Dioxygen is essential in life processes, as metalloenzymes activate dioxygen to carry out a variety of biological reactions including biotransformation of naturally occurring molecules, oxidative metabolism of xenobiotics, and oxidative phosphorylation. One primary goal in metalloenzyme research is to understand the structures of active sites and reactive intermediates with the mechanistic details of dioxygen activation occurring at those sites. Metalloenzymes use diverse active sites to activate dioxygen, such as mononuclear heme iron sites, nonheme monoiron and diiron sites, mononuclear and dinuclear copper sites, a heteronuclear heme-copper site, and other metal sites.
Despite this diversity of active sites, there is a common mechanistic hypothesis for oxygen activation emerging wherein oxygen first binds to a reduced metal center, thus forming a metal-peroxo intermediate, followed by 0-0 bond cleavage to form a high-valent metal-oxo oxidant that carries out substrate oxidation.