Streptomyces cattleya is unusual in that it elaborates organofluorine metabolites and in particular secretes fluoroacetate and 4-fluorothreonine as secondary metabolites when incubated in a medium supplemented with fluoride ions.[1, 2] The first committed step on the biosynthetic pathway to these fluorometabolites involves the enzymatic mediated synthesis of 5′-fluoro-5′-deoxyadenosine (5′-FDA) from S-adenosyl-L-methionine (SAM) and fluoride ions.[3-5] After depurination, 5′-FDA is metabolized first to 5-fluoro-5-deoxyribose-1-phosphate (5′-FRP)[6] and then to fluorocetaldehyde.[7] Fluoroacetaldehyde is converted separately into fluoroacetate [8] and 4-fluorothreonine [9](Scheme 1).

The fluorination enzyme, which has been the subject of a recent structural study,[10] has been isolated and fully characterized, the gene has been cloned, and the enzyme has been overexpressed. A stereochemical study that involved (5′ S)-[5′-2H]-SAM has indicated that the C–F bond of 5′-FDA replaces the C–S bond of SAM with an inversion of configuration.[11, 12] This observation, as well as a theoretical study [13] and structural analysis [10] all suggest an SN2 reaction mechanism for the fluorinase. Despite the high bonddissociation energy of the C–F bond (the strongest bond in organic chemistry [14]), we herein report that the fluorinase operates in the reverse direction and that it utilizes a chloride ion. Incubation of 5′-FDA with L-(13C-methyl) methionine resulted in the formation of 13C-methyl-SAM, which was identified by HPLC–ES-MS. A similar experiment with L-selenomethionine (L-Se-met) and 5′-FDA resulted in a sixfold more-efficient reaction to generate L-Se-SAM, which was readily identified through the mass-spectrometry fragmentation product of methylselenoadenosine (MeSeAdo) by its signature isotope fingerprint in HPLC–ES-MS. Comparison with the Vmax values that were calculated at saturating kinetics (assays run in each direction) indicated that the equilibrium lies in favor of 5′-FDA by a factor of three (see Scheme 2).