ja0722067_si_002.pdb (733.5 kB)
Synthetic Efficiency in Enzyme Mechanisms Involving Carbocations: Aristolochene Synthase
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posted on 2007-10-31, 00:00 authored by Rudolf K. Allemann, Neil J. Young, Shuhua Ma, Donald G. Truhlar, Jiali GaoAn intramolecular proton-transfer mechanism has been proposed for the carbocationic cyclization
of farnesyl pyrophosphate (FPP) to (+)-aristolochene catalyzed by aristolochene synthase. This novel
mechanism, which is based on results obtained by high-level ab initio molecular orbital and density functional
theory calculations, differs from the previous proposal in the key step of carbocation propagation prior to
the formation of the bicyclic carbon skeleton. Previously, germacrene A was proposed to be generated as
an intermediate by deprotonation of germacryl cation followed by reprotonation of the C6−C7 double bond
to yield eudesmane cation. In the mechanism proposed here the direct intramolecular proton transfer has
a computed barrier of about 22 kcal/mol, which is further lowered to 16−20 kcal/mol by aristolochene
synthase. An alternative pathway is also possible through a proton shuttle via a pyrophosphate-bound
water molecule. The mechanism proposed here is consistent with the observation that germacrene A is
not a substrate of aristolochene synthase. Furthermore, the modeled substrate−enzyme complex suggests
that Trp 334 and Phe 178 play key roles in positioning the substrate in the reactive orientation in the binding
pocket. This is consistent with experimental findings that mutations of either residue lead to pronounced
generation of aborted cyclization products.