Proximal effects in the modulation of NO synthase reactivity: a QM-MM study

M.L. FERNANDEZ; M. A. MARTI; A. CRESPO; D. A. ESTRIN

JOURNAL OF BIOLOGICAL INORGANIC CHEMISTRY

Springer

Lugar: Heidelberg; Año: 2005 vol. 6 p. 595 - 604

0949-8257

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 × 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.