Prokaryotic expression of the heme- and flavin-binding domains of rat neuronal nitric oxide synthase as distinct polypeptides: identification of the heme-binding proximal thiolate ligand as cysteine-415.

The heme- and flavin-binding domains of constitutive rat neuronal nitric oxide synthase (NOS) were expressed in Escherichia coli as distinct polypeptides with properties characteristic of the intact enzyme. The amino-terminal heme-binding domain (residues 1-714) was expressed using the expression vector pCW. The denatured molecular mass of the expressed protein was 80 kDa, and the protein was shown to be immunoreactive to rabbit anti-NOS IgG. The NOS hemoprotein exhibited a ferrous-carbon monoxide difference spectrum with a wavelength maximum at 445 nm. Spectral perturbation with L-arginine and BH4 elicited a type I difference spectrum, confirming the presence of binding sites for these molecules within the N-terminal NOS polypeptide. Site-directed mutagenesis was applied to the putative axial heme ligand, cysteine-415, generating the histidine mutant, which confirmed the identity of the proximal ligand. NOS flavoproteins, with (C1, residues 715-1429) and without (C2, residues 749-1429) an amino-terminal calmodulin-binding motif, were expressed using the vector pPROK-1. The C1 and C2 flavoproteins were immunoreactive to anti-NOS IgG and were sized at approximately 80 kDa. Both of the purified flavoproteins exhibited optical absorbance properties typical of a flavin prosthetic group, with wavelength maxima at 380 and 450 nm, and were competent in NADPH-dependent electron transfer to cytochrome c, with observed rates of approximately 2-4 mumol/min/mg. The bacterial expression of the NO synthase heme-binding oxygenase and flavoprotein oxidoreductase domains as isolated proteins with specific properties of the intact enzyme represents an important development in structure-function studies of this complex enzyme.