| Abstract: | Oxynitrilase containing 2-thioFAD C(2) = S] in place of FAD exhibits catalytic activity similar to that of native enzyme. Reaction of methyl methanethiolsulfonate with 2-thioFAD bound to oxynitrilase results in the formation of the corresponding flavin disulfide C(2)-SSCH3]. Normal flavin C(2) = O] is formed by reacting 2-thioFAD oxynitrilase with m-chloroperoxybenzoate or H2O2. Both reactions proceed via a spectrally detectable flavin 2-S-oxide intermediate C(2) = S+-O-], but sizable amounts of this intermediate accumulate only in the m-chloroperoxybenzoate reaction (about 40%). While similar reactions have been reported with free 2-thioflavin, kinetic and other data indicate that the oxynitrilase reactions occur with intact enzyme. This shows that the 2-position of the pyrimidine ring in the bound coenzyme is accessible to solvent. The data are consistent with previous studies on the reaction of peroxides with oxynitrilase-bound 5-deazaFAD which show that the pyrimidine ring is accessible at position 4. Analogous studies indicate that the pyrimidine ring is buried in the case of flavin bound to lactate oxidase, since the data indicate that both positions 2 and 4 are inaccessible to solvent. |
