What is the introduction of ETC?

Oxidative phosphorylation is a metabolic process in which energy is harnessed for the production of ATP. The process occurs in the mitochondria. Electrons released through the oxidation of glucose are shuttled into the oxidative phosphorylation supercomplex via FMNH2. The electrons are passed through a remarkable electron transport chain along and across the mitochondrial membrane. The electron transport chain releases minute amounts of energy with each electron transfer, and the transport is coupled to the pumping of protons across the mitochondrial membrane. Eventually, the electrons are delivered to molecular oxygen, which is reduced to water. Finally, the protons that have gathered on the edge of the mitochondrial membrane cascade back across, turning a molecular millwheel that drives the manufacture of ATP. The ATP is used to power processes throughout the cell.

The mitochondria are the site of metabolic activity in the eukaryotic cell. The citric acid cycle occurs within the mitochondrial matrix, catalysed by a range of metabolic enzymes. For this reason, mitochondria are sometimes called "the power plants of the cell". Oxidative phosphorylation plays a central role in this production, harvesting electrons from NADH and succinate to manufacture ATP.

The complexes involved in oxidative phosphorylation are embedded in the inner mitochondrial membrane. In the picture below, the lower portion of the outer mitochondrial membrane is visible at the top. The inner mitochondiral membrane stretches across the middle of the picture. Both membranes are formed by lipid bilayers. In contrast, both the intermembrane space and the mitochondrial matrix are aqueous environments. The complexes that take part in oxidative phosphorylation are labelled I-V in the picture.

Each complex is actually a collection of different proteins; Complex I alone is composed of over 40 protein subunits, but in the picture above each complex has been simplified to one monolithic block. Each complex has its own specialised role. Both Complex I and Complex II serve as entry points for electrons into the respiratory electron transport chain. Complex I accepts electrons from NADH, produced in glycolysis and the citric acid cycle. Complex II accepts electrons from succinate, which is one of the intermediates in the citric acid cycle. In fact, Complex II is an integral part of the citric acid cycle, since it carries out a key step in that process.

• Complex I and II are the entry points for electrons into the electron transport chain.

Both Complex I and Complex II release small amounts of energy as electrons roll energetically downhill to sites of higher and higher reduction potential. The electrons are then shuttled to the same acceptor, Complex III, via a lipid-soluble electron carrier molecule. The electron transport chain continues, releasing some more energy before the electrons are passed to the final destination in Complex IV. This time, the trip from Complex III to Complex IV is conducted via a hydrophilic protein, cyctochrome c. Electrons travel through Complex IV, back towards the matrix, and are accepted by molecular oxygen, resulting in its reduction to water.

• Small amounts of energy are released as electrons move to sites of higher reduction potential.
• Complex IV is the final destination of the electron transport chain.
• In Complex IV, electrons are used to convert O2 to H2O.

Complex I, III and IV all use the energy released from the electron transport chain to pump protons from the matrix into the intermembrane space. The proton gradient that results across the inner mitochondrial membrane is used to power ATP production in complex V. In addition, a couple of protons are consumed by Complex I and Complex II as they package electrons into the lipid-soluble carrier, ubiquinone/ubiquinol. However, Complex II does not transport any electrons all the way across the inner mitochondrial membrane.

• Complex I, III and IV all use the energy released from the electron transport chain to pump protons across the inner mitochondrial membrane.
• The proton gradient across the membrane drive a nano-millwheel used to manufacture ATP from ADP and inorganic phosphate.

In the following pages we will take a closer look at each of the complexes that take part in this process.

Visit an overview of oxidative phosphorylation at Henry Jakubowski's Biochemistry Online.

Electron Transport Chain

Electron Transport Chain Definition:

Electron Transport Chain (ETC) is the moving of electrons through a series of electron transporters that undergo a redox reaction. Hydrogen ions accumulate in the form of matrix space with the help of an electron transport chain. A concentration gradient creates in which diffusion of hydrogen ions occurs by passing through ATP synthase.

