Spectroscopic characterization of cytochrome P450 Compound I

► Spectroscopic studies on Compound I (Cpd I) of cytochrome P450 are reviewed. ► The spectroscopic methods are explained and data are related to the P450 mechanism. ► In conclusion, research will no further deal with proving the existence of Cpd I. ► Future studies will focus on uncovering structural requirements to trap Cpd I. The cytochrome P450 protein-bound porphyrin complex with the iron-coordinated active oxygen atom as Fe(IV) O is called Compound I (Cpd I). Cpd I is the intermediate species proposed to hydroxylate directly the inert carbon–hydrogen bonds of P450 substrates. In the natural reaction cycle of cytochrome P450 Cpd I has not yet been detected, presumably because it is very short-lived. A great variety of experimental approaches has been applied to produce Cpd I artificially aiming to characterize its electronic structure with spectroscopic techniques. In spite of these attempts, none of the spectroscopic studies of the last decades proved capable of univocally identifying the electronic state of P450 Cpd I. Very recently, however, Rittle and Green [9] have shown that Cpd I of CYP119, the thermophillic P450 from Sulfolobus acidocaldarius, is univocally a Fe(IV) O–porphyrin radical with the ferryl iron spin ( S = 1) antiferromagnetically coupled to the porphyrin radical spin ( S′ = 1/2) yielding a S tot = 1/2 ground state very similar to Cpd I of chloroperoxidase from Caldariomyces fumago. In this mini-review the efforts to characterize Cpd I of cytochrome P450 by spectroscopic methods are summarized.