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6.7: Flavin as a One-Electron Carrier

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    In chapter 15 we saw how a nicotinamide and flavin coenzymes can act as acceptors or donors of two electrons in hydride-transfer redox steps. Recall that it was mentioned that flavin, (but not nicotinamide) can also participate in single-electron transfer steps through a stabilized radical intermediate called a semiquinone.. Frey p. 162 fig 3-30; Silverman p. 122 sch. 3.34; J Phys Chem A. 2013, 117, 11136 fig 2)

    Reduced flavin (either FADH2 or FMNH2) has one electron acceptor becomes falvin semiquinone radical which has one electron acceptor becomes oxidized flavin (FAD or FMN).

    Note in this reaction that overall, flavin loses or gains two electrons and two protons, just like in the flavin-dependent redox reactions we saw in chapter 15. The difference here is that the electrons are transferred one at a time, rather than paired in the form of a hydride ion.

    Two important examples single-electron acceptor species in human metabolism are ubiquinone (coenzyme \(Q\)) and the oxidized form of cytochrome. Ubiquinone is a coenzyme that can transfer single electrons via a semiquinone state analogous to that of flavin, and cytochrome is a protein containing a 'heme' iron center which shuttles between the \(Fe^{+3}\) (oxidized) and \(Fe^{+2}\) (reduced) state.

    Oxidized ubiquinone (Coenzyme Q) reacts with H plus and an electron to produce ubiquinone semiquinone radical which reacts with H plus and an electron to produce reduced ubiquinone (QH2). Iron three plus (the heme center in cytochrome protein) reacts with one electron to produce Iron two plus.

    Further discussion of the mechanisms of single-electron flavin reactions is beyond our scope here, but when you study the 'respiratory chain' in a biochemistry course you will gain a deeper appreciation for the importance of flavin in single-electron transfer processes.

    This page titled 6.7: Flavin as a One-Electron Carrier is shared under a CC BY-NC-SA 4.0 license and was authored, remixed, and/or curated by Tim Soderberg.