Evaluation of the electrostatic effect of the 5'-phosphate of the flavin mononucleotide cofactor on the oxidation-reduction potentials of the flavodoxin from Desulfovibrio vulgaris (Hildenborough)

Two mutants of the Desulfovibrio vulgaris flavodoxin, T12H and N14H, were generated which, for the first time, place a basic residue within the normally neutral 5'-phosphate binding loop of the flavin mononucleotide cofactor binding site found in all flavodoxins. These,histidine residues were designed to form an ion pair with the dianionic 5'-phosphate, either altering its ionization state or offsetting its negative charge to allow evaluation of the magnitude of its electrostatic effect on the redox properties of the cofactor, The midpoint potential for the oxidized/semiquinone couple was not significantly altered in either mutant. However, the midpoint potentials for the semiquinoe/hydroquinone couple (E(sq/hq)) were less negative than that of the wild type, increasing by 28 and 15 mV relative to that of the wild type for the T12H and N14H mutants, respectively, at pH 6. P-31 NMR spectroscopy suggests that, just as for wild type, the phosphate group in each mutant does not change its ionization state between pH 6, and 8, Therefore, the small increases in midpoint potential must be linked to the protonation of the histidine residues, either through favorable interactions with the anionic hydroquinone or by the partial compensation of the charge on the 5'-phosphate. Values for the pK(a) of His12 and His14 in the oxidized flavodoxin were determined by H-1 NMR spectroscopy to be 6.71 and 6.93, respectively, which are only modestly elevated relative to the average value for histidines in proteins. This suggests that the histidines do not form strong ion-pairing interactions with the phosphate and/or that the effective charge on the 5'-phosphate may be substantially less than the reported formal dianionic charge. Either way, the data provide evidence for the rather weak electrostatic interaction between a charged group at this site and the anionic flavin hydroquinone. In contrast, E(sq/hq) reported for the apoflavodoxin-riboflavin complex, which lacks the 5'-phosphate group, is 180 mV less negative than that of the native flavodoxin, The re-evaluation of the redox and cofactor binding properties of the riboflavin complex generated values for the dissociation constants for the riboflavin complex in the oxidized, semiquinone, and hydroquinone oxidation states that are 2100-, 63000-, and 54-fold higher, respectively, than that for the naturally occurring flavin mononucleotide complex, The large redox potential shifts observed for both redox couples in the riboflavin complex are primarily the consequence of a decreased stabilization of the semiquinone rather than the result of the absence of the negative charge of the 5'-phosphate. It is concluded from this study that the negative charge on the phosphate group of the cofactor does not play a disproportionate role in decreasing E(sq/hq), at most contributing equivalently with the acidic amino acid residues clustered around the flavin to an unfavorable electrostatic environment for the formation of the flavin hydroquinone anion.