Section 26.8 Biosynthesis of Terpenes 019PROBLEM 14 SOLVEDGive the mechanism for the last step in the biosynthesis of isopentenyl pyrophosphate,
showing why ATP is required.SOLUTION In the last step of the biosynthesis of isopentenyl pyrophosphate, elimina-
tion of is accompanied by elimination of an group, which is a strong base and
therefore a poor leaving group. ATP is used to convert the OH group into a phosphate
group, which is easily eliminated because it is a weak base.PROBLEM 15Give the mechanisms for the Claisen condensation and aldol addition that occur in the first
two steps of the biosynthesis of isopentenyl pyrophosphate.Biosynthesis of Dimethylallyl Pyrophosphate
Both isopentenyl pyrophosphateand dimethylallyl pyrophosphateare needed for
the biosynthesis of terpenes. Therefore, some isopentenyl pyrophosphate is converted
to dimethylallyl pyrophosphate by an enzyme-catalyzed isomerization reaction. The
isomerization involves addition of a proton to the carbon of isopentenyl py-
rophosphate that is bonded to the greater number of hydrogens (Section 4.4) and elim-
ination of a proton from the carbocation intermediate in accordance with Zaitsev’s
rule (Section 11.2).
sp^2CO +++ADPOOOPO−OO−PO−OOOPO−OO− CO 2PO−HOOO−PO−OOO−PO−
OOPO−−O OOPO−adenosineO−+ H+OHCO + OATPOOOPO−OO−PO−OOPO−−OOPO−OOPO−adenosineO−CO 2 - OH? AU: Add-H+here?isopentenyl pyrophosphate dimethylallyl pyrophosphateOOOPO−OO−PO−OOOPO−OO− + H+PO−OOOPO−OO−PO−H+ +