The mystery of cytochrome P450 Compound I: A mini-review dedicated to Klaus Ruckpaul

The cytochrome P450 protein-bound porphyrin complex with the iron-coordinated active oxygen atom is called Compound I, which is presumably the intermediate species which hydroxylates inert carbon–hydrogen bonds of substrates. In this mini-review, the milestones in discovering Compound I of cytochrome P450 are summarized. It will be discussed what was known and suggested in the years before 1984, the year when Klaus Ruckpaul's first book about cytochrome P450 appeared, and compared with recent approaches and studies to catch and characterize this intermediate oxygen species in the reaction cycle of cytochrome P450. Although many studies have been undertaken before and after 1984 to characterize Compound I, its electronic structure and physicochemical properties are still a mystery. The conclusion from this review is that the knowledge about Compound I has significantly increased; however, we still ask the same questions. There is a need for improved experimental approaches, detection techniques, and theoretical simulations for future studies of cytochrome P450 Compound I. This mini-review is dedicated to Klaus Ruckpaul on the occasion of his 80th birthday.