Fig. 4.

SAXS investigation of the transition between lower and higher order structures of E. coli class Ia RNR. (A) Scattering curves measured as 0–175 μM dATP was titrated into a 6 μM solution of α₂ and β₂ in the presence of 1 mM CDP (red to violet) display isointensity points, suggesting a two-state transition. (B) Kratky representations of the scattering curves (Iq² vs. q) at 0 μM (red), 12 μM (green), and 40 μM (blue) dATP show a transition from a compact globular state, as indicated by the monomodal peak, to a large nonglobular state, indicated by a bimodal curve (36). (C) Fitting linear combinations of the α₂β₂ docking model (15) and the α₄β₄ ring to the titration data provided relative fractions of the two states. (D) Ab initio SAXS reconstructions of free subunits aligned with deposited crystal structures (5, 39), a compact α₂β₂ state aligned with the proposed docking model (15), and the α₄β₄ ring aligned with the crystal structure (SI Appendix, Tables S2 and S3, and Fig. S10). The small additional density observed in the molecular envelope of α₂β₂ can be explained by the presence of approximately 3% α₄β₄ (Fig. 4C).