In general, an electron acceptor is a chemical entity that can accept electrons from another chemical entity. By accepting the electrons, the acceptor is reduced, and thus prior to receiving the electrons the entity is called "an oxidized electron acceptor" and after receiving them it is called "a reduced electron acceptor".
Many reduced electron acceptors are able to transfer the electrons to other molecules, becoming oxidized in the process. Thus the distinction between an electron acceptor and an electron donor is often not determined by the actual chemical entity, but by the role it plays in a particular reaction. However, some acceptors in biological reactions only accept electrons and do not transfer them further. Those are often known as "terminal electron acceptors".
While in general every chemical reaction in which electrons are transferred includes an electron donor and an electron acceptor, several molecules are commonly used in metabolism solely for the purpose of electron transfer, and are referred to in biology as electron acceptors. Different biological electron acceptors differ in the number of electrons that they transfer and in their electron affinity.
Some of the common biological electron acceptors transfer only a single electron (e.g. cytochromes, blue copper proteins, and some ferredoxins), some transfer two electrons (e.g. some ferredoxins), some transfer a hydride ion ( e.g. NAD+ and NADP+), and some transfer two electrons and two protons ( quinones, FAD and FMN). In addition, a number of inorganic small molecules (e.g. oxygen, nitrate, iron (III), manganese (IV), sulfate) are often used as terminal electron acceptors.
Child Classes: α-NAD(P)+ (0), α-NAD(P)H (0), an unknown electron acceptor (0), an unknown single electron acceptor (0), an unknown two electrons acceptor (0), Membrane-Electron-Carriers (14), NAD(P)+ (2), NAD(P)H (2), NADHX (2)
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