In summary, the QM and MD simulations of HheC wild type and F12Y in complex with epoxide 1 and azide indicate that mutation F12Y changes the network of hydrogen bonding with azide, which has a big impact on the activation barrier, but also on the preorganization of the active site and the retention of both epoxide and azide at their required optimal positions for enhanced activity. In comparison, the equivalent tyrosine 18 in HheG wild type is not able to establish similar hydrogen bonding interactions with azide or T151 (equivalent to T131 in HheC) due to the much wider active site pocket of HheG. The latter might explain why mutation Y18F in HheG affects enzyme activity only slightly (vide infra). The extra flexibility of residue F12 in WT (highlighted with a double arrow) affects the productive binding of azide in the active site pocket, thus hampering the epoxide-ring opening reaction.