The electron transport chain is a collection of molecules embedded in the inner membrane of the mitochondrion in eukaryotic cells.
Most components of the chain are proteins, which exist in multiprotein complexes numbered I through IV. Electrons acquired from glucose by NAD+ during glycolysis and the citric acid cycle are transferred from NADH to the first molecule of the electron transport chain in complex I. This molecule is a flavoprotein, so named because it has a prosthetic group called flavin mononucleotide (FMN). In the next redox reaction, the flavoprotein returns to its oxidized form as it passes electrons to an iron-sulfur protein (Fe⋅S in complex I), one of a family of proteins with both iron and sulfur tightly bound. The iron-sulfur protein then passes the electrons to a compound called ubiquinone (Q). This electron carrier is a small hydrophobic molecule, the only member of the electron transport chain that is not a protein.
Most of the remaining electron carriers between ubiquinone and oxygen are proteins called cytochromes.
The electron transport chain has several types of cytochromes, each a different protein with a slightly different electron-carrying heme group. The last cytochrome of the chain, Cyt a3, passes its electrons to oxygen, which is very electronegative. Each oxygen atom also picks up a pair of hydrogen ions (protons) from the aqueous solution, neutralizing the -2 charge of the added electrons and forming water.
Another source of electrons for the transport chain is FADH2. FADH2 adds its electrons to the electron transport chain from within complex II, at a lower energy level than NADH does. The electron transport chain makes no ATP directly. Instead, it eases the fall of electrons from food to oxygen, breaking a large free-energy drop into a series of smaller steps that release energy in manageable amounts, step by step. How does the mitochondrion (or the plasma membrane, in the case of prokaryotes) couple this electron transport and energy release to ATP synthesis? The answer is a mechanism called chemiosmosis.
ChemiosmosisInETC
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24 сен 2024
