Properties of a high-potential flavin analog and its use as an active site probe with clostridial flavodoxin

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Abstract
The reduction potential of flavin bearing a methylsulfonyl moiety (MeSO2) in place of a methyl group at position 8 is increased by more than 150 mV as compared with normal flavin. This substitution is accompanied by a substantial increase in reactivity with various reductants, including NADH, and greatly (10(3)-fold) enhanced susceptibility toward nucleophilic attack by sulfite at N(5). 1,5-Dihydro-8-(methylsulfonyl)riboflavin exhibits two intense, well-resolved absorption bands (lambda max = 310, 362 nm) in a region where most other reduced flavins exhibit weak, characterless absorption. This unusual spectrum is attributable to a shift of pi-electron density from the N(5) atom into the benzene ring. It is observed only with reduced flavins bearing a strongly electronegative substituent (MeSO2, CN) at the 8-position. The effect is abolished by replacing the hydrogen at N(5) with a bulky group, like sulfite, which interferes with sp2 hybridization at N(5). Reaction of 8-MeSO2-substituted flavins with thiols results in nucleophilic displacement of MeSO2- in a reaction that is about 10(3)-fold faster than an analogous nucleophilic displacement reaction observed with 8-halo-substituted flavins. The flavin ring acts as a redox switch in controlling electrophilicity at the 8-position, as judged by the fact that the displacement reactions are observed only with the oxidized flavins. Initial studies to evaluate 8-MeSO2-substituted flavins as active site probes were conducted with flavodoxin from Clostridium beijerinckii MP. 8-MeSO2FMN is rapidly bound to apoflavodoxin, accompanied by absorbance and fluorescence changes similar to those observed for FMN binding. 1,5-Dihydro-8-MeSO2FMN flavodoxin exhibits spectral properties (lambda max = 323, 382 nm) similar to those of the corresponding free flavin, except for a bathochromic shift due to a change in the polarity of the flavin environment. As judged by peak resolution and intensity, the spectral properties of 1,5-dihydro-FMN flavodoxin (lambda max = 311, 362 nm) appear to lie about midway between those observed for the free 1,5-dihydro forms of FMN versus 8-MeSO2FMN. This suggests that the protein environment may favor enhanced resonance delocalization of pi-electron density into the benzene ring of bound 1,5-dihydro-FMN, as compared with the free flavin. This hypothesis is consistent with previous NMR studies and with a proposal that electron transfer from reduced flavodoxin to other redox proteins occurs through this region of the ring. 8-MeSO2FMN bound to flavodoxin reacts readily with exogenous thiols but does not react with sulfite.(ABSTRACT TRUNCATED AT 400 WORDS)
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