Mechanistic Characterisation of a Diterpene Synthase for Chryseojoostenes A–E from Chryseobacterium Joostei

A diterpene synthase from Chryseobacterium joostei was characterised and produces the five unique compounds chryseojoostenes A–E. Chryseojoostenes D and E were produced in too low amounts for isolation from the wildtype enzyme, but extensive site‐directed mutagenesis resulted in an enzyme variant in which the production of these compounds was enhanced. The biosynthesis of the enzyme products was investigated in detail through a combined experimental and computational approach, indicating a complex hydrogen scrambling during terpene cyclisation and a long‐range proton shift towards chryseojoostene E. Density functional theory (DFT) calculations revealed that a similar long range hydrogen shift is involved in the formation of an even fragment ion ( m / z 216), characterising the unique chemistry of the chryseojoostene skeleton. Further insights into the cyclisation mechanism were obtained by enzymatic conversion of two substrate analogs with reduced reactivity.