Electron Transport Chain

Electron transport chain or system (ETC or ETS) • • • • • • • • • •

It is known as a system as it is composed of number of components which interact with one another It is also known as chain because of a series of reactions ETC is located on the inner mitochondrial membrane Units of ETC There are numerous units 1. FMN (Flavin mononucleotide) = 1st electron acceptor 2. Fe-S (Iron-sulfur) is a protein 3. Ubiquinone 4. Five types of cytochromes i) Cyt. b ii) Cyt. C1 iii) Cyt. C

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iv)

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ATP synthase

Cyt. a

v)

Cyt. a3

Summary of ETC • In the final stage of cellular aerobic respiration, all the enzyme-catalyzed steps in the oxidative degradation of carbohydrates, fats and amino acids in aerobic cells trickle down to electron transport and oxidative phosphorylation (final stages of aerobic respiration). • In the above-mentioned stage, the flow of electrons from organic substrates to oxygen takes place with the simultaneous release of energy to generate ATPs. • The energy rich carbohydrates, fatty acids, and amino acids undergo a series of metabolic reactions (break down) and finally get oxidized to CO2 and H2. The reduced products of various metabolic intermediates are transferred to coenzymes NAD+ and FAD to produce, respectively, NADH and FADH2 which pass through the electron transport chain (ETC) to finally reduce oxygen to water.

• The electron movement through the ETC is associated with the loss of free energy. However, a proportion this free energy is utilized to generate ATP from ADP and Pi. • The mitochondria are the centers for metabolic oxidative reactions to generate reduced coenzymes (NADH and FADH2) which, in turn, are utilized in ETC to release energy in the form of ATP. • For this reason, the mitochondrion is rightly regarded as the powerhouse of the of the cell.

Models 1. 2. 3. 4.

Salisbury and Ross (1992) Heldt (1997) Hopkins and Huner (2009) Taiz et al. (2015)

Hopkins and Huner (2009) • 1.

2. 3.

4. •

There are four complexes of mitochondrial membrane: NADH-Ubiquinone oxidoreductase or NADHdehydrogenase complex Succinate-Ubiquinone oxidoreductase or Succinate dehydrogenase Cytochrome c reductase or Cytochrome b/c1 complex Cytochrome c oxidase or Cytochrome a/a3 complex UQ is soluble in lipids so it is mobile.

Salisbury and Ross (1992)

• Cytochromes and Fe-S proteins have the ability to receive and transfer only one electron at a time whereas FMN and UQ can receive and transfer two hydrogen ions and two electrons at a time.

• Cytochrome c oxidase has greater affinity to O2. It is the terminal enzyme where O2 is converted into H2O.

Model by Heldt (1997) This model was proposed by Heldt (German scientist) in 1997. This is the simplest model of ETC.

Complex II is not involved in energy transduction and its value is zero. Complex II is not essential for electron transport and remaining 3 complexes are necessary for electron transport. In this case jumping of electron downward releases energy which is utilized to move the hydrogen ions from matrix to intermembranous space.

Moller,2001 (modified by Rasmusson, 2004) It is the latest model. It was proposed by Moller in 2001. This model is valid till today.

Taiz et al., 2015

• This model also explains the inhibitors 1. Rotenone (pesticide, a fish killer), amytal (sedative), Piericidin (an antibiotic) (Complex I) 2. Diphenyl eneidonoium (DPI, a drug) (Complex II) 3. Salicylic hydroxamic acid (SHAM, a drug which can irreversibly inhibit enzymes) blocks alternative oxidase 4. Antimycin A is the active ingredient of Fintrol, a Fish poison used for eliminating invasive species in aquaculture (Complex III) 5. Cyanide (CN), azide or CO block Complex IV

Some electron Transport Enzymes are unique to plant mitochondria • These enzymes are only found in plant mitochondria and not in animals. 1.Two NAD(P)H dehydrogenases, both Ca2+ dependent, attached to outer surface of the inner mitochondrial membrane facing the inter-membrane space can oxidize cytosolic NADH and NADPH. Electrons from these external NAD(P)H dehydrogenases – NDex (NADH) and NDex (NADPH) enter the main electron transport chain at the level of the ubiquinone pool (Moller, 2001; Rasmusson et al., 2004) 2.Plant mitochondria have two pathways for oxidizing matrix NADH. 1. Through complex I: sensitive to inhibition by several compounds like Rotenone and Piericidin 2. Plants also have rotenone resistant dehydrogenase NDin (NADH). Functions as bypass.

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NAD(P)H dehydrogenase (NDin) is present on the matrix surface. Very little is known about this Most plants have alternative respiratory pathway which involves alternative oxidase which is insensitive to inhibition by cyanide, azide or CO

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ATP synthesis: Number of ATP depends on nature of electron donor. In experiments on isolated mitochondria, matrix NADH gives rise ADP:O ratio (Number of ATP synthesized per 2 electron transferred to O2) of 2.4 to 2.7. Succinate and external NADH 1.6 to 1.8

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4 H+

1 ATP