Fig. 1. Architectural variants of class I and class II terpene cyclases.

The central scheme displays the proposed evolution of terpene cyclases from ancestral mono-domain enzymes³. Structurally related domains are colored and labeled α, β or γ accordingly, while parentheses indicate the absence of an active site. Cartoon representations to the left (class I) and right (class II) visualize the structural domain arrangement in selected examples with the respective PDB entry code noted below. So far, single domain architectures have only been described in microbial class I terpene cyclases, while MstE is a potential ancestor of βγ-assemblies in class II enzymes. Proposedly, bifunctional αβγ diterpene cyclases formed through fusion of α and βγ precursors and later diversified into a broad array of specialized enzymes.