Taking into account that HheC mutant F12Y is not only more active in the dehalogenation and epoxide ring opening of several tested substrates, but exhibits also higher enantioselectivity and thermal stability compared to wild-type HheC, the question arises why mutation F12Y was not selected during natural evolution of this enzyme. For comparison, other native HHDHs such as HheB or HheG carry a tyrosine instead of a phenylalanine at the respective motif 1 position. One possible hypothesis might be that mutant F12Y is not superior in combination with HheC's natural substrate(s), as the epoxides and haloalcohols tested by us probably do not represent natural substrates of HheC. In agreement with this hypothesis, mutant F12Y is not generally more active independent of the used substrate, but was found to display a lower specific activity in the azidolysis of glycidyl phenyl ether. On the other hand, the gene of HheC is organized in an operon together with an epoxide hydrolase-encoding gene in the genome of A. radiobacter. Both enzymes were predicted to act together in the detoxification of harmful haloalcohols. Thus, HheC's activity in A. radiobacter likely needs to be harmonized with the respective activity of the epoxide hydrolase to prevent an accumulation of the epoxide intermediate, which is harmful itself due to its high reactivity with e.g. primary amines of lysine residues. A too high activity of HheC might therefore not be evolutionary beneficial.