. ATP and one FADH 2 yields 1 ATP.

Regulation of Electron Transport and Oxidative

Phosphorylation

Electron transport

  • Is regulated by the availability of ADP, Pi, oxygen (O 2 ), and

NADH.

  • Decreases with low levels of any of these compounds and

decreases the formation of ATP.

When a cell is active and ATP is consumed rapidly, the

elevated levels of ADP will activate the synthesis of ATP.

The activity of electron transport is strongly dependent on

the availability of ADP for ATP synthesis.

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Electron Transport Chain

Course: bs nursing

999+ Documents
Students shared 8659 documents in this course

University: Southwestern University PHINMA

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ELECTRON TRANSPORT CHAIN
The NADH and FADH2 produced in the citric cycle pass to the electron
transport chain, it is a biochemical reaction in which electrons and
hydrogen ions from NADH and FADH2 are passed to the intermediate
carriers and then ultimately react with molecular oxygen to produce
water. NADH and FADH2 are in the process of:
NADH + H+ NAD + 2H+ + 2e
FADH2 FAD+ 2H+2e
The enzymes and electron carriers for electron transport are located
along the inner membrane of the mitochondria.
The reduced coenzymes NADH and FADH2 produced from glycolysis,
oxidation of pyruvate, and the citric acid cycle are oxidized to provide
the energy for the synthesis of ATP.
In the electron transport system,
There are five protein complexes, which are numbered I, II, III, IV, and
V.

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