Diterpene cyclases and the nature of the isoprene fold

The structures and mechanism of action of many terpene cyclases are known, but no structures of diterpene cyclases have yet been reported. Here, we propose structural models based on bioinformatics, site-directed mutagenesis, domain swapping, enzyme inhibition, and spectroscopy that help explain the nature of diterpene cyclase structure, function, and evolution. Bacterial diterpene cyclases contain similar to 20 alpha-helices and the same conserved QW and DxDD motifs as in triterpene cyclases, indicating the presence of a beta gamma barrel structure. Plant diterpene cyclases have a similar catalytic motif and beta gamma-domain structure together with a third, alpha-domain, forming an alpha beta gamma structure, and in H+-initiated cyclases, there is an EDxxD-like Mg2+/diphosphate binding motif located in the gamma-domain. The results support a new view of terpene cyclase structure and function and suggest evolution from ancient (beta gamma) bacterial triterpene cyclases, to (beta gamma) bacterial and thence to (alpha beta gamma) plant diterpene cyclases.