Electron Transport Chain And Oxidative Phosphorylation Pdf
File Name: electron transport chain and oxidative phosphorylation .zip
Due to the existence of electron leak and proton leak, not all electrons in the ETC can be transferred to the final electron acceptor O 2 and the energy released by the transferred electrons cannot be completely coupled with ATP generation. However, both the ROS generated by electron leak and the UCPs implicated in proton leak play an important role in the physiology and pathology of cells. Therefore, it is extremely important to understand the process of electron transfer in the ETC and the mechanism of electron leak and proton leak.
- Oxidative Phosphorylation Impairment by DDT and DDE
- Oxidative Phosphorylation and Electron Transport
- Oxidative Phosphorylation
The electron transport chain is the last component of aerobic respiration and is the only part of glucose metabolism that uses atmospheric oxygen. Oxygen continuously diffuses into plant tissues typically through stomata , as well as into fungi and bacteria; however, in animals, oxygen enters the body through a variety of respiratory systems. Chemiosmosis: In oxidative phosphorylation, the hydrogen ion gradient formed by the electron transport chain is used by ATP synthase to form ATP.
The components have progressively more positive E 0 values leading to electron transfer and energy production for pumping protons out of the matrix. The components span the inner membrane asymmetrically to facilitate proton pumping. Components : b cytochromes, Fe-S-protein center, cytochrome c 1. Controlled influx of these protons produces "site one" for ATP production. Coenzyme Q transfers its electrons to cytochrome b and then to a heme Fe-S center prosthetic group of the cytochromes.
Oxidative Phosphorylation Impairment by DDT and DDE
Due to the existence of electron leak and proton leak, not all electrons in the ETC can be transferred to the final electron acceptor O 2 and the energy released by the transferred electrons cannot be completely coupled with ATP generation.
However, both the ROS generated by electron leak and the UCPs implicated in proton leak play an important role in the physiology and pathology of cells. Therefore, it is extremely important to understand the process of electron transfer in the ETC and the mechanism of electron leak and proton leak. In this review, the basic components of the ETC are discussed and the process of electron transfer in each complex, including the structure, composition and function of each complex is reviewed.
Moreover, proton leak is emphatically introduced, including the structure, tissue distribution, functions and regulatory factors of UCPs. The ETC, which is composed of transmembrane protein complexes I-IV and the freely mobile electron transfer carriers ubiquinone and cytochrome c, exists in the folded inner membranes called cristae Fig. The complexes must be assembled into a specifically configured supercomplex to function properly 2 , 3. To better understand the whole process of how electron transportation produces ATP via the ETC, it is necessary to know the ultrastructure and function of the individual complexes.
Generation of electron leaks and proton leaks in the electron transport chain. The red arrows indicate electron pathways. The black arrows represent substrate reactions.
The blue arrows show the proton circuit across the IMM. A number of studies have reported the structure of the bacterial mitochondrial CI using X-ray crystallography at a nearly atomic resolution 4 , 5. Mitochondria from the Bos taurus heart have been regarded as the best model for human CI 6 - 9.
These studies demonstrate that the L-shaped eukaryotic CI contains two domains: The membrane arm embedded in the inner membranes and the matrix arm protruding into the matrix. The two domains are mainly composed of 14 core subunits that are conserved from bacterial CI and are the core of the enzymatic reaction. There are 45 clearly identified proteins that participate in the formation of the core subunits.
The membrane arm contains seven hydrophobic subunits ND and ND4L , all of which are encoded by the mitochondrial genome. In addition, a large number of accessory subunits are arranged around the core subunits. The assembly of these modules has been reviewed in detail elsewhere An FMN bound at the cusp of the matrix arm could form FMNH 2 by accepting a pair of electrons derived from matrix NADH, which is primarily produced by the tricarbox-ylic acid Krebs cycle that continuously occurs in the matrix.
The ubiquinone binding site is located at the junction of the membrane arm and matrix arm, in which ubiquinone CoQ is reduced to ubiquinol QH 2.
The energy released by the transfer of a pair of electrons from NADH to CoQ in CI probably not definitively induce the pumping of four protons from the matrix into the intermembrane space 14 - Several hypotheses exist in current research: Ohnishi 18 proposed a hypothesis that two protons are indirectly pumped out in a conformation-coupled manner and that the other two protons are directly pumped out by the induction of ubiquinone redox.
Sazanov and Hinchliffe 4 hypothesized that three protons are indirectly pumped via three antiporter homologs, and the final proton is shifted in an unclear way. In addition, Tan et al 14 speculated that the conformation changes and the density of water molecules in the trans-membrane domain determine the proton translocation in CI.
However, how the energy transfers from the redox reaction to proton translocation are still unknown. As a part of the Krebs cycle, CII catalyzes the oxidation of succinate to fumarate. CII consists of four subunits A total of two of the subunits, the membrane-anchor proteins CybL and CybS, are hydrophobic, anchor the complex to the inner membrane, and contain the CoQ binding site. The other two subunits are located on the matrix side of the inner membrane and contain the binding site of the substrate succinate, three FeS clusters [ 2Fe-2S , 4Fe-4S and 3Fe-4S ], and a flavoprotein covalently bound to a FAD cofactor.
