Ionic strength dependence of the non-physiological electron transfer between flavodoxin and cytochrome C₅₅₃ from D. vulgaris

A hypothetical model for the non-physiological electron transfer complex between cytochrome c₅₅₃ (c₅₅₃) and the flavodoxin (fld) from the sulphate-reducing bacteria Desulfovibrio vulgaris has been recently published [1] based on rigid-body docking and refined by molecular dynamics. In this study, the functional validity of this model is tested by looking at the role of electrostatics in the non-physiological interprotein electron transfer between the two proteins at different ionic strengths. The results are compared with the electron transfer between fld and cytochrome c from horse heart (hhc). Second-order rate constants (k₂) were measured for both non-physiological systems at different ionic strengths: a complex, bell-shaped behaviour is observed for the k₂ of the c₅₅₃/fld redox pair with an optimum rate at I=58 mmol l^-1, whereas under the same conditions the k₂ for hhc/fld decreased monotonically with increasing ionic strength. Results from the electron transfer kinetics are rationalised in terms of reorganisational effects of an ensemble of conformations of the electron transfer competent c₅₅₃/fld complexes, consistent with the published model.