Proximal effects in the modulation of nitric oxide synthase reactivity: a QM-MM study

FERNÁNDEZ ML, MARTÍ MA, CRESPO A, ESTRIN DA

JOURNAL OF BIOLOGICAL INORGANIC CHEMISTRY

Springer

Año: 2005 vol. 10 p. 595 - 604

0949-8257

Abstract Nitric oxide synthases (NOS) are heme proteinsthat have a cysteine residue as axial ligand, which generatesnitric oxide (NO). The proximal environment,specifically H-bonding between tryptophan (Trp) 178and thiolate, has been proposed to play a fundamentalrole in the modulation of NOS activity. We analyzed themolecular basis of this modulation by performing electronicstructure calculations on isolated model systemsand hybrid quantum-classical computations of the activesites in the protein environment for wild-type and mutant(Trp 178 · Gly) proteins. Our results show that in theferrous proteins NO exhibits a considerable trans effect.We also showed that in the ferrous (Fe+2) mutant NOSthe absence of Trp, experimentally associated to a protonatedcysteine, weakens the Fe–S bond and yields fivecoordinate complexes. In the ferric (Fe+3) state, the NOdissociation energy is shown to be slightly smaller in themutant NOS, implying that the Fe+3–NO complex has ashorter half-life. We found computational evidence suggestingthat ferrous NOS is favored in wild-type NOSwhen compared to the Trp mutant, consistently with thefact that Trp mutants have been shown to accumulateless Fe+2–NO dead end species. We also found that theheme macrocycle showed a significant distortion in thewild-type protein, due to the presence of the nearby Trp178. This may also play a role in the subtle tuning of theelectronic structure of the heme moiety.