Crystal structure and mechanism of a bacterial fluorinating enzyme

Fluorine is the thirteenth most abundant element in the earth's crust, but fluoride concentrations in surface water are low and fluorinated metabolites are extremely rare1,2. The fluoride ion is a potent nucleophile in its desolvated state, but is tightly hydrated in water and effectively inert. Low availability and a lack of chemical reactivity have largely excluded fluoride from biochemistry: in particular, fluorine's high redox potential precludes the haloperoxidase-type mechanism3,4 used in the metabolic incorporation of chloride and bromide ions. But fluorinated chemicals are growing in industrial importance, with applications in pharmaceuticals, agrochemicals and materials products5,6,7. Reactive fluorination reagents requiring specialist process technologies are needed in industry and, although biological catalysts for these processes are highly sought after, only one enzyme that can convert fluoride to organic fluorine has been described8. Streptomyces cattleya can form carbon–fluorine bonds9 and must therefore have evolved an enzyme able to overcome the chemical challenges of using aqueous fluoride. Here we report the sequence and three-dimensional structure of the first native fluorination enzyme, 5′-fluoro-5′-deoxyadenosine synthase, from this organism. Both substrate and products have been observed bound to the enzyme, enabling us to propose a nucleophilic substitution mechanism for this biological fluorination reaction.