To determine a dynamic functional model of full-length nitric oxide synthase (NOS), we are coupling Small Angle X-ray Scattering (SAXS) with our crystal structures of individual NOS components. SAXS is a solution scattering method that is well suited to capturing protein conformational changes. Our lab has solved crystal structures of all three parts of NOS enzymes: NOS oxygenase, NOS reductase and NOS calmodulin (CaM) peptide with CaM bound. Based on our crystal structures and biochemical data, we propose that the NOS FMN domain swings from the electron-accepting “IN” to an electron-donating “OUT” conformation. In the electron-accepting conformation observed in our crystal structure, the FMN is buried and positioned to accept an electron from redox partner FAD. We propose that the entire FMN domain swings “OUT” from the remainder of the reductase domain to transfer electrons to the active site heme. We are combining scattering data with atomic resolution crystal structures and molecular dynamics simulations to identify snapshots of NOS in solution. We analyze and compare FMN domain movement in different isozymes, making use of overlapping truncated constructs and post-translationally modified proteins. Our SAXS results are consistent with a large swinging movement in the FMN domain that could enable electron transfer from FMN to heme.