Structure of 2-Methylisoborneol
Synthase from <i>Streptomyces coelicolor</i> and Implications
for the Cyclization of a Noncanonical <i>C</i>-Methylated
Monoterpenoid Substrate
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Abstract
The crystal structure of 2-methylisoborneol synthase
(MIBS) from <i>Streptomyces coelicolor</i> A3(2) has been
determined in complex with substrate analogues geranyl-<i>S</i>-thiolodiphosphate and 2-fluorogeranyl diphosphate at 1.80 and 1.95
Å resolution, respectively. This terpenoid cyclase catalyzes
the cyclization of the naturally occurring, noncanonical <i>C</i>-methylated isoprenoid substrate, 2-methylgeranyl diphosphate, to
form the bicyclic product 2-methylisoborneol, a volatile C<sub>11</sub> homoterpene alcohol with an earthy, musty odor. While MIBS adopts
the tertiary structure of a class I terpenoid cyclase, its dimeric
quaternary structure differs from that previously observed in dimeric
terpenoid cyclases from plants and fungi. The quaternary structure
of MIBS is nonetheless similar in some respects to that of dimeric
farnesyl diphosphate synthase, which is not a cyclase. The structures
of MIBS complexed with substrate analogues provide insights regarding
differences in the catalytic mechanism of MIBS and the mechanisms
of (+)-bornyl diphosphate synthase and <i>endo</i>-fenchol
synthase, plant cyclases that convert geranyl diphosphate into products
with closely related bicyclic bornyl skeletons, but distinct structures
and stereochemistries