Nitric oxide synthase (NOS) is a
homodimeric flavohemoprotein
responsible
for catalyzing the oxidation of l-arginine (l-Arg)
to citrulline and nitric oxide. Electrons are supplied for the reaction
via interdomain electron transfer between an N-terminal heme-containing
oxygenase domain and a FMN-containing (sub)domain of a C-terminal
reductase domain. Extensive attention has focused on elucidating how
conformational dynamics regulate electron transfer between the domains.
Here we investigate the impact of the interdomain FMN–heme
interaction on the heme active site dynamics of inducible NOS (iNOS).
Steady state linear and time-resolved two-dimensional infrared (2D
IR) spectroscopy was applied to probe a CO ligand at the heme within
the oxygenase domain for full-length and truncated or mutated constructs
of human iNOS. Whereas the linear IR spectra of the CO ligand were
identical among the constructs, 2D IR spectroscopy revealed variation
in the frequency dynamics. The wild-type constructs that can properly
form the FMN/oxygenase docked state due to the presence of both the
FMN and oxygenase domains showed slower dynamics than the oxygenase
domain alone. Introduction of the mutation (E546N) predicted to perturb
electrostatic interactions between the domains resulted in measured
dynamics intermediate between those for the full-length and individual
oxygenase domain, consistent with perturbation to the docked/undocked
equilibrium. These results indicate that docking of the FMN domain
to the oxygenase domain not only brings the FMN cofactor within electron
transfer distance of the heme domain but also modulates the dynamics
sensed by the CO ligand within the active site in a way expected to
promote efficient electron transfer.