Biological oxidation

electron transport chain and mechanism

1. BIOLOGICAL OXIDATION DEVIPRIYA P V M PHARM

2. contents Electron transport chain (ETC) and its mechanism. Oxidative phosphorylation & its mechanism Substrate level phosphorylation Inhibitors ETC and oxidative phosphorylation / Uncouplers

3. Biological Oxidation Oxidation is the loss of electrons and reduction is the gain of electrons. oxidation-reduction is applicable to biological systems also. The transfer of electrons from the reduced coenzymes through the respiratory chain to oxygen is known as biological oxidation. The enzymes involved in biological oxidation are: 1. Oxidases 2. Dehydrogenases 3. Hydroperoxidases 4. Oxygenases .

4. Electron transport chain (ETC) Energy-rich molecules, such as glucose, are metabolized by a series of oxidation reactions and yield CO 2 and water. The metabolic intermediates of these reactions donate electrons to specific coenzymes, nicotinamide adenine dinucleotide (NAD+) and flavin adenine dinucleotide (FAD), to form the energy-rich reduced forms, NADH and FADH 2 . These reduced coenzymes donate a pair of electrons to a specialized set of electron carriers, collectively called the electron transport chain The ETC is located in the inner mitochondrial membrane . The inner mitochondrial membrane have 5 enzyme complexes- complex I, II, II , IV and V. The complexes I- IV are carriers of electrons while complex V is responsible for ATP synthesis.

5. There are five distinct carriers that participate in the ETC. (1) Nicotinamide nucleotides: NAD + is reduced to NADH + H + by dehydrogenases with the removal of 2 hydrogen atoms from the substrates (glyceraldehyde-3phosphate, pyruvate,isocitrate , α - ketoglutarate and malate ). (2) Flavoproteins : NADH dehydrogenase reductase is a flavoprotein with FMN as the prosthetic group. The coenzyme FMN accepts 2 electrons and a proton to form FMNH 2 . Succinate dehydrogenase is also a flavoprotein with FAD as the coenzyme. This accept 2 hydrogen atoms from succinate (3) Iron- sulfur proteins: The iron-sulfur ( FeS ) proteins exist in the oxidized (Fe 3+ ) or reduced (Fe 2+ ) state and they participates in the transfer of electron

6. (4) Coenzyme Q ( ubiquinone ): It can accept electrons from FMNH 2 produced in the ETC by NADH dehydrogenase or FADH 2 produced outside ETC (5) Cytochromes : cytochromes are conjugated proteins containing heme group The electrons are transported from coenzyme Q to cytochromes (in the order) b, c1, c, a and a3.

7. Overview of biological oxidation (ETC- Electron transport chain).

8. Oxidative phosphorylation The transport of electrons through the ETC is linked with the release of free energy. The process of synthesizing ATP from ADP and Pi coupled with the ETC is known as oxidative phosphorylation . The complex V of the inner mitochondrial membrane is the site of oxidative phosphorylation . There are three reactions in the ETC that are exergonic to result in the synthesis of 3 ATP molecules. Oxidation of FMNH2 by coenzyme Q. Oxidation of cytochrome b by cytochrome c1 . Cytochrome oxidase reaction.

9. MECHANISM OF OXIDATIVE PHOSPHORYLATION. Chemical coupling hypothesis : According to chemical coupling hypothesis, during the course of electron transfer in respiratory chain, a series of phosphorylated high-energy intermediates are first produced which are utilized for the synthesis of ATP. Chemiosmotic hypothesis : Proton gradient : The inner mitochondrial membrane is impermeable to protons (H+) and hydroxyl ions (OH-). The transport of electrons through ETC is coupled with the translocation of protons (H+) across the inner mitochondrial membrane (coupling membrane) from the matrix to the intermembrane space. The pumping of protons results in a proton gradient which results in the synthesis of ATP from ADP and Pi. ATP synthase , present in the complex V, utilizes the proton gradient for the synthesis of ATP.

10. Rotary model for ATP generation: The changes in the mitochondrial membrane proteins leads to the synthesis of ATP. Structure of mitochondrial ATP synthase complex Inner mitochondrial membrane

11. substrate level phosphorylation ATP may be directly synthesized during substrate oxidation in the metabolism. The high-energy compounds such as phosphoenolpyruvate and 1,3-bisphosphoglycerate (intermediates of glycolysis) and succinyl CoA (of citric acid cycle) can transfer high-energy phosphate to ultimately produce ATP.

12. Inhibitors of ETC (1)NADH and coenzyme Q : Fish poison rotenone, barbituate drug Amytal and antibiotic piercidin A inhibit this site. (2)Between cytochrome b and c1 : Antimycin A -an antibiotic, British antilewisite (BAL)-an antidote. (3)Inhibitors of cytochrome oxidase : Carbon monoxide, cyanide, hydrogen sulphide and azide effectively inhibit cytochrome oxidase . Carbon monoxide reacts with reduced form of the cytochrome while cyanide and azide react with oxidized form. Cyanide is a potent inhibitor of ETC. It binds to Fe3+ of cytochrome oxidase blocking mitochondrial respiration leading to cell death.

13. Inhibitors of oxidative phosphorylation / Uncouplers Uncouplers are compounds that can uncouple (or delink) the electron transport from oxidative phosphorylation They increase the permeability of inner mitochondrial membrane to protons (H+), so that ATP synthesis does not occur. The uncouplers allow oxidation of substrates without ATP formation. Eg : 2,4-dinitrophenol (DNP), dinitrocresol , pentachlorophenol, trifluorocarbonylcyanide phenylhydrazone ( FCCP). Physiological substances like thermogenin , thyroxine and long chain free fatty acids The antibiotic Oligomycin prevents the mitochondrial oxidation as well as phosphorylation . Plant toxin Atractyloside is an uncoupler