| Abstract
| - An intramolecular proton-transfer mechanism has been proposed for the carbocationic cyclizationof farnesyl pyrophosphate (FPP) to (+)-aristolochene catalyzed by aristolochene synthase. This novelmechanism, which is based on results obtained by high-level ab initio molecular orbital and density functionaltheory calculations, differs from the previous proposal in the key step of carbocation propagation prior tothe formation of the bicyclic carbon skeleton. Previously, germacrene A was proposed to be generated asan intermediate by deprotonation of germacryl cation followed by reprotonation of the C6−C7 double bondto yield eudesmane cation. In the mechanism proposed here the direct intramolecular proton transfer hasa computed barrier of about 22 kcal/mol, which is further lowered to 16−20 kcal/mol by aristolochenesynthase. An alternative pathway is also possible through a proton shuttle via a pyrophosphate-boundwater molecule. The mechanism proposed here is consistent with the observation that germacrene A isnot a substrate of aristolochene synthase. Furthermore, the modeled substrate−enzyme complex suggeststhat Trp 334 and Phe 178 play key roles in positioning the substrate in the reactive orientation in the bindingpocket. This is consistent with experimental findings that mutations of either residue lead to pronouncedgeneration of aborted cyclization products.
|
