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PubMed1969

The electron transport system in nitrogen fixation by Azotobacter. I. Azotoflavin as an electron carrier.

A flavoprotein, named azotoflavin, was isolated from an extract of Azotobacter vinelandii cells, which linked the reducing power generated by illuminated spinach chloroplasts to the Azotobacter nitrogen-fixing enzyme complex. The photoreduction of the yellow azotoflavin by chloroplasts produced a stable, free-radical semiquinone, blue in color, with properties similar to those described by other investigators for an Azotobacter flavoprotein of unknown biological function.

Azotobacter↗
PubMed1976

Nicotinamide-dependent one-electron and two-electron (flavin) oxidoreduction: thermodynamics, kinetics, and mechanism.

1. Biological nicotinamide-dependent oxidoreduction consists of reversible 2e- oxidoreduction of substrates. A mechanism involving subsequent 1e- steps is shown to be very unfavourable due to the high energy of the nicotinamide radical. 2. Free energy relationships provide a convenient tool, allowing one to differentiate between hydride transfer and hydrogen atom transfer. It is concluded that biological nicotinamide-dependent, as well as flavin-nicotinamide oxidoreduction, proceed via hydride transfer but not via hydrogen atom transfer. 3. In flavin-nicotinamide oxidoreduction, flavin-nicotinamide charge transfer complexes are very likely the catalytic intermediates, preceding transfer of hydride ion. The energy of the long-wavelength charge transfer transition of zwitterionic oxidized-nicotinamide/reduced-flavin complexes is strongly dependent on polarity. It is maximal in a highly polar environment. 4. 5-Deazaflavins show the high thermodynamic radical instability of nicotinamides. They have to be considered as nicotinamide analog 2e- oxidoreductants rather than flavin analogs, therefore, lacking the ability to catalyze reversible 1e- oxidoreduction, essential for many flavoenzymes.

Binding Sites↗