Flavin-containing monooxygenases (FMOs) The fundamental property of FMOs is their ability to stabilise the flavin-hydroperoxide. Stabilisation of the intermediate requires the presence of NADP+, which remains tightly bound to the enzyme throughout the catalytic cycle. FMOs represent an ideal system to study the structural enzymology of flavin-hydroperoxide formation and, more generally, to gain insight into the reaction of flavoproteins with oxygen. Three-dimensional structure of FMO NADP generates the cavity hosting the flavin-hydroperoxide intermediate Flavin-hydroperoxide Alfieri et al. (2008) Proc. Natl. Acad. USA 195, 6572