Respiratory Chain PDF

Respiratory chain Electron transport chain (ETC) The respiratory chain is one of the pathways involved in oxidative phosphorylation It catalyzes the steps by which electrons are transported from reduced coenzyme (NADH+H+) or reduced ubiquinone (QH2) to molecular oxygen to form...

Respiratory chain Electron transport chain (ETC) The respiratory chain is one of the pathways involved in oxidative phosphorylation It catalyzes the steps by which electrons are transported from reduced coenzyme (NADH+H+) or reduced ubiquinone (QH2) to molecular oxygen to form water The redox reactions “oxidation-reduction” are accompanied by release of free energy(this reaction is strongly exergonic) Most of the energy released is used to establish a proton gradient across the inner mitochondrial membrane which is then ultimately used to synthesize ATP with the help of ATP synthase (syntheses of high energy phosphate bond for conversion of ADP to ATP) Components of the respiratory chain ETC The electron transport chain ETC consists of: 1. Three protein complexes (complexes I, III, and IV) which are integrated into the inner mitochondrial membrane 2. Two mobile electrons carrier; ubiquinone (coenzyme Q ) and cytochrome C 3. Succinate dehydrogenase, of the tricarboxylic acid cycle TCA, is also assigned to the respiratory chain as complex II 4. ATP synthase is sometimes referred to as complex V, although it is not involved in electron transport All of the complexes in the respiratory chain are made up of numerous polypeptides and contain a series of different protein bound redox coenzymes; These include flavins (FMN or FAD in complexes I and II), iron–sulfur clusters (in I, II, and III), and heme groups (in II, III, and IV) Electrons enter the respiratory chain in various different ways A. Complex I: NADH dehydrogenase  Contains enzyme called NADH dehydrogenase  Its coenzyme is FMN and contains several Fe/S cluster  It oxidizes NADH+H+ into NAD, electrons pass via FMN and Fe/S clusters to ubiquinone CoQ to form CoQH2 B. Complex II: succinate dehydrogenase  Contains enzyme called flavoprotein dehydrogenase e.g. succinate dehydrogenase of TCA and acyl CoA dehydrogenase of fatty acid oxidation  Its coenzyme is FAD and contains Fe/S cluster and heme group  It catalyze transfer of electrons from FADH2 to CoQ to form CoQH2 C. Complex III: Ubiquinol-cytochrome c reductase  Ubiquinol (CoQ)passes electrons on to complex III, which transfers them via two heme b groups “cyt b”, one Fe/S cluster, and heme c1 “cyt c1” to the small heme protein cytochrome C. 1  D. Complex IV: Cytochrome C Oxidase  Cytochrome C then transports the electrons to complex IV  Cytochrome C oxidase contains redox-active components in the form of two copper centers (Cu A and Cu B) and hemes a and a3(Cyt a & Cyt a3), through which the electrons finally reach oxygen  As the result of the two-electron reduction of O2, the O2– anion is produced, and this is converted into water by binding of two protons 2H+ The electron transfer is coupled to the formation of a proton gradient by complexes I, III, and IV E. Complex V: H+ transporting ATP synthase ATP synthesis  Proton transport via complexes I, III, and IV takes place from the matrix into the inter membrane space  When electrons are being transported through the respiratory chain, the H+ concentration in this space increases i. e., the pH value there is reduced by about one pH unit  For each H2O molecule formed, around 10 H+ ions are pumped into the inter membrane space  If the inner membrane is intact, ATP synthase can allow protons to flow back into the matrix. This is the basis for the coupling of electron transport to ATP synthesis  The energy obtained in this process is used to establish a proton gradient across the inner mitochondrial membrane 2   ATP synthesis is ultimately coupled to the return of protons from the intermembrane space into the matrix.  O2 reduction and ATP formation also take place in the matrix. ATP synthase The ATP synthase (complex V) that transports H+ is a complex molecular machine The enzyme consists of two parts—a proton channel (Fo, for “oligomycin-sensitive”) that is integrated into the membrane; and a catalytic unit (F1) that protrudes into the matrix. The catalytic cycle can be divided into three phases, through each of which the three active sites pass in sequence 3 Regulation The need to coordinate the production and consumption of ATP is already evident from the fact that the total amounts of coenzymes in the organism are low A. Respiratory control  The simple regulatory mechanism which ensures that ATP synthesis is “automatically” coordinated with ATP consumption is known as respiratory control  It is based on the fact that the different parts of the oxidative phosphorylation process are coupled via shared coenzymes and other factors  If a cell is not using any ATP, there is no any ADP will be available in the mitochondria  Without ADP, ATP synthase is unable to break down the proton gradient across the inner mitochondrial membrane. This in turn inhibits electron transport in the respiratory chain, which means that NADH+H+ can no longer be reoxidized to NAD+ Finally, the resulting high NADH/NAD+ ratio inhibits the tricarboxylic acid cycle  Conversely, high rates of ATP utilization stimulate nutrient degradation and the respiratory chain via the same mechanism B. Uncouplers  Substances that functionally separate oxidation and phosphorylation from one another are referred to as uncouplers  They break down the proton gradient by allowing H+ ions to pass from the inter membrane space back into the mitochondrial matrix without the involvement of ATP synthase  Uncoupling effects are produced by mechanical damage to the inner membrane or by lipid-soluble substances that can transport protons through the membrane  Example: 1. 2,4-dinitrophenol (DNP) 2. Thermogenin (uncoupling protein-1, UCP-1) 3. Thyroxin 4. Bilirubin 5. Ca++ 6. Arsinate N.B Sites of inhibition of the respiratory chain by specific drugs, chemicals, and antibiotics.  Barbiturates such as amobarbital inhibit electron transport via Complex I by blocking the transfer from Fe-S to CoQ. At sufficient dosage, they are fatal.  Antimycin A and dimercaprol inhibit the respiratory chain at Complex III 4  The classic poisons H2S, carbon monoxide, and cyanide inhibit Complex IV and can therefore totally arrest respiration  Malonate is a competitive inhibitor of Complex II  The antibiotic oligomycin completely blocks oxidation and phosphorylation by blocking the flow of protons through ATP synthase C. Regulation of the tricarboxylic acid cycle(TCA)  The most important factor in the regulation of the cycle is the NADH/NAD+ ratio  In addition to pyruvate dehydrogenase (PDH) and α-keto glutarate dehydrogenase, citrate synthase and isocitrate dehydrogenase are also inhibited by NAD+ deficiency or an excess of NADH+H+ 5