Flavodoxin properties

The above considerations provide a rationale for the redox properties of flavodoxin which function between the flavin hydroquinone and neutral semiquinone redox forms. Further studies are required to determine whether similar properties exist in flavoproteins in which both redox couples (PF/PFl- and PFI/PFIH2) are operative and in situations where the anionic semiquinone rather than the neutral form is functional. 

The properties of the semiquinone from of the ETF isolated from the methylotrophic bacterium resemble those of the bacterial flavodoxins with the exception that flavodoxins form neutral semiquinones whereas this ETF forms an anionic semiquinone. Nearly quantitative anionic semiquinone formation is observed either in the presence of excess dithionite or when excess trimethylamine and a catalytic amount of trimethylamine dehydrogenase are added. Of interest is the apparent stability of the anionic semiquinone towards oxidation by O2 but not to oxidizing agents such as ferricyanide. This appears to be the first example of an air-stable protein-bound anionic flavin semiquinone. Future studies on the factors involved in imparting this resistance to O2 oxidation by the apoprotein are looked forward to with great interest. 

According to recent data, the property of dithionite as an electron donor for nitrogenase is different from that of the natural donor flavodoxin (Burgess and Lowe, 1996). Flavodoxin from Azotobacter vinelandii has the redox potential equal to -0.515 V for the reversible transition between the semiquinone and hydroquinone forms of flavodoxin. Unlike dithionite, flavodoxin can reversibly reduce the [Fe4S4]+l cluster Av2 by one electron to the [Fe4S4]° state in which all iron ions exist in the ferrous form. It is assumed that, under natural conditions, two electrons can transfer from Av2 to Avl. Flavodoxin reduces both Av2 bound to Avl and free Av2 in a solution. The apparent rate constants of these reactions are 400 s 1 and > 1000 s"1, respectively (Duyvis et al. 1998). 

Lawson RJ, von Wachenfeldt C, Haq I, Perkins J, Munro AW. Expression and characterization of the two flavodoxin proteins of Bacillus subtilis, YkuN and YkuP biophysical properties and interactions with cytochrome P450. Biochemistry 2004 43 12390-12409. 

The electron transfer, as observed in vitro between these two proteins, is believed to occur through the formation of a speciflc complex between the two proteins, being the interaction mainly electrostatic in nature. The nature and properties of the protein-protein complex (stoichiometry, interaction sites, association constants) were probed. Integration and correlation of the experimental results obtained from magnetic resonance studies on protein-protein titrations with the available structural and biochemical data is presented. A structural model for a hypothetical ternary complex, formed between one molecule of flavodoxin and two molecules of cytochrome C3, is proposed using the available X-ray structures of the isolated proteins and, when required, model structures predicted by homology modeling. 

Bui SH, McLean KJ, Cheesman MR, Bradley JM, Rigby SEJ, Levy CW, Leys D, Munro AW (2012) Unusual spectroscopic and ligand binding properties of the cytochrome P450-flavodoxin fusion enzyme XplA. J Biol Chem 287 19699 19714... 

Dubourdieu M, le Gall J, Favaudon V (1975) Physicochemical properties of flavodoxin from Desulfo-vibrio vulgaris. Biochim Biophys Acta 376 519-532... 

Hanley SC, Ost TW, Daff S (2004) The unusual redox properties of flavocytochrome P450 BM3 flavodoxin domain. Biochem Biophys Res Commun 325 1418 1423... 

Curley GP, Carr MC, Mayliew SG, Voordouw G (1991) Redox and flavin-binding properties of recombinant flavodoxin from Desulfovibrio vulgaris (hildenborough). Eur JBiochem 202 1091-1100... 

Wild-type flavodoxin dimerizes by intermolecular disulfide bond formation between cysteine residues. In order to investigate the redox properties of mono-... 

Studies of the properties of immobilized 168 flavodoxin and of the replacement of ferredoxin as an electron-carrier... 

Mayhew, S. G., G. P. Foust, and V. Massey Oxidation-Reduction Properties of Flavodoxin from Peptostreptococcus elsdenii. J. Biol. Chem. 244, 803 (1969). 

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