Spectroscopic characterization of five- and six-coordinate ferrous-NO heme complexes. Evidence for heme Fe-proximal cysteinate bond cleavage in the ferrous-NO adducts of the Trp-409Tyr/Phe proximal environment mutants of neuronal nitric oxide synthase.
Nitric oxide synthases (NOS) are a family of cysteine thiolate-ligated heme-containing monooxygenases that catalyze the NADPH-dependent two-step conversion of L-arginine to NO and L-citrulline. During the catalysis, a portion of the NOS heme forms an inhibitory complex with self-generated NO that is subsequently reverted back to NO-free active enzyme under aerobic conditions, suggesting a downstream regulator role of NO. Recent studies revealed that mutation of a conserved proximal tryptophan-409, which forms one of three hydrogen bonds to the heme-coordinated cysteine thiolate, to tyrosine or phenylalanine considerably increases the turnover number of neuronal NOS (nNOS). To further understand these properties of nNOS on its active site structural level, we have examined the oxygenase (heme-containing) domain of the two mutants in close comparison with that of wild-type nNOS with UV-visible absorption, magnetic circular dichroism, and electron paramagnetic resonance spectroscopy. Among several oxidation and ligation states examined, only the ferrous-NO adducts of the two mutants exhibit spectra that are markedly distinct from those of parallel derivatives of the wild-type protein. The spectra of the ferrous-NO mutants are broadly similar to those of known five-coordinate ferrous-NO heme complexes, suggesting that these mutants are predominantly five coordinate in their ferrous-NO states. The present results are indicative of cleavage of the Fe-S bond in the nNOS mutants in their ferrous-NO state and imply a significant role of the conserved tryptophan in stabilization of the Fe-S bond.
Voegtle
Heather L
HL
Department of Chemistry and Biochemistry, University of South Carolina, 631 Sumter Street, Columbia, South Carolina 29208, USA.
Sono
Masanori
M
Adak
Subrata
S
Pond
Alycen E
AE
Tomita
Takeshi
T
Perera
Roshan
R
Goodin
David B
DB
Ikeda-Saito
Masao
M
Stuehr
Dennis J
DJ
Dawson
John H
JH
D002942:Circular Dichroism; D003545:Cysteine / Q000737:chemistry*; D004578:Electron Spin Resonance Spectroscopy; D005290:Ferric Compounds / Q000737:chemistry; D005296:Ferrous Compounds / Q000737:chemistry*; D006418:Heme / Q000737:chemistry*; D006801:Humans; D007506:Iron-Sulfur Proteins / Q000737:chemistry*; D008024:Ligands; D016297:Mutagenesis, Site-Directed; D009569:Nitric Oxide / Q000737:chemistry*; D019001:Nitric Oxide Synthase / Q000737:chemistry* / Q000235:genetics; D052248:Nitric Oxide Synthase Type I; D010084:Oxidation-Reduction; D010649:Phenylalanine / Q000235:genetics; D017434:Protein Structure, Tertiary; D012997:Solvents; D013056:Spectrophotometry, Ultraviolet; D014364:Tryptophan / Q000235:genetics; D014443:Tyrosine / Q000235:genetics
D005290:Ferric Compounds; D005296:Ferrous Compounds; D007506:Iron-Sulfur Proteins; D008024:Ligands; D012997:Solvents; D009569:Nitric Oxide; D014443:Tyrosine; D006418:Heme; D010649:Phenylalanine; D014364:Tryptophan; C496320:NOS1 protein, human; D019001:Nitric Oxide Synthase; D052248:Nitric Oxide Synthase Type I; D003545:Cysteine