Oxidative phosphorylation

Oxidative phosphorylation (UK /ɒkˈsɪd.ə.tɪv/, US /ˈɑːk.sɪˌdeɪ.tɪv/ or electron transport-linked phosphorylation) is the metabolic pathway in which cells use enzymes to oxidize nutrients, thereby releasing energy which is used to produce adenosine triphosphate (ATP). In most eukaryotes, this takes place inside mitochondria. Almost all aerobic organisms carry out oxidative phosphorylation. This pathway is probably so pervasive because it is a highly efficient way of releasing energy, compared to alternative fermentation processes such as anaerobic glycolysis.(Volts) NADH + Q + 5 H matrix + ⟶ NAD + + QH 2 + 4 H intermembrane + {displaystyle {ce {NADH + Q + 5H+_{matrix}-> NAD+ + QH2 + 4H+_{intermembrane}}}}     (1) Succinate + Q ⟶ Fumarate + QH 2 {displaystyle {ce {{Succinate}+ Q -> {Fumarate}+ QH2}}}     (2) ETF red + Q ⟶ ETF ox + QH 2 {displaystyle {ce {ETF_{red}{}+ Q -> ETF_{ox}{}+ QH2}}}     (3) QH 2 + 2 Cyt c ox + 2 H matrix + ⟶ Q + 2 Cyt c red + 4 H intermembrane + {displaystyle {ce {QH2{}+ 2 Cyt, c_{ox}{}+ 2H+_{matrix}-> Q{}+ 2 Cyt, c_{red}{}+ 4H+_{intermembrane}}}}     (4) 4 Cyt c red + O 2 + 8 H matrix + ⟶ 4 Cyt c ox + 2 H 2 O + 4 H intermembrane + {displaystyle {ce {4Cyt,c_{red}{}+O2{}+8H+_{matrix}->4Cyt,c_{ox}{}+2H2O{}+4H+_{intermembrane}}}}     (5)(Volts) ADP + P i + 4 H intermembrane + ↽ − − ⇀ ATP + H 2 O + 4 H matrix + {displaystyle {ce {ADP + P_i + 4H+_{intermembrane}<=> ATP + H2O + 4H+_{matrix}}}}     (6) O 2 → e − O 2 ∙ _ Superoxide → e − O 2 2 − Peroxide {displaystyle {ce {O2->{underset {Superoxide}{O2^{underline {ullet }}}}->{underset {Peroxide}{O2^{2-}}}}}}     (7) Oxidative phosphorylation (UK /ɒkˈsɪd.ə.tɪv/, US /ˈɑːk.sɪˌdeɪ.tɪv/ or electron transport-linked phosphorylation) is the metabolic pathway in which cells use enzymes to oxidize nutrients, thereby releasing energy which is used to produce adenosine triphosphate (ATP). In most eukaryotes, this takes place inside mitochondria. Almost all aerobic organisms carry out oxidative phosphorylation. This pathway is probably so pervasive because it is a highly efficient way of releasing energy, compared to alternative fermentation processes such as anaerobic glycolysis. During oxidative phosphorylation, electrons are transferred from electron donors to electron acceptors such as oxygen, in redox reactions. These redox reactions release energy, which is used to form ATP. In eukaryotes, these redox reactions are carried out by a series of protein complexes within the inner membrane of the cell's mitochondria, whereas, in prokaryotes, these proteins are located in the cells' intermembrane space. These linked sets of proteins are called electron transport chains. In eukaryotes, five main protein complexes are involved, whereas in prokaryotes many different enzymes are present, using a variety of electron donors and acceptors. The energy released by electrons flowing through this electron transport chain is used to transport protons across the inner mitochondrial membrane, in a process called electron transport. This generates potential energy in the form of a pH gradient and an electrical potential across this membrane. This store of energy is tapped when protons flow back across the membrane and down the potential energy gradient, through a large enzyme called ATP synthase; this process is known as chemiosmosis. The ATP synthase uses the energy to transform adenosine diphosphate (ADP) into adenosine triphosphate, in a phosphorylation reaction. The reaction is driven by the proton flow, which forces the rotation of a part of the enzyme; the ATP synthase is a rotary mechanical motor. Although oxidative phosphorylation is a vital part of metabolism, it produces reactive oxygen species such as superoxide and hydrogen peroxide, which lead to propagation of free radicals, damaging cells and contributing to disease and, possibly, aging (senescence). The enzymes carrying out this metabolic pathway are also the target of many drugs and poisons that inhibit their activities. It is the terminal process of cellular respiration in eukaryotes and accounts for high ATP yield. Oxidative phosphorylation works by using energy-releasing chemical reactions to drive energy-requiring reactions: The two sets of reactions are said to be coupled. This means one cannot occur without the other. The flow of electrons through the electron transport chain, from electron donors such as NADH to electron acceptors such as oxygen, is an exergonic process – it releases energy, whereas the synthesis of ATP is an endergonic process, which requires an input of energy. Both the electron transport chain and the ATP synthase are embedded in a membrane, and energy is transferred from electron transport chain to the ATP synthase by movements of protons across this membrane, in a process called chemiosmosis. In practice, this is like a simple electric circuit, with a current of protons being driven from the negative N-side of the membrane to the positive P-side by the proton-pumping enzymes of the electron transport chain. These enzymes are like a battery, as they perform work to drive current through the circuit. The movement of protons creates an electrochemical gradient across the membrane, which is often called the proton-motive force. It has two components: a difference in proton concentration (a H+ gradient, ΔpH) and a difference in electric potential, with the N-side having a negative charge. ATP synthase releases this stored energy by completing the circuit and allowing protons to flow down the electrochemical gradient, back to the N-side of the membrane. The electrochemical gradient drives the rotation of part of the enzyme's structure and couples this motion to the synthesis of ATP. The two components of the proton-motive force are thermodynamically equivalent: In mitochondria, the largest part of energy is provided by the potential; in alkaliphile bacteria the electrical energy even has to compensate for a counteracting inverse pH difference. Inversely, chloroplasts operate mainly on ΔpH. However, they also require a small membrane potential for the kinetics of ATP synthesis. In the case of the fusobacterium Propionigenium modestum it drives the counter-rotation of subunits a and c of the FO motor of ATP synthase. The amount of energy released by oxidative phosphorylation is high, compared with the amount produced by anaerobic fermentation. Glycolysis produces only 2 ATP molecules, but somewhere between 30 and 36 ATPs are produced by the oxidative phosphorylation of the 10 NADH and 2 succinate molecules made by converting one molecule of glucose to carbon dioxide and water, while each cycle of beta oxidation of a fatty acid yields about 14 ATPs. These ATP yields are theoretical maximum values; in practice, some protons leak across the membrane, lowering the yield of ATP.