Definition of the electron transport chain
The electron transport chain is a group of proteins that transfer electrons across a membrane within mitochondria to form a proton gradient that drives the creation of adenosine triphosphate (ATP). ATP is used by the cell as energy for the metabolic processes of cellular functions.
Where does the electron transport chain occur?
During the process, a proton gradient is created when protons are pumped from the mitochondrial matrix to the intermembrane space of the cell, which also helps drive ATP production. The use of a proton gradient is often referred to as a chemiosmotic mechanism that drives ATP synthesis, as it relies on a higher concentration of protons to generate the “proton motive force”. The amount of ATP created is directly proportional to the number of protons pumped across the inner mitochondrial membrane.
The electron transport chain involves a series of redox reactions that rely on protein complexes to transfer electrons from one molecule donor molecule to an acceptor molecule. As a result of these reactions, the proton gradient is produced, which enables the conversion of mechanical work into chemical energy, allowing the synthesis of ATP. The complexes are embedded in the inner mitochondrial membrane called the cristae at eukaryotes. Enclosed by the inner mitochondrial membrane is the matrix, which is where the necessary enzymes are found such as the pyruvate pyruvate dehydrogenase and pyruvate carboxylase. The process can also be found in photosynthetic eukaryotes in the thylakoid membrane of chloroplasts and in prokaryotes, but with modifications.
The by-products of other cycles and processes, such as the citric acid cycle, the oxidation of amino acids and the oxidation of fatty acidsare used in the electron transport chain. As seen in the general redox reaction,
2 H + + 2 E + + ½ O 2 → H 2 O + ENERGY
energy is released in an exothermic reaction when electrons pass through the complexes; three ATP molecules are created. Phosphate located in the matrix is imported across the proton gradient, which is used to create more ATP. The process of generating more ATP through the phosphorylation of ADP is called oxidative phosphorylationsince the energy from oxygenation of the hydrogen is used throughout the electron transport chain. The ATP generated from this reaction continues to fuel most of the cellular reactions necessary for life.
Steps in the electron transport chain.
In the electron transfer chain, electrons move along a series of proteins to generate an expulsion-type force to move hydrogen ions, or protons, across the mitochondrial membrane. The electrons begin their reactions at Complex I, continue to Complex II, pass through Complex III and cytochrome c through the coenzyme Q, and finally Complex IV. The complexes themselves are proteins of complex structure embedded in the phospholipid membrane. They are combined with a metal ion, such as the ironto assist with proton ejection into the intermembrane space, as well as other functions. The complexes also undergo conformational changes to allow openings for transmembrane movement of protons.
These four complexes actively transfer electrons from an organic metabolite, such as glucose. When the metabolite is broken down, two electrons and a hydrogen ion are released and then the coenzyme NAD + the collects to convert to NADH, releasing a hydrogen ion into the cytosol .
NADH now has two electrons that are passed to a more mobile molecule, ubiquinone (Q), in the first protein complex (Complex I). Complex I, also known as NADH dehydrogenase, pumps four hydrogen ions from the matrix into the intermembrane space, establishing the proton gradient. In the next protein, Complex II or succinate dehydrogenase, another electron carrier and coenzyme, succinate is oxidised to fumarate, which causes FAD (flavin adenine dinucleotide) to be reduced to FADH. 2 . The transport molecule, FADH 2, is oxidised again, donating electrons to Q (becoming QH 2 ), while releasing another hydrogen ion into the cytosol. While Complex II does not contribute directly to the proton gradient, it serves as another source of electrons.
Complex III, or cytochrome c reductase, is where the Q cycle takes place. There is an interaction between Q and cytochromes, which are molecules composed of iron, to continue the electron transfer. During the Q cycle, ubiquinol (QH 2 ) previously produced donates electrons to ISP and cytochrome b is converted to ubiquinone. ISP and cytochrome b are proteins found in the matrix which then transfers the electron it received from ubiquinol to cytochrome c1. Cytochrome c1 then transfers it to cytochrome c, which moves the electrons to the last complex (Note: unlike ubiquinone (Q), cytochrome c can only transport one electron at a time). Ubiquinone is then reduced back to QH 2restarting the cycle. In the process, another hydrogen ion is released into the cytosol to further create the proton gradient.
The cytochromes then extend to Complex IV or cytochrome c oxidase. Electrons are transferred one at a time to the cytochrome c complex. The electrons, in addition to the hydrogen and the oxygenreact to form water in an irreversible reaction. This is the last complex that translocates four protons across the membrane to create the proton gradient that develops ATP in the end.
As the proton gradient is set up, the ATP synthase F 1 F 0 sometimes referred to as Complex V, generates ATP. The complex is composed of several subunits that bind protons released in previous reactions. As the protein turns, the protons return to the mitochondrial matrix, allowing ADP to bind to free phosphate to produce ATP. For each complete turn of the protein, three ATP are produced, concluding the electron transport chain.