The assembly steps of the four subunits are detailed elsewhere Electron transport in CII is not accompanied by the translocation of protons. CIII is commonly referred to as a cytochrome bc l complex, or CoQ-cytochrome c reductase and transfers the electrons carried by QH 2 to cytochrome c.
CIII is a symmetrical dimer with 11 subunits per monomer The catalytically active subunits are cytochrome b b L and b H , cytochrome c 1 and a high-potential 2Fe-2S cluster wrapped by an iron-sulphur protein There are two CoQ binding sites on both ends of cytochrome b embedded in the inner membrane of the mitochondria, one of which is the QH 2 oxidation site Q o located at the cytoplasmic side, which is related to the low potential cytochrome b L.
The other is the Q - reduction site Q i on the side of the matrix, which is related to the high potential cytochrome b H QH 2 is oxidized to ubisemiquinone QH - after transferring an electron to the 2Fe-2S cluster and two protons are concurrently released into the mitochondrial intermembrane space IMS from the matrix The 2Fe-2S cluster transfers this electron to cytochrome c 1 , from which it is transferred to cytochrome c, a mobile electron carrier.
To complete the Q-cycle, the second QH 2 molecule is oxidized at the Q o site while displacing the other two protons. CIV, also known as cytochrome c oxidase, transfers electrons from cytochrome c to the terminal electron acceptor O 2 to generate H 2 O. Mammalian CIV consists of 13 different subunits containing four redox-active metal centers, namely, Cu A , heme a Fe a and a binuclear center composed of heme a 3 Fe a3 and Cu B 29 , Subunit I contains three of the four cofactors, heme a and the binuclear center, which transfers electrons from heme a to O 2 Subunit III stabilizes the other two core proteins and is mainly involved in proton pumping 33 , Cytochrome c, similar to CoQ, is a mobile electron carrier that is loosely connected to the outer surface of the inner mitochondrial membrane by electrostatic interactions, allowing it to interact with the cytochrome c 1 of CIII and to accept electrons The reduced cytochrome c moves along the surface of the membrane and interacts with subunit II of CIV by electrostatic interactions, simultaneously transmitting electrons to the Cu A site of subunit II, and then the electrons are passed from heme a to the binuclear center of subunit I 29 , 39 , where the O 2 is reduced to H 2 O.
A total of four electrons at a time from cytochrome c are almost simultaneously transferred to bind dioxygen; eight protons in total are removed from the matrix, of which half are used to form the two water molecules and the other four are pumped across the membrane into the IMS The F 0 domain, located in the inner mitochondrial membrane, contains a subunit c-ring, including one of each of the subunits a, b, d, F6 and oligomycin sensitivity-conferring protein OSCP as well as the accessory subunits e, f, g and A6L 41 , The subunits b, d, F6 and OSCP form the peripheral stalk, which is located on one side of the complex.
A number of additional subunits e, f, g and A6L , which all span the membrane, are associated with the c-ring subunit. In conclusion, the entire composition of each individual complex has been well described over the past century and it is now widely accepted that these complexes must establish interactions and form supercomplexes to perform their function.
Due to the application of cryo-electron microscopy, a greater understanding of the high-resolution structure of these complexes has been gained 45 - Mitochondria are a main source of cellular ROS. Under physiological conditions, 0. The occurrence of numerous diseases and hypoxia are closely related to the increase of ROS production. Hernansanz-Agustin et al 53 found that acute hypoxia produces a superoxide burst during the first few minutes in arterial endothelial cells and CI mainly participated in this process.
ROS are exclusively produced in the matrix, because the flavoprotein is located on the matrix side of the inner mitochondrial membrane In addition, any contribution by site II F can be dampened by the occupation of the CII flavoprotein site by dicarboxylic acids, particularly oxaloacetate, malate and succinate, which blocks the access of oxygen to site II F , where it would form ROS 21 , CIII transfers electrons through the Q-cycle.
Muller et al 60 built two models explaining how superoxide can reach the matrix. This permanent and stable oxidant molecule, which freely disperses through the outer membrane of mitochondria, acts as an intracellular signaling molecule, physiologically functioning via the direct modification of amino acids It is important to note that the binu-clear center structure of CIV is crucial for the nonsequential transfer of the three electron equivalents 39 , In the past, it was believed that ROS were exclusively harmful to cells.
However, recent studies have demonstrated that ROS appear to be very important second messengers that mediate different intracellular pathways 50 , 61 , ROS act through the oxidative modification of numerous types of proteins, particularly receptors, kinases, phosphatases, caspases, ion channels and transcription factors Under hypoxic conditions, ROS activate AMPK, which can upregulate cytoprotective autophagy by inhibiting downstream mammalian target of rapamycin activity ROS are also necessary for long-term potentiation, a phenomenon of synaptic plasticity widely regarded as one of the main molecular mechanisms that form the basis of learning and memory 77 , The signaling pathways involved in the different cell fates in which the mitochondrial production of ROS has been implicated.
