Function

The primary function of ( )-epi-alpha-bisabolol synthase is to catalyze the cyclization and rearrangement of farnesyl diphosphate (FPP), a 15-carbon prenyl diphosphate precursor, into the specific sesquiterpene alcohol ( )-epi-alpha-bisabolol. This enzymatic reaction typically involves the cleavage of the diphosphate group, followed by a series of carbocationic rearrangements and subsequent quenching by water or an intramolecular proton abstraction to yield the final hydroxylated product. The precise stereochemistry indicated by "epi-" specifies a particular epimer of alpha-bisabolol, meaning it differs from other alpha-bisabolol isomers in the configuration at one of its chiral centers. The placeholder "( )-" often denotes a specific enantiomeric form (e.g., (+) or (-)) that might be determined or context-dependent for the "epi" variant.

Classification

As a synthase, this enzyme falls under the general EC (Enzyme Commission) class of lyases (EC 4), specifically those forming carbon-carbon bonds. More precisely, it is classified as a terpene synthase, and within that, a sesquiterpene synthase, due to its use of the C15 precursor farnesyl diphosphate.

Biological Significance

Alpha-bisabolol, in its various stereoisomeric forms, is a naturally occurring sesquiterpene alcohol found in several plant species, most notably in Matricaria chamomilla (German chamomile) and Myoporum crassifolium. Different isomers of alpha-bisabolol, such as (-)-alpha-bisabolol (levomenol), are known for their biological activities, including anti-inflammatory, anti-irritant, anti-microbial, and antioxidant properties. The biosynthesis of specific bisabolol stereoisomers is crucial for the overall chemical profile and medicinal properties of plants. An enzyme like ( )-epi-alpha-bisabolol synthase would contribute to the diversity of bisabolol isomers produced by an organism, potentially leading to distinct biological effects or serving specific ecological roles within the plant.

Mechanism

Terpene synthases operate through a complex mechanism involving multiple steps:

Ionization: Cleavage of the diphosphate group from FPP to generate a highly reactive allylic carbocation.

Cyclization: Rearrangements and cyclizations of the carbocationic intermediate, often involving electrophilic attack on double bonds.

Rearrangements: Further skeletal rearrangements, hydride shifts, or Wagner-Meerwein shifts to form diverse cyclic structures.

Quenching: Termination of the carbocation cascade by attack of a nucleophile (like water for alcohol products) or deprotonation, leading to the final product.

The specific sequence of these steps and the precise folding of the carbocation intermediate within the enzyme's active site determine the exact stereochemistry and structure of the ( )-epi-alpha-bisabolol product.