Carbon monoxide heme iron, binding

HRP C contains two different types of metal center (i.e., iron(III) protoporphyrin IX-heme group and two calcium atoms) that are fundamental for the integrity of the enzyme. The heme group is attached to the enzyme at His 170 by a coordinate bond between the histidine side-chain NE2 atom and the heme iron atom. The second axial coordination site is unoccupied in the resting state of the enzyme but available to hydrogen peroxide during enzyme turnover. Small molecules such as carbon monoxide, cyanide, fluoride, and azide bind to the heme iron atom at this distal site, giving six-coordinated PX complexes. 

Figure 7.66 The heme moiety of the hemoglobin molecule showing the binding of the oxygen molecule to the iron atom. As shown in the diagram, carbon monoxide binds at the same site. Abbreviation His, side chain of the amino acid histidine. Source From Ref. 18.

At the present time we have no certain knowledge of the state of the heme in these 450 nm species. We do not know if there are heme aggregates although they are unlikely. It is therefore reasonable to look at systems where the haem is aggregated as well as those where it is not in order to see how the absorption spectra can be mimic-ed. It seems reasonable to assume that the iron is low-spin in the carbon monoxide, isocyanide, and nitric oxide complexes as no high-spin iron complexes of this type are known. In the high-spin or low-spin state it may be that the thiol is weakly bound, if at all, for Fe(II) heme in models or in hemoglobin does not bind to thiols. In an attempt to understand these spectra we shall use a semi-empirical approach based on the theoretical discussion in the previous article (52) and elaborated in what follows immediately. Only Fe(II) complexes will be analysed as the Fe(III) proteins have been previously examined (52). 

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