Properties of the complexes of riboflavin 3',5'-bisphosphate and the apoflavodoxins from Megasphaera elsdenii and Desulfovibrio vulgaris

Megasphaera elsdenii and Desulfovibrio vulgaris apoflavodoxins have been reconstituted with riboflavin 3',5'-bisphosphate. Several biochemical and biophysical properties of the complexes have been investigated and the results are compared with the properties of the native proteins. The dissociation constant of the modified complex of M. elsdenii flavodoxin is increased by a factor of about 23 by comparison with that of the native protein. The rate constant for the formation of the complex of M. elsdenii flavodoxin is about 26 times lower than that for the native protein. The redox potential of the transition between the oxidized and semiquinone state is similar to that of the native protein. On the other hand, the redox potential of the semiquinone-hydroquinone transition is about 20 mV more negative than that of the native protein. Absorbance and circular dichroic spectra of the protein-bound artificial prosthetic group and the protein-bound natural prosthetic group are very similar. In both the oxidized and in the fully reduced state only minor differences in interaction between the isoalloxazine ring and the apoprotein for the two flavin derivatives are found by 13C and 15N NMR. 31P-NMR studies show that the 5'-phosphate group of the two flavin derivatives is bound in the same way and that it is dianionic in the complex. In contrast, the 3'-phosphate group in riboflavin 3',5'-bisphosphate is monoanionic or even neutral when bound to the protein. The 3'-phosphate group is also close to or on the surface of the protein. Desulfovibrio vulgaris apoflavodoxin has an affinity for riboflavin 3',5'-bisphosphate which is 10 times higher as compared to Megasphaera elsdenii apoflavodoxin (Ka = 10(8) M-1). Also the association rate constant of Desulfovibrio vulgaris apoprotein and riboflavin 3'5'-bisphosphate is found to be 10 times faster than for the Megasphaera elsdenii flavodoxin reaction. The dissociation behaviour of native Desulfovibrio vulgaris flavodoxin measured under identical conditions as for the riboflavin 3',5'-bisphosphate analog gives a value (Kd approximately equal to 0.2 nM) which is considerably lower than reported earlier [Dubourdieu, M., MacKnight, M. L. & Tollin, G. (1974) Biochem. Biophys. Res. Commun. 60, 649-655]. The results are discussed in the light of the existing crystallographic data of flavodoxins and the recently proposed theory on the regulation of the redox potential in flavoproteins [Moonen, C. T. W., Vervoort, J. & Müller, F. (1984) in Flavins and flavoproteins, pp. 493-496, Walter de Gruyter, Berlin].