Nitric oxide (NO), recently found to play many physiological roles, is generated by the catalysis of L-arginine and O2 to L-citrulline and NO by nitric oxide synthases (NOSs). Resonance Raman spectra from the heme of resting, reduced, and CO-bound forms of rat brain NOS firmly establish that the enzyme belongs to the P-450 class of enzymes. The electron density marker line (v4) in the Raman spectrum of ligand-free ferrous NOS has a low frequency (1347 cm-1), indicating a thiolate axial ligand on the heme. The assignment of a thiolate axial ligand is confirmed in the CO-bound form of the enzyme by the frequency of the Fe-C-O bending mode at 562 cm-1. The heme in resting NOS is five-coordinate high spin and thereby differs from the resting state of most substrate-free P450s, which are predominantly six- coordinate low spin. The frequency of the Fe-CO stretching mode in the CO- bound enzyme at 491 cm-1, identified by isotope substitution, is higher than that in substrate-free P-450s. Thus, in the ferric and the CO-bound forms of the enzyme, the sixth-ligand binding site on the heme is restricted by steric or hydrophobic interactions. In addition, the Fe-CO stretching mode is broad (30 cm-1) and may be resolved into two overlapping lines of equal intensity, indicating that the heme domains can adopt two distinct conformations.
|ジャーナル||Journal of Biological Chemistry|
|出版ステータス||Published - 1993|
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