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Mitochondrial electron transport chain

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• The mitochondrial electron transport chain is composed of three main

membrane-associated electron carriers flavoproteins (FMN, FAD),

cytochromes, and quinones (coenzyme Q, also known as ubiquinone

because it is a ubiquitous quinone in biological systems).

• All these electron carriers reside within the inner membrane of the

mitochondria and operate together to transfer electrons from donors, like

NADH and FADH2, to acceptors, such as O2. The, electrons flow from

carriers with more negative reduction potentials to those with more

positive reduction potentials and eventually combine with O2 and H to

form water.

• However, the mitochondrial electron transport system is arranged into

four enzyme complexes of carriers, each capable of transporting electrons

part of the way to O2 (Fig. 24.5). Coenzyme Q and cytochrome c connect

the complexes with each other.• The four enzyme complexes of carriers are:

NADH-Q oxidoreductase, succinate-Q-

reductase, Q-cytochrome c oxidoreductase, and

cytochrome c oxidase. These complexes are the

enzyme complex and each of them consists of

different prosthetic groups (Table 24.2).The whole process of mitochondrial electron transport can be represented in brief in

the following manner:• 1. Electrons donated by NADH enter the chain at complex I (NADH-Q-oxidoreductase) and pass through

a flavoprotein (FMN) to a series of iron-sulphur-proteins (FeS) and then to ubiquinone (Q).

• 2. Electrons donated by succinate enter the chain at Complex II (succinate-Q-reductase) and pass through

a flavoprotein (FAD) and FeS centres and then to ubiquinone (Q).

• 3. Ubiquinone (Q) serves as a mobile carrier of electrons received from complexes I and II and passes

them to complex III (Q-cytochrome c oxidoreductase).

• 4. Complex III called Q-cytochrome c oxidoreductase or cytochrome bc1 complex passes the electrons

through its prosthetic groups Cyt bL (Heme bL), Cyt bH (heme bH), FeS, and Cyt cL (Heme cL) to

cytochrome c.

• 5. Cytochrome c (Cyt c), a mobile connecting link between complex III and IV, passes electrons to

complex IV (cytochrome c oxidase). The latter carries electrons through its prosthetic groups Cyt a (Heme

a), Cyt a3 (Heme a3) CuA and CuB and transfers them to molecular oxygen, reducing it to H2O.

• 6. Electron flow through complexes I, III and IV is accompanied by proton flow from the mitochondrial

matrix (which becomes negatively charged) to inter membrane space or cytosolic side (which becomes

positively charged). The number of protons (H+) moved across the membrane at each site per pair of

electrons transported is still somewhat uncertain; the current consensus is that at least 10 protons move

outward during NADH oxidation.

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