Elucidation of Reaction Mechanisms of Biological Nitric Oxide by Vibrational Spectroscopy

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Soluble guanylate cyclase (sGC) was isolated from bovine lung and its resonance Raman (RR) spectra were investigated for the reduced, CO-bound (CO-sGC), NO-bound (NO-sGC), oxidized, and oxidized NO-bound forms in the presence and absence of GTP.The RR spectra of sGC and CO-sGC,including the Fe-His stretch at 204 cm^<-1> and Fe-CO stretch at 473 cm^<-1>, were unaltered by binding of GTP,but apparent RR spectra of NO-sGC in the presence of GTP changed with time and concentrations of GTP.In the absence of GTP,the RR bands of N-O stretch (nu_<NO>) and Fe-No stretch (nu_<Fe-NO>) were observed at 1679 and 521 cm^<-1>, respectively. In its presence, two types of RR spectra were obtained, but after all GTP was exhausted by the enzymatic reaction, the spectrum for the absence of GTP was restoed. In one type of RR spectra, which appeared prior to the other, no NO-isotope sensitive band was detected and is heme spectrum was of a five-coordinate ferric high-spin type. In the other type, two nu_<NO> bands were observed at 1700 and 1681 cm^<-1>, which exhibited ^<15>NO isotopic frequency shifts of 21 and 34 cm^<-1>, respectively. Structural implications of the two types of RR spectra arizing from reaction intermediates have been discussed.