Ionic strength dependence of the non-physiological electron transfer between flavodoxin and cytochrome c(553) from D-vulgaris

A hypothetical model for the non-physiological electron transfer complex be tween cytochrome c(553) (c(553)) and the flavodoxin (fld) from the sulphate -reducing bacteria Desulfovibrio vulgaris has been recently published [1] b ased 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 bet ween the two proteins at different ionic strengths. The results are compare d 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-physi ological systems at different ionic strengths: a complex, bell-shaped behav iour 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 t he 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.