Elucidation of the Differences between the 430- and 455-nm Absorbing Forms of P450-Isocyanide Adducts by Resonance Raman Spectroscopy

Abstract

Cytochromes P450 (P450s) 1 belong to a superfamily of thiolate-coordinated heme proteins and are involved in the oxidative metabolism of various endogenous and exogenous organic compounds, catalyzing monooxygenase reactions. The x-ray crystallographic structures of five bacterial P450s currently available (1-5) suggest that the general features of a protein structure are essentially conserved in all P450s, although many specific sites vary within individual molecules (2-5). It is well known that alkyl and phenyl isocyanide adducts of ferrous P450 enzymes from several sources such as microsomes (6 -10), mitochondria (11), and also certain fungi (12) exhibit two Soret bands around 430 and 455 nm. The relative intensity of these two bands has been found to be sensitive to pH. The 455 nm band increases with increasing pH at the expense of the 430 nm band. These observations have been interpreted to mean that the ferrous isocyanide derivatives of P450s exist in two interconvertible forms, the 430-and 455-nm forms hereafter, which are in a pH-dependent equilibrium (6 -12).Studies of ferrous isocyanide derivatives of P450s, mostly of the hepatic microsomal P450s, have revealed several aspects of the 430-and 455-nm forms. For instance, the 430-nm form gives ␣ and ␤ bands around 560 and 530 nm, while the 455-nm form exhibits them around 580 and 555 nm (8, 9). It has also been found that the pK a with respect to the equilibrium between the 430-and 455-nm forms varies over P450 species (9, 12).The reality in the differences between the two interconvertible forms, however, remains elusive. This is partly due to the fact that ferrous isocyanide derivatives of many P450s undergo a gradual denaturation, particularly under acidic as well as alkaline conditions (6, 7). Therefore, observations on the derivatives have been successful in a pH range from 6 to 8 at which populations of the 430-and 455-nm forms in the equilibrium are in the same order of magnitude (6 -9). Such a limitation on the experimental conditions makes it difficult to investigate each of these interconvertible forms individually.Among P450 species so far studied, P450nor (CYP 55A1 in the systematic nomenclature system (13)) purified from denitrifying fungus Fusarium oxysporum (14) exhibits the spectral properties characteristic of P450 (14 -16) and belongs to the P450 superfamily (17). Nevertheless, this P450 has no monooxygenase activity. Instead P450nor catalyzes the reduction of NO to N 2 O by accepting electrons directly from NADH (14). Especially interesting is that P450nor has the highest pK a value for the equilibrium between the 430-and 455-nm forms of the ferrous isocyanide derivatives (12). Although the pK a value was not determined exactly, it has been observed that ethyl, n-butyl, and t-butyl isocyanide adducts of ferrous P450nor are essentially in the 430-nm form in pH ranges below 7.25 (12). Therefore, the ferrous isocyanide derivatives of P450nor are capable of being studied as the best representative of the 430-nm form.On the other hand, P450cam...

Nitric oxide reductase (P450) from

“…In the oxidized ferric state, imidazole derivatives show spectral changes only in the mM concentration range. In contrast, good affinities were observed with alkylated isocyanides [24,25]. Such compounds, when bound to P450 CAM , resulted in Soret bands at 455 nm, but some other P450s exhibit bands at 430 and 455 nm in a pH-dependent equilibrium.…”

Cytochrome P450 Is Present in Both Ferrous and Ferric Forms in the Resting State within Intact Escherichia coli and Hepatocytes

“…4). Although only a small peak was observed at ϳ430 nm in the oxidized samples, characteristic absorbance peaks at ϳ455 nm (both enzymes) and ϳ430 nm (CYP176A1) were observed in spectra obtained from the reduced and native samples, indicating binding of nBIC to the Fe(II) form (30). The ratio of these peaks differed between the native and reduced spectra for the same P450 form, which is consistent with the pH effect previously observed (30) and the ability of dithionite to decrease the pH (31).…”

Spectroscopic investigation of isonitrile complexes of ferric and ferrous microperoxidase 8

“…However, it has been shown [23] that two specific cytochromes P450, P450 nor and P450 cam , only gave, respectively, the 426 and the 453-nm absorbing Fe II -isocyanide species. More recently, X-ray diffraction studies [34] as well as resonance Raman spectroscopy studies [35] have shown that in the structure of the 453 absorbing species, P450 cam Fe II -CNnBu, the Fe-C-N geometry was bent with an angle of 159°. This led to a better overlap between the d-orbitals of the iron and the p* orbitals of the nBuNC ligand and facilitated the d-p* back donation to the ligand which then acted as a pure pacceptor.…”