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

Huiying Li,C.S Raman,Charles B Glaser,Eric Blasko,Tish A Young,John F Parkinson,Marc Whitlow,Thomas L Poulos

doi:10.1074/jbc.274.30.21276

Huiying Li, C.S Raman + Show 6 more

Open Access

https://doi.org/10.1074/jbc.274.30.21276

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Abstract

The crystal structures of the heme domain of human inducible nitric-oxide synthase (NOS-2) in zinc-free and -bound states have been solved. In the zinc-free structure, two symmetry-related cysteine residues form a disulfide bond. In the zinc-bound state, these same two cysteine residues form part of a zinc-tetrathiolate (ZnS(4)) center indistinguishable from that observed in the endothelial isoform (NOS-3). As in NOS-3, ZnS(4) plays a key role in stabilizing intersubunit contacts and in maintaining the integrity of the cofactor (tetrahydrobiopterin) binding site of NOS-2. A comparison of NOS-2 and NOS-3 structures illustrates the conservation of quaternary structure, tertiary topology, and substrate and cofactor binding sites, in addition to providing insights on isoform-specific inhibitor design. The structural comparison also reveals that pterin binding does not preferentially stabilize the dimer interface of NOS-2 over NOS-3.

Highlights

The crystal structures of the heme domain of human inducible nitric-oxide synthase (NOS-2) in zinc-free and -bound states have been solved
Residues 83–502 form a continuous stretch of electron density in the zinc-bound structure, including the loop region (Pro122-Pro135) preceding the ␣1 helix that is disordered in the Nitric-oxide synthases (NOS)-3 structure
Zinc Tetrathiolate Center—As in the murine NOS-2 structure [8], the human NOS-2 structure reveals a disulfide bond formed between symmetry-related Cys115 residues at the dimer interface (Fig. 2)

Summary

MATERIALS AND METHODS

Expression and Purification of NOS-2—A cDNA fragment containing the first 703 amino acids of human inducible NOS-2 was obtained by polymerase chain reaction of mRNA made from cytokine-stimulated human colorectal adenocarcinoma DLD-1 cells (11new). This fragment was sub-cloned via intermediate vectors into a pUC18 derivative to place the NOS-2 heme domain under control of the TAC promoter. E. coli (strain pNOS115-W3110) cells were grown in “Terrific Broth” in a 10 liter fermenter. Seventy-five grams of E. coli cell pellet was thawed, and 500 ml of 40 mM BTP, pH 7.8, 10% glycerol, containing 0.3 g of lysozyme, 80 ␮l of benzonase, 1 mg of leupeptin, 5 mg of aprotinin, 24 mg of Pefabloc SC, 1 mg of pepstatin, and 0.1 ml of Me2SO were added. The NOS-2 bound Ni-nitrilotriacetic acid resin was placed in a 2.5-cm diameter column and washed with 20 mM imidazole in the column buffer

Bond length Bond angle

RESULTS AND DISCUSSION

Hydrogen bonds