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 c(553) (c(553)) 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 fi-om horse heart (hhc), Second-order rate constants (k(2)) were measured for both non-physiological systems at different ionic strengths: a complex, bell-shaped behaviour is observed for the k(2) of the c(553)/fld redox pair with an optimum rate at I=58 mmol l(-1), whereas under the same conditions the k(2) 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 c(553)/fld complexes, consistent with the published model.