The Electron Transport Chain (ETC) is the part of glucose metabolism which uses atmospheric oxygen. Oxygen continuously diffuses and enters the body through the respiratory system. The electron transport chain is the last component of respiration in a series of redox reactions. Redox reaction arranges a bucket bridge by through electrons pass fastly from one end to the other. There are four protein complexes in the ETC and work together for accessory electron carriers, is known as Electron Chain Transport. The electron chain lies in the multiple copies of the inner mitochondrial membrane of eukaryotes, and the plasma membrane of prokaryotes.

Where Does the Electron Transport Chain Occur?

Electron Transport Chain (ETC) is the series of electron transporter which moves electrons undergo redox reaction. The Electron Transport Chain (ETC) occurs in the inner region of the mitochondrial membrane.

The mitochondrial membrane has two forms, the outer membrane and the lower or inner membrane with cisternae (folds). The electron transport chain is a series of proteins of Transmembrane present in the inner membrane. The electron’s shuttle between those proteins that are used for pumping protons to space lies between the inner membrane and outer membrane. This process generates a gradient that is used to produce ATP.

Steps of the Electron Transport Chain:

Electrons move in a series of proteins in Electron Transport Chain (ETC), to move hydrogen ions across the mitochondrial membrane. The electrons start from their reaction in Complex I, continue toward Complex II, transferred to Complex III, and cytochrome c via Coenzyme Q, and then finally reached to Complex IV. The complex structure embedded proteins in the phospholipid membrane. These are combined with the help of metal ion. These complexes do some conformational changes to permit opening for transmembrane of protein movement. These complexes work for transferring electrons from the organic metabolite. When metabolic breaks down, then one hydrogen ion and two electrons released and coenzyme NAD+ pick up to become NADH, for releasing a hydrogen ion into the cytosol.

• Complex I:

In the complex, I (first protein complex), the two electrons of NADH has to pass onto more mobile molecule, ubiquinone (Q). Complex I is also known as NADH dehydrogenase, which works for the pumping of four hydrogen ions from the matrix into the intermembrane space.

• Complex II:

In Complex II, the next protein which is also known as succinate dehydrogenase is another electron carrier and coenzyme. In this complex, succinate is oxidized into fumarate, which caused by the reducing of FAD (flavin adenine dinucleotide) into FADH2. After this, FADH2 is deoxidized, and donate electrons to Q (which becomes QH2), while the process of releasing another hydrogen ion into the cytosol is continuing. It works as another source for electrons.

• Complex III:

Complex III is also known as cytochrome c reductase. This is the step where the Q cycle occurs. There is a connection between Q and cytochrome, which are composed of molecule iron, to continue the process of transfer of electrons. During the Q cycle, the ubiquinol donates electrons to ISP and cytochrome b become ubiquinone. ISP and cytochrome b are proteins that present in the matrix to transfer the electron it receives from ubiquinol to cytochrome c1. Then, cytochrome c1 transfers electrons to cytochrome c that moves the electrons next complex. Ubiquinone again reduces to QH2 for restarting the cycle. In this phenomenon, another hydrogen ion releases into the cytosol for the creation of the proton gradient.

• Complex IV:

Complex IV is the last protein, also known as cytochrome c oxidase. In this complex, the electrons transfer one at a time. In addition to hydrogen and oxygen, the electrons react to form water in the irreversible reaction. Complex IV is the last complex which translocates four protons in the membrane for the creation of proton gradient which creates ATP at the end.

Sometimes, establishing of the proton gradient, F1 F0 ATP synthase refers to as Complex IV, and develop the ATP. The complex is made of several subunits that bind to releasing protons in prior reactions. As proteins rotate, protons brought back to the mitochondrial matrix, permit ADP to bind phosphate for the production of ATP. For every turn, the production of three ATP occurs, concluding the Electron Chain Transport.