Hemoproteins absorption spectra

The P-450 enzyme comprises a unique class of hemoprotein that has protoporphyrin IX at the active site as a prothetic group (Fig. 1), an unusual cysteine thiolate ligand to the heme iron, and a characteristic absorption band at 450 nm. The reduced form of F-450 readily binds CO to form a CO complex (la-d). In 1962, Omura and Sato showed P-450 to be a hemoprotein based on the ethyl isocyanate difference spectrum of the reduced pigment (2). At the same time. Mason and colleagues also concluded that P-450 was a low-spin ferric hemoprotein by EPR measurements (J). 

Compared to other hemoproteins the spectra are unu >ial with respect to the blue-shift and absorption decrease upon reduction. Addition of carbon monoxide to the reduced hemoprotein results in formation of the 447 nm Soret absorption band which appears at 450 nm in a difference spectrum and gave its name to this cytochrome. 

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