Flavin-Dependent Oxido-Reduction

It has already been stated that flavin fulfils the requirements of a quinonoid system. It exhibits thermodynamic reversibility of one-electron as well as two-electron transfer. Basing his terms on electrochemical data, Michaelis 128) was right in postulating that a transfer of redox equivalents to and from flavin occurs stepwise, one electron at a time. It must, therefore, be emphasized that flavin can react this way, but need not do so, and this defines the uniqueness of flavin among redox-biocatalysts. This uniqueness becomes more convincing, if we consider electron carriers located next to flavin in biological redox chains, such as nicotinamides and ubiquinones 78)  

Nowadays, a characteristically modified flavin, 5-deazaflavin (33,142), has gained much interest in flavin enzymology because it can replace natural flavin at many enzyme sites. This derivative, however, is no quinoid system and its radical has no thermodynamic stability. It is, therefore, a mutilated flavin retaining the 2e -transfer (i.e. nicotinamidelike) properties only (cf. Table 2). The thia-analog, however, is an O2-stable dihydro derivative, which exhibits a stable radical, but no reversibly reducible oxidized form. Hence, it is also a mutilated flavin, but one which retains the e -transferase activity only. 

As to the mechanism of these oxido-reductions, we have to ask the following questions  

The bold lines separate the main subclasses of -transferases (including flavodoxins as pure e -transferases), oxidases (including oxygenases in the absence of oxene-acceptor) and, finally, oxygenases (including luciferase) in the presence of oxene acceptor) 

Block position Redox input -equiv. output Class name Rac type lical yield 02- rate Reduction product F1-02- Ci structure omplex absorption (nm) 

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Finally, one has to ask Is flavin-dependent oxido-reduction a ping-pong mechanism (eq. 2)... 

Thanks are due to numerous members of the world-wide flavin community for communication of recent data and critical evaluation, sometimes denial of ideas brought forth in this article. In spite of this, the reviewer did not hesitate to articulate what he thinks are chemically sound speculations on the molecular biology of flavin-dependent oxido-reduction . The future will decide. 

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