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Electron Transport Mechanism

1. H atoms. A hydrogen atom consists of one proton and one electron. Protons are soluble in the water of the cell, but electrons are not. According to the chemiosmotic hypothesis hydrogen carriers alternate with electron carriers. When an electron carrier interacts with a hydrogen atom, it accepts only the electron. The proton is released into the aqueous solution of the cell.

2. NAD+. Hydrogen atoms liberated in the dehydrogenation reactions of aerobic glycolysis, oxidation of pyruvic acid and the krebs cycle are accepted by nicotinamide adenine dinucleotide (NAD+) (except in one dehydrogenation reaction, succinate to fumarate). The NAD+ molecule normally carries a positive electric charge, and hence the plus sign. Each molecule of NAD+ accepts two electrons and one proton and is reduced to NADH.

3. FMN. NADH donates two electrons and a proton to flavin mononucleotide (FMN), and is oxidized back to NAD+. FMN accepts the two electrons and the proton from NADH, and another proton from the internal medium, and is reduced to FMNH2 .

4. Fe-S. FMNH2 now given up two protons and two electrons and is oxidized to FMN. The two electrons are acquired by the iron-sulphur (Fe-S) proteins, which however, cannot accept the protons. The two protons are transported outside the mitochondrion.

5. The quinone cycle. The iron-sulphur proteins donate the pair of electrons to ubiquinone (UQ), also called coenzyme Q (CoQ, Q). Ubiquinone can exist in three possible states of oxidation, quinone, semiquinone and hydroquinone.

(i) Quinone (Q) is the fully oxidized form, with two oxygen atoms connected to the ring by double bonds.
(ii) Semiquinone (QH) is the semi-reduced form of ubiquinone, with a hydrogen atom attached to one oxygen.
(iii) Hydroquinone (QH2) is the fully reduced form, with hydrogen atoms attached to both oxygen.

6. Cytochrome c oxidase Coenzyme Q is a hydrogen carrier, while the cytochromes are electron carrier. After coenzyme Q, the hydrogens are split up into electrons and protons. The electrons pass down cytochromes b and c and are accepted by cytochrome c oxidase. The protons are released into the aqueous environment.
Cytochrome c oxidese contains 4 units which can carry one electron each. Thus 4 electrons are stored in the enzyme prior to discharge. The 4 electrons (4e-) combine with 4 protons (4H+) form the aqueous medium and one molecule of oxygen to form 2 molecules water.

7. Oxidative phosphorylation. It will be seen that for each pair of electrons transferred from NADH down the respiratory chain to oxygen, six protons are translocated across the mitochondrial membrane from the inside to the outside. This increases the proton concentration outside the membrane, and sets up a proton gradient. The resulting electric potential forces the protons through complex V back into the mitochondrion, and provides energy for the synthesis of ATP.

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