title: Proximal effects in the modulation of nitric oxide synthase reactivity: a QM-MM study.
abstract: Nitric oxide synthases (NOS) are heme proteins that have a cysteine residue as axial ligand, which generates nitric oxide (NO). The proximal environment, specifically H-bonding between tryptophan (Trp) 178 and thiolate, has been proposed to play a fundamental role in the modulation of NOS activity. We analyzed the molecular basis of this modulation by performing electronic structure calculations on isolated model systems and hybrid quantum-classical computations of the active sites in the protein environment for wild-type and mutant (Trp 178 x Gly) proteins. Our results show that in the ferrous proteins NO exhibits a considerable trans effect. We also showed that in the ferrous (Fe(+2)) mutant NOS the absence of Trp, experimentally associated to a protonated cysteine, weakens the Fe-S bond and yields five coordinate complexes. In the ferric (Fe(+3)) state, the NO dissociation energy is shown to be slightly smaller in the mutant NOS, implying that the Fe(+3)-NO complex has a shorter half-life. We found computational evidence suggesting that ferrous NOS is favored in wild-type NOS when compared to the Trp mutant, consistently with the fact that Trp mutants have been shown to accumulate less Fe(+2)-NO dead end species. We also found that the heme macrocycle showed a significant distortion in the wild-type protein, due to the presence of the nearby Trp 178. This may also play a role in the subtle tuning of the electronic structure of the heme moiety.
authors.affiliation: Departamento de Química Inorgánica, Analítica y Química Física/INQUIMAE-CONICET, Facultad de Ciencias Exactas y Naturales, Universidad de Buenos Aires, Ciudad Universitaria, Pabellón II, Buenos Aires, C1428EHA, Argentina.
authors.forename: M Laura
authors.initials: ML
authors.lastname: Fernández
authors.forename: Marcelo A
authors.initials: MA
authors.lastname: Martí
authors.forename: Alejandro
authors.initials: A
authors.lastname: Crespo
authors.forename: Darío A
authors.initials: DA
authors.lastname: Estrin
mesh_terms: D001665:Binding Sites; D019295:Computational Biology; D006418:Heme / Q000737:chemistry*; D006420:Hemeproteins / Q000737:chemistry* / Q000235:genetics; D006801:Humans; D006860:Hydrogen Bonding; D007501:Iron / Q000737:chemistry; D008958:Models, Molecular; D052250:Nitric Oxide Synthase Type III / Q000737:chemistry* / Q000235:genetics; D014364:Tryptophan / Q000737:chemistry* / Q000235:genetics
chemical_list: D006420:Hemeproteins; D006418:Heme; D014364:Tryptophan; D007501:Iron; C496314:NOS3 protein, human; D052250:Nitric Oxide Synthase Type III