Structure of Geranyl Diphosphate <i>C</i>-Methyltransferase from <i>Streptomyces coelicolor</i> and
Implications for the Mechanism of Isoprenoid Modification
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Abstract
Geranyl diphosphate <i>C</i>-methyltransferase
(GPPMT)
from <i>Streptomyces coelicolor</i> A3(2) is the first methyltransferase
discovered that modifies an acyclic isoprenoid diphosphate, geranyl
diphosphate (GPP), to yield a noncanonical acyclic allylic diphosphate
product, 2-methylgeranyl diphosphate, which serves as the substrate
for a subsequent cyclization reaction catalyzed by a terpenoid cyclase,
methylisoborneol synthase. Here, we report the crystal structures
of GPPMT in complex with GPP or the substrate analogue geranyl <i>S</i>-thiolodiphosphate (GSPP) along with <i>S</i>-adenosyl-l-homocysteine in the cofactor binding site, resulting
from <i>in situ</i> demethylation of <i>S</i>-adenosyl-l-methionine, at 2.05 or 1.82 Å resolution, respectively.
These structures suggest that both GPP and GSPP can undergo catalytic
methylation in crystalline GPPMT, followed by dissociation of the
isoprenoid product. <i>S</i>-Adenosyl-l-homocysteine
remains bound in the active site, however, and does not exchange with
a fresh molecule of cofactor <i>S</i>-adenosyl-l-methionine. These structures provide important clues about the molecular
mechanism of the reaction, especially with regard to the face of the
2,3 double bond of GPP that is methylated as well as the stabilization
of the resulting carbocation intermediate through cation−π
interactions