Squalene hopene cyclases: highly promiscuous and evolvable catalysts for stereoselective CC and CX bond formation.

We review here how the inherent promiscuous nature, as well as the evolvability of terpene cyclase enzymes enables new applications in chemistry. We mainly focus on squalene hopene cyclases, class II triterpene synthases that use a proton-initiated cationic polycyclization cascade to form carbopolycyclic products. We highlight recent findings to demonstrate that these enzymes are capable of activating different functionalities other than the traditional terminal isoprene C=C-group as well as being compatible with a wide range of nucleophiles beyond the 'ene-functionality'. Thus, squalene hopene cyclases demonstrate a great potential to be used as a toolbox for general Brønsted acid catalysis.