FIGURE 4.

iNOS_ox crystal structures before (top, PDB 3E7M) and after (bottom, PDB 3NQS) nitrosylation, showing the ZnS₄ (left), dimer interface (middle), and pterin sites (right). A, in the absence of NO donor, the iNOS_ox ZnS₄ is occupied and ordered as shown by F_o − F_c omit electron density for the zinc and its four cysteine ligands (Cys-104a, Cys-104b, Cys-109a, Cys-109b). B, after CysNO treatment during crystallization, the iNOS_ox ZnS₄ is disrupted and residues 100 to 107 are disordered. F_o − F_c omit electron density observed for Cys-109 from each subunit indicates that the inter-atomic distance between sulfur atoms (5.7 Å) is too long for a ZnS₄ cluster or disulfide bond (expected distance S-S is 2.0 Å) formation, but would accommodate two S-NO modifications. C, pterin and D, N-NO pterin (after reaction with CysNO) cofactors (green tubes with atom-colored spheres for oxygen and nitrogen atoms) both hydrogen bond with adjacent heme and are tightly packed between two iNOS_ox subunits (purple/gold surfaces) at the dimer interface. E, pterin and F, N-NO pterin in F_o − F_c omit electron density contoured at 3σ (blue) and 6σ (gray) are distinguishable by additional protruding electron density for the NO moiety (identified by arrow) covalently attached to position N5.