The One Ring: A Monocycle Producing Class II Diterpene Cyclase from Isodon leucophyllus

Area of Science:

• Biochemistry
• Enzymology
• Plant Science

Background:

• Class II diterpene cyclases (DTCs) typically produce bicyclic diterpenoids from geranylgeranyl pyrophosphate (GGPP).
• Monocyclic diterpenoids were previously thought to arise from carotenoid degradation, not direct enzymatic cyclization.
• The catalytic mechanism of DTCs, particularly the role of the pyrophosphate head group, is incompletely understood.

Purpose of the Study:

• To report the discovery and characterization of a novel DTC from Isodon leucophyllus.
• To elucidate the unique catalytic mechanism of this enzyme in producing monocyclic diterpenoids.
• To investigate the evolutionary origins and distribution of this novel DTC activity.

Main Methods:

• Gene discovery and heterologous expression of the novel DTC (IleuTPS).
• Site-directed mutagenesis to probe the function of the unusual E366XDD motif.
• Isotope labeling, quantum chemical calculations, and molecular docking (TerDockin) to determine the reaction mechanism.
• Phylogenetic analysis to infer evolutionary history.

Main Results:

• Discovery of IleuTPS, the first reported DTC producing monocyclic (+)-trixagyl pyrophosphate.
• IleuTPS possesses an E366XDD motif, distinct from the canonical DXDD, where glutamate acts as a catalytic base.
• Mutational analysis and mechanistic studies revealed an unusual fragmentation process leading to monocycle formation.
• Evolutionary analysis suggests early gene duplication and neofunctionalization, with similar activities found broadly across plants.

Conclusions:

• This discovery reveals a novel enzymatic pathway for direct monocyclic diterpenoid biosynthesis.
• The unusual catalytic mechanism of IleuTPS highlights the critical role of catalytic bases in determining product outcome.
• The findings necessitate a re-evaluation of diterpenoid biosynthesis and the functional roles of DTCs in nature.