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OXIDATIVE PHOSPHORYLATION Electron transport chain (ETC) reaction Definition Components of the ETC Mechanism of oxidation phosphorylation (formation of ATP) Chemiosmotic hypothesis The flow sheet of respiration, showing the origin of the pairs of hydrogen atom removed by dehydrogenases with the transfer of their electrons (2е-) to the electron transport chain which carries them to oxygen . Reduction of each atom of oxygen requires 2е- +2H+.Energy set free during transport of a pair of electron from NADH to oxygen is harnessed to cause the coupled synthesis of three of ATP from ADP and phosphate in the process of oxidation phosphorylation. The electron transport chain is shown here in the abbreviation form. Definition The oxidation of a substrate with accompanying phosphorylation of ADP to ATP is known as oxidation phosphorylation. This process takes place in mitochondria as the enzymes concerned with Citric acid cycle are located in the space between the cristae and the enzyme concerned electron transport are arranged in appropriate patter in cristae themselves. Oxidation phosphorylation is fundamental to all aspect of cellular life in aerobic organisms since it is their main source of useful energy. Oxidation phosphorylation involves the reduction of O2to H2O with electrons donated by NADH and FADH2 . Components of the ETC Pyridine nucleotides Dehydrogenases enzymes cannot catalyze the removal of hydrogen from substrate unless low molecular weight cofactors are also present to act as hydrogen acceptors. There are frequently two pyridine nucleotides : 1- Nicotinamide adenine dinucleotide (NAD) 2- Nicotinamide adenine dinucleotide phosphate (NADP) Components of the ETC Flavoproteins Reversible hydrogen acceptors are flavoprotein, which consist of protein moiety and a prosthetic group containing riboflavin. There are two flavin prosthetic group 1- Flavin mononucleotides (FMN). 2- Flavin dinucleotides (FDN). Components of the ETC Coenzyme Q An important part of the chain of compounds involved in cellular respiration. Associated with the lipoprotein part of mitochondria. Coenzyme Q is found widely in living materials and has therefore given the name of ubiquinone (latin ubique, every where). Components of the ETC Cytochromes Cytochrome means cellular pigment. Cytochromes are redox catalysts of major important in cellular respiration. They act by reversibly donating and accepting ELECTRONS (not hydrogen). Electrons are transported by the haem iron which undergoes a change of valency in the process. There are many cytochromes designated a, b , c. The electron transport chain Mechanism of oxidation phosphorylation (formation of ATP) There are 3 hypothesis regarding the mechanism of oxidation phosphorylation: 1- Chemical coupling hypothesis. 2- Conformational coupling hypothesis. 3- Chemiosmotic hypothesis. 1- Chemical coupling hypothesis. • It proposes that the electron transport is coupled ATP synthesis by sequence of consecutive reaction in which a high energy intermediate is formed by electron transport and subsequently is cleaved and donates its energy to generate ATP. 2- Conformational coupling hypothesis. • It proposes that the transport of electron along the respiratory chain causes a conformational change in protein component of inner membrane to yield a high-energy form. The conformational change so produced is transmitted to F0F1ATPase molecule, causing it to become energized. Relaxation of the energized F0F1ATPase to its normal conformation has proposed to provide the energy for the synthesis of ATP and its release from the enzyme F0F1ATPase . 3- Chemiosmotic hypothesis. • It postulates that electron transport, pumps H+ from the matrix across the inner mitochondria membrane to the outer aqueous phase thus generating a H+gradient across the inner membrane. The osmotic energy inherent in its gradient was postulated to drive the energy requiring synthesis ATP. • This mechanism mainly involves the following 3 important stages: 1- Ionization of hydrogen, 2- Electron transport chain. 3- Formation of water. Hydrogen atom ionized to hydrogen ion H+ . Electrons removed from hydrogen atom enter an electron transport chain. This ETC include flavoprotein, iron sulfide protein, ubiquinone and the cytochromes b,c1, c ,a and A3. Each electron gets shuttled from one of these acceptors to the next until it finally reaches cytochrome A3 which called cytochrome oxidase because it is capable, by giving 2 electron, of causing elemental oxygen to combine with hydrogen ions to form water. The next step in the oxidation phosphorylation is to convert ADP to ATP by a large protein in inner membrane called ATPase (F0F1ATPase)