Acyl-Carbon Bond Cleaving Cytochrome P450 Enzymes: CYP17A1, CYP19A1 and CYP51A1
Cytochrome P450 (P450 or CYP) enzymes in their resting state contain the heme-iron in a high-spin FeIII state. Binding of a substrate to a P450 enzyme allows transfer of the first electron, producing a FeII species that reacts with oxygen to generate a low-spin iron superoxide intermediate (FeIII–O–O•) ready to accept the second electron to produce an iron peroxy anion intermediate (a, FeIII–O–O−). In classical monooxygenation reactions, the peroxy anion upon protonation fragments to form the reactive Compound I intermediate (Por•+FeIV=O), or its ferryl radical resonance form (FeIV–O•). However, when the substrate projects a carbonyl functionality, of the type b, at the active site as is the case for reactions catalyzed by CYP17A1, CYP19A1 and CYP51A1, the peroxy anion (FeIII–O–O−) is trapped, yielding a tetrahedral intermediate (c) that fragments to an acyl-carbon cleavage product (d plus an acid). Analogous acyl-carbon cleavage reactions are also catalyzed by certain hepatic P450s and CYP125A1 from Mycobacterium tuberculosis. A further improvisation on the theme is provided by aldehyde deformylases that convert long-chain aliphatic aldehydes to hydrocarbons. CYP17A1 is involved in the biosynthesis of corticoids as well as androgens. The flux toward these two classes of hormones seems to be regulated by cytochrome b 5, at the level of the acyl-carbon cleavage reaction. It is this regulation of CYP17A1 that provides a safety mechanism, ensuring that during corticoid biosynthesis, which requires 17α-hydroxylation by CYP17A1, androgen formation is avoided (Fig. 4.1).
KeywordsAromatase Sterol 14α-demethylase 17α-hydroxylase-17,20-lyase P450 2B4 CYP125A1 Aldehyde deformylase Cytochrome b5 in CYP17A1 catalysis
Note added in proof
Recently another alternative to the mechanism of Scheme 4.11 has been suggested (Yoshimoto FK, Guengerich FP, J Am Chem Soc 136: 15016–25).
We acknowledge with pleasure the help of Dr. Peter Lee-Robichaud in writing the section on the interaction of CYP17A1 with cytochrome b 5, an area in which he has made seminal contributions.
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