Synthesis and Enzymatic Studies of Bisubstrate Analogues for Farnesyl Diphosphate Synthase

Farnesyl
diphosphate synthase catalyzes the sequential chain elongation
reactions between isopentenyl diphosphate (IPP) and dimethylallyl
diphosphate (DMAPP) to form geranyl diphosphate (GPP) and between
IPP and GPP to give farnesyl diphosphate (FPP). Bisubstrate analogues
containing the allylic and homoallylic substrates were synthesized
by joining fragments for IPP and the allylic diphosphates with a C–C
bond between the methyl group at C3 in IPP and the Z-methyl group at C3 in DMAPP (3-OPP) and GPP (4-OPP), respectively. These constructs placed substantial
limits on the conformational space available to the analogues relative
to the two substrates. The key features of the synthesis of bisubstrate
analogues 3-OPP and 4-OPP are a regioselective C-alkylation of the dianion of 3-methyl-3-buten-1-ol (5), a Z-selective cuprate addition of alkyl
groups to an α,β-alkynyl ester intermediate, and differential
activation of allylic and homoallylic alcohols in the analogues, followed
by a simultaneous displacement of the leaving groups with tris­(tetra-n-butylammonium) hydrogen diphosphate to give the corresponding
bisdiphosphate analogues. The bisubstrate analogues were substrates
for FPP synthase, giving novel seven-membered ring analogues of GPP
and FPP. The catalytic efficiencies for cyclization of 3-OPP and 4-OPP were similar to those for chain elongation
with IPP and DMAPP.