The oxidative phosphorylation helps to remove electrons from biological fuels such as glucose, fatty acids etc which further go through a series of electron carrier system named the Mitochondrial Electron Transport Chain (ETC) located in IMM, generating energy in the form of ATP and ultimately these electrons reduce the molecular oxygen to water. Mitochondria, the cellular power house, are the major contributor to oxidative stress being the site of oxygen consumption. Mitochondria controlling the electron transport processīeing an extraordinary complex organelle, mitochondria contains their own DNA, a dual membrane: Outer Mitochondrial Membrane (OMM) and Inner Mitochondrial Membrane (IMM) which gives rise to two compartments the Inter Membrane Space (IMS) located between OMM and IMM, and the Matrix (M) located in the internal space created by the IMM itself. The energetic function of the isolated mitochondria can be assessed by polarographic measurement of oxygen consumption under various conditions which in turn confirm that the addition of Adenosine Di-Phosphate (ADP) to the isolated mitochondria increases the electron flow and oxygen consumption until the excess ADP is phosphorylated to ATP. As shown in figure 1, the proton movement through the F1FoATP synthase couples the energy-releasing reactions of oxidation to the energy-storing reaction of phosphorylation. Complex V (ATP synthase) allows protons to flow back into the mitochondrial matrix by using proton motive force and uses the released energy to synthesize ATP. The electron flux is coupled to the translocation of protons (H+) into the intermembrane space at three coupling sites (complexes I, III, and IV) which create a transmembrane gradient. Electrons are transferred to oxygen via the energy-transducing complexes for the concomitant energy transduction into ATP: complexes I, III, and IV for are used for succinate dehydrogenase complexes III and IV for FADH2 derived from the oxidation pathway via the Electron-Transfer Flavoprotein (ETF) and the ETF-CoQ oxidoreductase system CoQ (a lipoidal quinone) and cytochrome c (a low molecular-weight hemoprotein) act as major shuttles between the complexes. Indeed, the respiratory chain catalyzes the oxidation of fuel molecules such as malate or succinate by oxygen which serves as the terminal electron acceptor in the chain of oxidation-reduction reactions that occur in the inner mitochondrial membrane. Complex III (Coenzyme Q: Cytochrome C-Oxidoreductase,) containing 11 subunits helps to transfer electrons from CoQ to cytochrome c while the complex IV (Cytochrome-C Oxidase), composed of two cytochromes (a and a3), two copper atoms, and 13 different protein subunits, catalyzes the transfer of reducing equivalents from cytochrome c to molecular oxygen. Complex I, NADH:ubiquinone oxidoreductase consists of 25-28 different polypeptides, seven of which are encoded by mitochondrial DNA (mtDNA), carry reducing equivalents from NADH to the Co-Enzyme Q (CoQ) complex II (the succinate- CoQ reductase) contains five polypeptides, including the Flavin Adenine Dinucleotide (FAD)-dependent succinate dehydrogenase and a few non-heme-iron-sulfur centers, carry the reducing equivalents from the reduced flavin adenine di-nucleotide (FADH2) to CoQ. This chain is widely known to consist of five functional units or complexes embedded in the inner mitochondrial membrane. Mitchell P, proposed the theory of coupling of phosphorylation to electron and hydrogen transfer by a chemiosmotic type of mechanism. The respiratory chain or electron transport chain uses electron flow to create a proton gradient (proton motive force) that is used to drive Adenosine Tri-Phosphate (ATP) synthesis. The respiratory chain or electron transport chain Current review focused on the physiological impact of electron/ proton leakage particularly in the eukaryotic cells based on the previous reports emphasized on the prospects of the eukaryotic mitochondrion as a modulator of proton and electron leakage and finally attempted to assess the regulatory mechanisms of such electron/ proton leakage. Thus, the mitochondrial leakage and the physiological effect of leaked protons and electrons grow up with future interest in energy metabolism. In eukaryotes, mitochondrion is known to be the major site of the ROS generation in different pathological processes which may further cause cell damages as evident through the ischemia-reperfusion (I/R) injury, respiratory diseases, cell apoptosis, and even the onset of cancer. The finding of electron leakage during the electron transport within the mitochondrial membrane (in eukaryotes) or in the cell membrane of the prokaryotes is an important issue for the accumulation of the Reactive Oxygen Species (ROS) in the cytosol which in turn induce the probable aging of cells.
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