The fluorinase enzyme (EC 2.5.1.63, also known as adenosyl-fluoride synthase) catalyzes the reaction between fluoride ion and the co-factor S-adenosyl-L-methionine to generate L-methionine and 5'-fluoro-5'-deoxyadenosine, the first committed product of the fluorometabolite biosynthesis pathway. The fluorinase was originally isolated from the soil bacterium Streptomyces cattleya, but homologues have since been identified in a number of other bacterial species, including Streptomyces sp. MA37, Nocardia brasiliensis and Actinoplanes sp. N902-109. This is the only known enzyme capable of catalysing the formation of a carbon-fluorine bond, the strongest single bond in organic chemistry.

A homologous chlorinase enzyme, which catalyses the same reaction with chloride rather than fluoride ion, has been isolated from Salinospora tropica, from the biosynthetic pathway of salinosporamide A.

The fluorinase catalyses an S_N2-type nucelophilic substitution at the C-5' position of SAM, while L-methionine acts as a neutral leaving group. The fluorinase-catalysed reaction is estimated to be between 10⁶ to 10¹⁵ times faster than the uncatalysed reaction, a significant rate enhancement. Despite this, the fluorinase is still regarded as a slow enzyme, with a turnover number (k_cat) of 0.06 min⁻¹. The high kinetic barrier to reaction is attributed to the strong solvation of fluoride ion in water, resulting in a high activation energy associated with stripping solvating water molecules from aqueous fluoride ion, converting fluoride into a potent nucleophile within the active site.

The reaction catalysed by the fluorinase is reversible, and upon incubation of 5'-fluoro-5'-deoxyadenosine and L-methionine with the fluorinase, SAM and fluoride ion are produced. Replacing L-methionine with L-selenomethionine results in a 6-fold rate enhancement of the reverse reaction, due to the increased nucleophilicity of the selenium centre compared to the sulfur centre.

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