OXIDATIVE PHOSPHORYLATION AND ELECTRON TRANSPORT CHAIN (ETC)

OXIDATIVE PHOSPHORYLATION AND ELECTRON TRANSPORT CHAIN (ETC)

OXIDATIVE PHOSPHORYLATION

• Oxidative phosphorylation is the process in which ATP is formed as a result of the transfer of electrons from NADH or FADH₂ to O₂ by a series of electron carriers. This process, which takes place in mitochondria, is the major source of ATP in aerobic organisms.
• Oxidative phosphorylation is linked with the Electron transport chain (ETC).

ELECTRON TRANSPORT CHAIN

• Electron transport chain (ETC) is the final common pathway in aerobic cells by which electrons derived from various substrates are transferred to Oxygen.
• The transfer of electrons from one electron carrier to the next releases energy, some of which is used to generate ATP from ADP through a process called Chemiosmosis.
• ETC is a series of highly organized Oxidation–Reduction reaction.
• In Eukaryotes, ETC takes place in the Mitochondria but in Prokaryotes, ETC takes place in Plasma membrane.

Carrier Molecules in Electron Transport Chain (ETC)

• There are three classes of carrier molecules in Electron transport chains.
• The first are Flavoproteins. These proteins contain flavin, a co-enzyme derived from Riboflavin (Vitamin B₂) and are capable of performing alternating oxidations and reductions. One important flavin coenzyme is flavin mononucleotide (FMN).
• The second class of carrier molecules are Cytochromes, proteins with an iron-containing group (heme) capable of existing alternately as a reduced form (Fe²⁺) and an oxidized form (Fe³⁺). The Cytochromes involved in electron transport chains include Cytochrome b (cyt b), Cytochrome e, (cyt e,), Cytochrome e (cyt e), Cytochrome a (cyt a) and Cytochrome a₃ (cyt a₃).
• The third class is known as Ubiquinones, or Co-enzyme Q, symbolized Q. These are small non-protein carriers.

Multiprotein Complexes in Electron Transport Chain (ETC)

(i) Complex - I (NADH-CoQ Reductase)(Molecular weight – 8,50,000; Polypeptide chain subunits – 16 to 25)

• Complex - I has two functions: (i) Electron Transfer and (ii) Act as a proton pump.
• The system catalyses transfer of two electrons from NADH to small lipid soluble CoQ via FMN and FeS clusters.
• From FMN.H₂ electrons are transferred to a group of FeS proteins. Fe atoms of FeS protein oscillate between Fe²⁺ and Fe³⁺. The electrons are then transferred to CoQ.
• The Complex – I process is accompanied by pumping of Protons from Mitochondrial matrix into intermembrane space.
• Complex – I permits one ATP Formation (Site – 1)

(ii) Complex - II (Succinate - CoQ Reductase) (Molecular weight – 1,25,000; Polypeptide chain subunits – 4)

• In Complex – II, flow or transfer of electrons from Succinate to CoQ occurs via FAD.H₂.
• Complex II consists of four protein sub-units: (i) Succinate dehydrogenase (SDHA), (ii) Succinate dehydrogenase (Ubiquinone), (iii) Iron - sulfur subunit, mitochondrial, (SDHB), (iv) Succinate dehydrogenase complex sub-unit C, (SDHC) and (v) Succinate dehydrogenase complex, sub-unit D, (SDHD).
• The small energy change does not allow “Succinate-CoQ reductase” system to pump protons across the mitochondrial membrane, hence this protein complex does not contribute to proton gradient. Hence no ATP is formed.

(iii) Complex - III (CoQ – Cytochrome – c Reductase) (Molecular weight – 2,50,000; Polypeptide chain subunits – 8)

• Complex - III has two functions: (i) Proton pump and (ii) Catalyses transfer of electrons.
• The Complex - III system catalyses transfer of electrons from CoQ.H₂ to Cyt-c via Cyt-b and Cyt-c₁.
• The Complex - III system also acts as a proton pump. It is believed that 4 (four) protons are pumped across the mitochondrial membrane during the oxidation.
• The energy change permits ATP formation (Site - II).

(iv) Complex - IV (Cytochrome – c Oxidase) (Molecular weight – 3,00,000; Polypeptide chain subunits – 7)

• Complex - IV has two functions: (i) Proton pump and (ii) Catalyses transfer of electrons to molecular O₂ to form H₂O.
• Complex - IV is the terminal component of ETC. It catalyses the transfer of electrons from Cyt-c to molecular O₂ via Cyt-a, Cu⁺⁺ ions and Cyt-a₃.
• ATP was produced in this system (Site –III)