Crystal structures of zinc-free and -bound heme domain of human inducible nitric-oxide synthase - Implications for dimer stability and comparison with endothelial nitric-oxide synthase

The crystal structures of the heme domain of human inducible nitric-oxide s ynthase (NOS-2) in zinc-free and -bound states have been solved. In the zin c-free structure, two symmetry-related cysteine residues form a disulfide b ond. In the zinc-bound state, these same two cysteine residues form part of a zinc-tetrathiolate (ZnS4) center indistinguishable from that observed in the endothelial isoform (NOS-3). As in NOS-3, ZnS4 plays a key role in sta bilizing intersubunit contacts and in maintaining the integrity of the cofa ctor (tetrahydrobiopterin) binding site of NOS-2, A comparison of NOS-2 and NOS-3 structures illustrates the conservation of quaternary structure, ter tiary topology, and substrate and cofactor binding sites, in addition to pr oviding insights on isoform-specific inhibitor design. The structural compa rison also reveals that pterin binding does not preferentially stabilize th e dimer interface of NOS-2 over NOS-3.