Interactions between substrates and the haem-bound nitric oxide of ferric and ferrous bacterial nitric oxide synthases

We report here the resonance Raman spectra of the FeIII–NO and FeII–NO complexes of the bacterial NOSs (nitric oxide synthases) from Staphylococcus aureus and Bacillus subtilis. The haem–NO complexes of these bacterial NOSs displayed Fe–N–O frequencies similar to those of the mammalian NOSs, in presence and absence of L-arginine, indicating that haem-bound NO and L-arginine had similar haem environments in bacterial and mammalian NOSs. The only notable difference between the two types of NOS was the lack of change in Fe–N–O frequencies of the FeIII–NO complexes upon (6R) 5,6,7,8-tetrahydro-L-biopterin binding to bacterial NOSs. We report, for the first time, the characterization of NO complexes with NOHA (Nω-hydroxy-L-arginine), the substrate used in the second half of the catalytic cycle of NOSs. In the FeIII–NO complexes, both L-arginine and NOHA induced the Fe–N–O bending mode at nearly the same frequency as a result of a steric interaction between the substrates and the haem-bound NO. However, in the FeII–NO complexes, the Fe–N–O bending mode was not observed and the νFe−NO mode displayed a 5 cm−1 higher frequency in the complex with NOHA than in the complex with L-arginine as a result of direct interactions that probably involve hydrogen bonds. The different behaviour of the substrates in the FeII–NO complexes thus reveal that the interactions between haem-bound NO and the substrates are finely tuned by the geometry of the Fe-ligand structure and are relevant to the use of the FeII–NO complex as a model of the oxygenated complex of NOSs.