Same Substrate, Many Reactions: Oxygen Activation in Flavoenzymes

Over time, organisms have evolved strategies to cope with the abundance of dioxygen on Earth. Oxygen-utilizing enzymes tightly control the reactions involving O2 mostly by modulating the reactivity of their cofactors. Flavins are extremely versatile cofactors that are capable of undergoing redox reactions by accepting either one electron or two electrons, alternating between the oxidized and the reduced states. The physical and chemical principles of flavin-based chemistry have been investigated widely. In the following pages we summarize the state of the art on a key area of research in flavin enzymology: the molecular basis for the activation of O2 by flavin-dependent oxidases and monooxygenases. In general terms, oxidases use O2 as an electron acceptor to produce H2O2, while monooxygenases activate O2 by forming a flavin intermediate and insert an oxygen atom into the substrate. First, we analyze how O2 reaches the flavin cofactor embedded in the protein matrix through dedicated access pathways. Then we approach O2 activation from the perspective of the monooxygenases, their preferred intermediate, the C(4a)-(hydro)peroxyflavin, and the cases in which other intermediates have been described. Finally, we focus on understanding how the architectures developed in the active sites of oxidases promote O2 activation and which other factors operate in its reactivity.