Hemoproteins reversible binding

Cytochrome P450 monooxygenases, which are protoheme proteins, present an unusually red-shifted Soret band for the reduced CO complex at 450 nm [167]. Upon denaturation by various treatments, the cytochromes are always able to bind CO reversibly by losing their unique spectral properties as well as their catalytic activities. The spectral properties of the reduced carbonylated complexes of the denaturated form called P420 are identical to those of the dioxygen-carrying hemoproteins (Amax = 420 nm). These facts indicate that the unusual spectral properties of cytochrome P450 closely relate to its function. 

As early as the late 1960s, it was demonstrated that, like virtually all hemoproteins, the CYPs bind -NO (Miyake et aL, 1968), and this has been used in numerous studies as a spectroscopic probe to examine the heme ligand environment using EPR spectroscopy (Miyake et aL, 1968 Saprin et aL, 1977 O Keeffe etaL, 1978). It has been shown that supraphysiological amounts of nitrogen oxides inhibit CYP activity, by both reversible and irreversible mechanisms (Wink etaL, 1993b Khatsenko etaL, 1993 Stadler et aL, 1994). It is thus possible that endogenous -NO production inhibits activity via this mechanism. It is also possible that -NO could inhibit spectral... 

The hemin thus formed has the shape of a flat disk with the iron in the center and its two remaining valencies directed at right angles from the plane of the disk. In the hemoproteins one or both are in all probability bound to the protein component. Under certain conditions, e.g., in acid or alkaline solution, a reversible or irreversible splitting of one or both iron bindings to the protein component may take place. In this connection, however, heme is not always entirely freed from the protein component, but may remain bound by other groups than the iron. 

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