Electron Transport Chain

ELECTRON TRANSPORT SYSTEM

 

Introduction:

The majority of the energy conserved during catabolism reactions occurs near the end of the metabolic series of reactions in the electron transport chain. The electron transport or respiratory chain gets its name from the fact electrons are transported to meet up with oxygen from respiration at the end of the chain. The overall electron chain transport reaction is:

2 H⁺ + 2 e⁺ + 1/2 O₂ ---> H₂O + energy

Notice that 2 hydrogen ions, 2 electrons, and an oxygen molecule react to form as a product water with energy released in an exothermic reaction. This relatively straight forward reaction actually requires eight or more steps. The energy released is coupled with the formation of three ATP molecules per every use of the electron transport chain.

 

Complex I

In Complex I (NADH dehydrogenase, also called NADH:ubiquinone oxidoreductase; two electrons are removed from NADH and transferred to a lipid-soluble carrier, ubiquinone (Q). The reduced product, ubiquinol (QH₂) freely diffuses within the membrane, and Complex I translocates four protons (H⁺) across the membrane, thus producing a proton gradient. Complex I is one of the main sites at which premature electron leakage to oxygen occurs, thus being one of the main sites of production of harmful superoxide.
 The pathway of electrons occurs as follows:
NADH is oxidized to NAD⁺, by reducing Flavin mononucleotide to FMNH₂ in one two-electron step. FMNH₂ is then oxidized in two one-electron steps, through a semiquinone intermediate. Each electron thus transfers from the FMNH₂ to an Fe-S cluster, from the Fe-S cluster to ubiquinone (Q). Transfer of the first electron results in the free-radical (semiquinone) form of Q, and transfer of the second electron reduces the semiquinone form to the ubiquinol form, QH₂. During this process, four protons are translocated from the mitochondrial matrix to the intermembrane space.

Complex II

In Complex II (succinate dehydrogenase; EC 1.3.5.1) additional electrons are delivered into the quinone pool (Q) originating from succinate and transferred (via FAD) to Q. Complex II consists of four protein subunits: SDHA, SDHB, SDHC, and SDHD. Other electron donors (e.g., fatty acids and glycerol 3-phosphate) also direct electrons into Q (via FAD).

Complex III

In Complex III (cytochrome bc₁ complex; EC 1.10.2.2), the Q-cycle contributes to the proton gradient by an asymmetric absorption/release of protons. Two electrons are removed from QH₂ at the Q_O site and sequentially transferred to two molecules of cytochrome c, a water-soluble electron carrier located within the intermembrane space. The two other electrons sequentially pass across the protein to the Q_i site where the quinone part of ubiquinone is reduced to quinol. A proton gradient is formed by two quinol (4H+4e-) oxidations at the Q_o site to form one quinol (2H+2e-) at the Q_i site. (in total six protons are translocated: two protons reduce quinone to quinol and four protons are released from two ubiquinol molecules).
Cytochromes are very similar to the structure of myoglobin or hemoglobin. The significant feature is the heme structure containing the iron ions, initially in the +3 state and changed to the +2 state by the addition of an electron. The CoQH₂ (yellow)passes along the 2 electrons first to cytochrome (blue) b1 heme (magenta), then b2 heme , then to an iron-sulfur protein (green), then to cytochrome c1 (red with black heme), and finally to cytochrome c (not shown). Co Q is represented by the inhibitor antimycin (yellow) in the graphic
When electron transfer is reduced (by a high membrane potential or respiratory inhibitors such as antimycin A), Complex III may leak electrons to molecular oxygen, resulting in superoxide formation.

Complex IV

In Complex IV (cytochrome c oxidase;), sometimes called cytochrome A3, four electrons are removed from four molecules of cytochrome c and transferred to molecular oxygen (O₂), producing two molecules of water. At the same time, four protons are translocated across the membrane, contributing to the proton gradient. The activity of cytochrome c oxidase is inhibited by cyanide.

Coupling with oxidative phosphorylation

According to the chemiosmotic coupling hypothesis, proposed by Nobel Prize in Chemistry winner Peter D. Mitchell, the electron transport chain and oxidative phosphorylation are coupled by a proton gradient across the inner mitochondrial membrane. The efflux of protons from the mitochondrial matrix creates an electrochemical gradient (proton gradient). This gradient is used by the F_OF₁ ATP synthase complex to make ATP via oxidative phosphorylation. ATP synthase is sometimes described as Complex V of the electron transport chain.^{[citation needed]} The F_O component of ATP synthase acts as an ion channel that provides for a proton flux back into the mitochondrial matrix. This reflux releases free energy produced during the generation of the oxidized forms of the electron carriers (NAD⁺ and Q). The free energy is used to drive ATP synthesis, catalyzed by the F₁ component of the complex.^{[citation needed]}
Coupling with oxidative phosphorylation is a key step for ATP production. However, in specific cases, uncoupling the two processes may be biologically useful. The uncoupling protein, thermogenin—present in the inner mitochondrial membrane of brown adipose tissue—provides for an alternative flow of protons back to the inner mitochondrial matrix. This alternative flow results in thermogenesis rather than ATP production and generates heat.^{[citation needed]} Synthetic uncouplers (e.g., 2,4-dinitrophenol) also exist, and, at high doses, are lethal

Summary

In the mitochondrial electron transport chain electrons move from an electron donor (NADH or QH₂) 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.
The reactions catalyzed by Complex I and Complex III work roughly at equilibrium. This means that these reactions are readily reversible, by increasing the concentration of the products relative to the concentration of the reactants (for example, by increasing the proton gradient). ATP synthase is also readily reversible. Thus ATP can be used to build a proton gradient, which in turn can be used to make NADH. This process of reverse electron transport is important in many prokaryotic electron transport chains