Abstract
Calmodulin (CaM) binding to nitric oxide synthase (NOS) enables a conformational change, in which the FMN domain shuttles between the FAD and heme domains to deliver electrons to the active site heme center. A clear understanding of this large conformational change is critical, since this step is the rate-limiting in the NO production. In this study, molecular dynamics simulations were conducted on a model of an oxygenase/FMN (oxyFMN) construct of human inducible NOS (iNOS). This is to investigate the structural rearrangements and the domain interactions before and after the FMN–heme interdomain electron transfer (IET). We carried out simulations on the iNOS oxyFMN•CaM complex models in [Fe(III)][FMNH
–
] and [Fe(II)][FMNH
•
] oxidation states, the pre- and post-IET states. The comparison of the dynamics and conformations of the iNOS construct at the two oxidation states has allowed us to identify key factors related to facilitating the FMN–heme IET process. The computational results demonstrated that the conformational change is redox-dependent. Predictions of the key interacting sites in optimal interdomain FMN/heme docking are well supported by experimental data in the literature. An intra-subunit pivot region is predicted to modulate the FMN domain motion and correlate with existence of a bottleneck in the conformational sampling that leads to the electron transfer-competent state. Interactions of the residues identified in this work are proposed to ensure that the FMN domain moves with appropriate degrees of freedom and docks to proper positions at the heme domain, resulting in efficient IET and nitric oxide production.
Synopsis.
Comparison of the dynamics and structures of an iNOS construct at two oxidation states identified key factors related to the FMN–heme interdomain electron transfer (IET) process. The computational results demonstrated that the FMN domain motion is redox dependent. Interactions of the residues identified in this work are proposed to ensure that the FMN domain moves with appropriate degree of freedom and docks to proper positions at the heme domain, resulting in efficient IET and NO production.
