Radical Intermediates in the Catalytic Cycles of Cytochrome P-450
Cytochrome P-450 is the term used for an unusual and very large family of heme proteins that have a cysteine thiolate ligated to the fifth coordination site of heme iron, and in the reduced state form unique CO complexes that have a Soret band at around 450 nm.1 This family of cytochromes consists of constitutive forms, such as the steroid-oxygenating enzymes, and inducible forms that oxygenate a large variety of naturally occurring and foreign compounds.2 Unlike most enzyme catalysts, this versatile class of heme proteins catalyzes a variety of both oxidative and reductive reactions. Oxidatively, they are capable of effecting both aliphatic and aromatic hydrocarbon hydroxylation, olefin epoxidation, dealkylation at N, O, and S atoms, N-oxidation, sulfoxidation, peroxidation, and carbon-carbon bond cleavage.1,3 Reductively, they are capable of electron transfer reactions to azo, nitro, N-oxide, epoxide, peroxy, and alkyl halide groups, thus justifying the term “cytochrome” for these catalysts. Substrates for P-450 include endobiotics like steroids, fatty acids, and prostaglandins, and xenobiotics like drugs, pesticides, and petroleum products of aliphatic or aromatic nature.
KeywordsCatalytic Cycle Heme Iron Heme Protein Superoxide Radical Anion Reductive Cleavage
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