Figure 2

Electron density at the cystene-ligation center and switch point for swapped and unswapped structures. Simulated annealed _A-weighted F_obs - F_calc omit maps shown for the two structures (2.7 and 2.35 Å resolution for swapped and unswapped, respectively) at the disulfide/zinc center and the swapping switch point. (A) A disulfide bond between two symmetry-related Cys109 residues links the two subunits (yellow and white) in the swapped iNOS_ox structure (electron density contours: purple at 2, red at 4). (B) The Cys109 disulfide is replaced by a tetrathiolate zinc center in unswapped iNOS_ox (contours: purple at 3, red at 6, cyan at 11). (C and D) The switch point for domain swapping of the N-terminal hook. Omit electron density indicates two distinctly different conformations for residues 104–107 in the swapped [(C) 2.0 purple contours, 4.0 red contours] and unswapped [(D) 2.2 purple contours, 5.0 red contours] iNOS_ox structures. The swapped conformation (yellow bonds) and unswapped conformation (white bonds) are shown superimposed on both electron density maps.