Characterized Plant Sesquiterpene Synthases

Background

Sesquiterpene synthases (STSs) in plants catalyze the formation of a vast array of sesquiterpenes, C15 chemical compounds made from C5 isoprene units. The reaction cascade of an STS is initiated by metal-mediated removal of the diphosphate anion in the FPP substrate, leading to the formation of a transoid (2E,6E)-farnesyl cation (farnesyl cation) which, due to its constrained structure, can undergo cyclization only at its C10-C11 double bond leading to cations 1 or 2 in Figure 1. However, the farnesyl cation can also isomerize to form a cisoid (2Z,6E)-farnesyl cation (nerolidyl cation). The nerolidyl cation, in addition to a C1-attack on the C10-C11 double bond to form cations 3 or 4, can also undergo cyclization at its C6-C7 double bond, forming cations 5 or 6. These carbocations undergo multiple further skeletal rearrangements, cyclizations, hydride or methyl shifts, and other modifications to form the final products of the enzyme. Along with this myriad of cyclic products, acyclic sesquiterpenes can also be formed from either the farnesyl or the nerolidyl cation through proton loss or addition of water. This schematic of carbocations derived from FPP can be used to divide sesquiterpenes produced by plants into seven groups - both based on their parent cation (farnesyl or nerolidyl) and the first cyclization that occurs (by attack of the carbocation on the 10,1-; 11,1-; 6,1-; or 7,1- double bond; or acyclic). For an STS enzyme, the carbocation of its major product is then used to determine its group in the database presented below. A sesquiterpene's parent cation and cyclization were obtained using the scheme detailed by IUBMB's Enzyme Nomenclature on Sesquiterpenoid Biosynthesis.

Database of Characterized Plant Sesquiterpene Synthases

UPDATE Jan 2021: Added 42 new STSs
The database has 307 enzymes covering 113 species and 117 sesquiterpene products. For each enzyme, we provide:

1. the UniProt accession, linked to the corresponding UniProt entry page
2. the PubMed ID of the paper detailing its experimental characterization, linked to the corresponding PubMed page
3. the species from which the enzyme is obtained, linked to the corresponding UniProt Taxonomy page
4. the parent cation of the major product (see Figure 1)
5. the first cyclization undergone by the major product (see Figure 1)
6. the major products, linked to their corresponding PubChem entries
7. the minor products, linked to their corresponding PubChem entries

More details can be found in An analysis of characterized plant sesquiterpene synthases (BioRXiv preprint link).