FOUND AT LAST: TWO LONG-sought-after reactive intermediates in the activation of dioxygen by nonheme iron enzymes. After years of speculation, two new structures—one of an enzyme containing O 2 bound in a side-on fashion to the active-site iron and the other of a synthetic compound featuring an Fe(IV)=O unit—provide direct evidence for the existence of these two intermediates [ Science , 299 , 1039 and 1037 (2003)]. The controlled oxidation of organic substrates by O 2 is a key reaction in many biological transformations. But without a catalyst, these reactions are extremely unfavorable. Many enzymes, therefore, turn to an iron-porphyrin cofactor called heme to activate O 2 for reaction with organic molecules. But many others—including enzymes required to biosynthesize molecules like serotonin and penicillin as well as enzymes that bacteria use to convert pollutants into usable carbon sources—rely on just a single iron atom to do the job. Although the mechanistic details are still being 'ironed out,'the ...