The fluorinase from Streptomyces cattleya and chlorinase from Salinispora tropica have a commonality in that they mediate nucleophilic reactions of their respective halide ions to the C-5′ carbon of S-adenosyl-L-methionine (SAM). These enzyme reactions fall into the relatively small group of S(N)2 substitution reactions found in enzymology. These enzymes have some homology to a larger class of proteins expressed by the duf-62 gene, of which around 200 representatives have been sequenced and deposited in databases. The duf-62 genes express a protein which mediates a hydrolytic cleavage of SAM to generate adenosine and L-methionine. Superficially this enzyme operates very similarly to the halogenases in that water/hydroxide replaces the halide ion. However structural examination of the duf-62 gene product reveals a very different organisation of the active site suggesting a novel mechanism for water activation.