The amount of ROS generated as a result of a stimulus determines whether ROS play beneficial or harmful roles, which means different physiological or pathological pathways are activated.
A large amount of ROS cause lipid peroxidation, DNA damage, protein oxidation, irreversible impairment of mitochondria, insufficient ATP generation and, eventually, cell death In addition, it is widely known that the ROS burst during reperfusion plays a critical role in ischemia-reperfusion injury The pathologies in which the mitochondrial production of reactive oxygen species has been implicated.
Under routine circumstances, a small number of protons do not pass through ATP synthase and instead flow directly into the mitochondrial matrix across the inner mitochondrial membrane, without the generation of ATP, in a process known as proton leak. In the concept of 'respiratory state' proposed by Chance and Williams 16 , mitochondrial respiration persists in the absence of ADP state 4 and reflects the oxygen consumption of proton leak.
It was found that the proton leak of the inner mitochondrial membrane demonstrated nonohmic conductivity Proton leak consists of two parts: Basal proton leak and inducible proton leak.
Basal proton leakage is not regulated and is related to the lipid bilayer of the inner mitochondrial membrane and the adenine nucleotide translocase ANT. Basal proton leak has an important relationship with the basal metabolic rate BMR in mammals at rest.
The lower the BMR of a species, the weaker the basal proton conductance. Studies have demonstrated that the extent of basal proton leak among species has a phylogenetic relationship , UCP1, which is abundant in brown adipose tissue BAT , may also be involved in basal proton leak , although there remains controversy The majority of the induced proton leak is catalyzed by UCPs.
UCPs belong to the family of mitochondrial anion carrier proteins, through which the protons can reflux into the matrix. In addition to the role of uncoupling, UCPs may also participate in other processes, such as the regulation of calcium homeostasis, ion transportation or synaptic plasticity , UCP1 can also be detected in the beige adipocytes of white adipose tissue WAT during thermal acclimation under specific conditions The genetic deletion of UCP1 severely inhibits cold adaptive thermogenesis and diet-induced adrenergic thermogenesis, and UCP1-null mice develop fatal hypothermia upon cold exposure , UCP1 has also been found in thymocytes and demonstrated to be involved in the maturation and fate determination of developing T-cells - Sale et al found that UCP1 is expressed in islets and associates with the acute insulin response to glucose.
UCP1-catalyzed proton leak could be activated by long chain free fatty acids and inhibited by purine nucleotides There are currently three models for the regulated mechanism of UCP1-implicated proton leak - The UCP3 gene is mainly expressed in skeletal muscle, BAT and heart , and has also been detected in the thymus, spleen and skin cells Therefore, mild uncoupling is a feedback mechanism adopted by the body to prevent excessive ROS in the mitochondria, which was termed 'uncoupling to survive' In addition to the function of reducing the generation of ROS, UCP3 has been demonstrated to be involved in exporting mitochondrial fatty acid anions to the cytoplasm, thereby protecting the mitochondrial against lipid peroxide-induced damage , UCP4 was first detected in the brain, but it has recently been found in adipocytes In addition, UCP4 also plays a predominant role in insect mitochondria On the other hand, UCP5, which is not limited to the brain, is also expressed in the testis, uterus, kidney, lung, stomach, liver and heart Although UCP4 and UCP5 may play an unconfirmed role in the neural system, their function for reducing oxidative stress is clear , Oxidative stress has been proven to be involved in both neurodegenerative diseases and aging, so the UCP-dependent reduction of ROS in the nervous system has the potential to be neuroprotective in diseases such as Alzheimer's disease, Parkinson's disease and amyotrophic lateral sclerosis ,
Oxidative Phosphorylation and Electron Transport
You have just read about two pathways in cellular respiration—glycolysis and the citric acid cycle—that generate ATP. However, most of the ATP generated during the aerobic catabolism of glucose is not generated directly from these pathways. Rather, it is derived from a process that begins with moving electrons through a series of electron transporters that undergo redox reactions: the electron transport chain. This causes hydrogen ions to accumulate within the matrix space. Therefore, a concentration gradient forms in which hydrogen ions diffuse out of the matrix space by passing through ATP synthase.
Electron Transport &. Oxidative Phosphorylation. Dr. Kevin Electron. Transport. Inner. Mitochondrial. Membrane Fungi, Protozoa. Short-circuits System.
In eukaryotes , this takes place inside mitochondria. Almost all aerobic organisms carry out oxidative phosphorylation. This pathway is so pervasive because it releases more energy than alternative fermentation processes such as anaerobic glycolysis.
Every day, we build bones, move muscles, eat food, think, and perform many other activities with our bodies. All of these activities are based upon chemical reactions. However, most of these reactions are not spontaneous i.
Electron Transport — the oxidation phase of Oxidative Phosphorylation 3. As charges move, the work that is done can be used to make ATP. In biologic systems, the cells use electron transport chain to transfer electrons stepwise from substrates to oxygen.
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