A docked state conformational dynamics model to explain the ionic strength dependence of FMN – heme electron transfer in nitric oxide synthase

The FMN−heme interdomain electron transfer (IET) in nitric oxide synthase (NOS) is a key stage of the electron transport chain, which supplies the catalytic heme site(s) with the NADPH-derived electrons. While there is a recognition that this IET depends on both the electron tunneling and the conformational dynamics, the detailed mechanism remains unclear. In this work, the IET kinetics were measured by laser flash photolysis for a bidomain oxygenase/FMN (oxyFMN) construct of human inducible NOS (iNOS) over the ionic strength range from 0.1 to 0.5 M. The forward (heme → FMN, k _ETf ) and backward (FMN → heme, k _ETb ) intrinsic IET rate constants were determined from the analysis of the observed IET rates using the additional information regarding the conformational dynamics obtained from the FMN fluorescence lifetime measurements and theoretical estimates. Both k _ETf and k _ETb exhibit a bell-shaped dependence on the ionic strength, I, with the maximum rates corresponding to I ~ 0.2 M. This dependence was explained using a new model, which considers the effect of formation of pairs between the protein surface charged residues and solution ions on the docked state dynamics. The trial simulations of the intrinsic IET rate dependences using this model show that the data can be reproduced using reasonable energetic, structural, and chemical parameters. The suggested model can explain both the monophasic and biphasic ionic strength dependences and can be used to rationalize the interprotein/interdomain electron transfer rates for other types of protein systems where the docked state is sufficiently long-lived.