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Oxidative Phosphorylation & Its Mechanism and Substrate Level Phosphorylation

It is called electron transport linked phosphorylation because it explains how nutrients are used during oxidation to release energy.

Oxidative Phosphorylation and It's Mechanism

It is called electron transport linked phosphorylation because it explains how nutrients are used during oxidation to release energy. Oxidative phosphorylation leads to ATP formation.. Eukaryotes utilize it as the final step of cell respiration as well as an important cellular energy conversion process. Eukaryotic cells or prokaryotic cells undergo oxidative phosphorylation in their mitochondrial inner membranes.

Substrate-Level Phosphorylation and Oxidative Phosphorylation

An energy-rich compound, which is phosphorylated by the coupled reaction, is transferred to ADP so that ATP can be synthesized by substrate phosphorylation. The phosphate group in GDP is recharged to produce GTP.

Phosphorylation at the substrate level and oxidative phosphorylation are both similar

Substrate-level phosphorylation and oxidative phosphorylation share the common property that ATP is their ultimate product.

Oxidative phosphorylation and substrate-level phosphorylation are two different modes of phosphorylation

The energy needed to convert ADP into ATP in oxidative phosphorylation is provided by a different source than in substrate-level phosphorylation. To phosphorylate ADP into ATP, a substrate level phosphorylation is used to use energy from the coupled reaction. A pair of coupled reactions is seen as occurring simultaneously in oxidative phosphorylation. The oxidative phosphorylation period involves the transfer of energy from ADP to ATP produced by oxidative reactions.

Phosphorylation by Oxidative Oxidation

Oxidative phosphorylation produces bulk ATP, which is the major source of energy for living organisms. Oxidative phosphorylation is highly used to regulate apoptosis to the formation of ROS – Reactive Oxygen Species.

Phosphorylation by Oxidation

In order for a hydroelectric dam to function, water falls into it and generates potential energy, which is then converted into kinetic energy, which is later converted into electrical energy. ADP is converted into ATP during oxidative phosphorylation by a process known as chemiosmosis, just as electricity is produced from hydroelectric dams.

NADH, a coenzyme with a very high transfer electrical potential is produced in eukaryotes during catabolism, such as glycolysis or the citric acid cycle. During mitochondrial respiration, a hydrogen ion is pumped through the inner mitochondrial membrane by an electron transferred from NADH to oxygen via an electron transport chain (ETC) in the mitochondrial matrix. In the mitochondrial inner membrane, hydrogen ions are found in greater concentration in the intermembrane space than in the matrix, causing an electrochemical gradient to form. In the inner mitochondrial membrane, the proton-motive force is captured by ATP synthase when hydrogen ions pass across the electrochemical gradient. This is how chemiosmosis occurs.

On the inner membrane of the mitochondria, there is a series called the electron transport chain.

NADH – Coenzyme Q Qxidoreductase

In the electron transport chain, NADH dehydrogenase, complex I or NADH oxidoreductase plays the primary role of transporting protons as the first enzyme. NADH is oxidized by coenzyme-catalyzed reactions. The mitochondrial matrix pumps four protons into the intermembrane space in response to two electrons passing through complex I.

Succinic-Coenzyme Q Qxidoreductase

In the electron transport chain, succinate dehydrogenase, or succinate oxidoreductase, allows protons to travel through the second enzyme. It catalyzes metabolic reactions by reducing coenzyme Q10 into ubiquinone (QH2) and oxidizing succinic acid to fumarate. This reaction consumes far less energy than the oxidation of ATP because it involves electron transfer and proton pumping. Electrons from electron transfer flavin, which is found in the mitochondrial matrix, are used by electron transfer flavin-coenzyme Q oxidoreductase to reduce Q10.

Coenzyme Q-cytochrome C reductase

In addition to the reduction of cytochrome c and ferritin, complex III also catalyzes the oxidation of QH2 by coenzyme Q-cytochrome C reductase. The electron is carried by cytochrome C in this reaction. When QH2 is oxidized by Coenzyme Q1 reductase, it produces Coenzyme Q10. It leads to the generation of proton gradients, the result of protons moving across the membrane.

Cytochrome C oxidase

Cytochrome c oxidase, also known as complex IV, is the last protein complex in the electron transport chain. Essentially, it acts as the last link in the electron transport chain - taking electrons to the final electron receptor oxygen - reducing oxygen to water - causing protons to move between the membrane and the cytosol. Protons that are pumped out directly and those that are consumed by oxygen reduction led to a higher proton gradient at the end of the reaction.

Proton concentration gradient drives the ATP synthase to phosphorylate ADP to produce ATP.

It is the cytochrome c oxidase, also known as complex IV, that makes up the final protein complex of the electron transport chain. In the final reaction of electron transfer, electrons are transferred from the final electron receptor oxygen to water, which oxidizes into protons that are diffused to the surface of the membrane. Protons that are pumped out directly and those that are consumed by oxygen reduction led to a higher proton gradient at the end of the reaction. Neither of these reactions pumps out protons. After the NADH2 electron transport chain, the subsequent reactions are nearly identical.

In bacteria and archaea, electron transfer enzymes are abundant, which can metabolize chemical substrates from a wide range of sources. Electron transport is also driven by the energy released from oxidation of the substrate to move protons across the membrane in prokaryotes. This gradient is used to drive the ATP synthase to generate ATP. Unlike bacteria, archaea utilize a wide variety of substrates as electron donors and electron receivers. Prokaryotes also need this to adapt to different environments and grow.

Substrate Level Phosphorylation

The phosphate group of adenosine diphosphate (ADP) is transferred from a reactive intermediate to adenosine triphosphate (ATP) during substrate-level phosphorylation. The process occurs in the cytoplasm (in glycolysis) under aerobic and anaerobic conditions. The oxidation and phosphorylation here are not coupled as they are in oxidative phosphorylation. ADP and phosphate groups are transferred to 1,3-bisphosphoglycerate in order to yield 3-phosphoglycerate; phosphate groups are transferred to pyruvate in order to yield pyruvate in the pay-off phase. In the citric acid cycle, the guanosine triphosphate (GTP) is produced as a by-product of a two-cycle process of substrate-level phosphorylation (per cycle).

Activated skeletal muscles and the brain also exhibit substrate-level phosphorylation. The enzyme creatine phosphokinase converts phosphate from phosphocreatine into ADP to make ATP. As ATP is released, chemical energy is released. A second option, as well as the substrate-level phosphorylation occurring during glycolysis and Krebs cycle, is to create ATP through oxidative phosphorylation. As a result of oxidative phosphorylation, NADH is transformed into NAD+, generating 2.5 ATPs, and FADH2 produces 1.5 ATPs.
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Ankur Choudhary is India's first professional pharmaceutical blogger, author and founder of Pharmaceutical Guidelines, a widely-read pharmaceutical blog since 2008. Sign-up for the free email updates for your daily dose of pharmaceutical tips.
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