4-Thioflavins as active site probes of flavoproteins. General properties

4-Thioflavins (oxygen at position 4 replaced by sulfur) have been studied as potential active site probes of flavoproteins. They react readily with thiol reagents, with large spectral changes, which should be useful for testing the accessibility of the flavin 4-position in flavoproteins. They have an oxidation-reduction potential at pH 7 of -0.055 V, approximately 0.15 V higher than that of native flavins. The spectral characteristics in the fully reduced state show two clear absorption bands, dependent on the ionization state (pK = 4.5). The lowest energy band of the neutral dihydroflavin has a maximum at approximately 485 nm while that of the anion is approximately 425 nm. This should be useful in defining the ionization state of the reduced flavin in flavoproteins. The spectral characteristics of the semiquinoid forms of 4-thioflavins have been determined bound to the apoproteins of flavodoxin and D-amino acid oxidase. The neutral radical has an absorption maximum at 730 nm, while the anion radical has an unusually sharp peak at 415 nm. The reduced forms of 4-thioflavins, free and enzyme bound, react with O2 to regenerate oxidized 4-thioflavin. Reduced 4-thio-FAD p-hydroxybenzoate hydroxylase, however, in its reaction with O2, undergoes a substantial conversion to the native FAD-enzyme. 4-Thioflavins are unusually susceptible to attack by nucleophiles such as hydroxylamine and amines to form the respective 4-hydroxyimino- and 4-aminoflavins, offering the possibility of forming stable covalent flavin-protein linkages with suitably positioned protein residues. Thiols also react with 4-thioflavins, promoting their conversion to the normal (4-oxo) flavin coenzymes. Such reactivity has been found with the apoenzymes of glucose oxidase and lactate oxidase, providing evidence for a thiol residue in the active site of these enzymes.