Comparing the temperature dependence of FMN to heme electron transfer in full length and truncated inducible nitric oxide synthase proteins.

The FMN-heme interdomain (intraprotein) electron transfer (IET) kinetics in full length and oxygenase/FMN (oxyFMN) construct of human iNOS were determined by laser flash photolysis over the temperature range from 283 to 304K. An appreciable increase in the rate constant value was observed with an increase in the temperature. Our previous viscosity study indicated that the IET process is conformationally gated, and Eyring equation was thus used to analyze the temperature dependence data. The obtained magnitude of activation entropy for the IET in the oxyFMN construct is only one-fifth of that for the holoenzyme. This indicates that the FMN domain in the holoenzyme needs to sample more conformations before the IET takes place, and that the FMN domain in the oxyFMN construct is better poised for efficient IET.