Cht Hoc C

ch3 HoC C

Condensation carbonium ion Geranyl or farnesyl pyrophosphate

FIGURE 2-14 (A) Conversion of mevalonic acid (51) into the intermediate 3-phosphomevaIonic acid 5-pyrophosphate (53), which is then decarboxylated to yield the nucleophilic 3-isopentenyl pyrophosphate

(1). Structure 1 is then isomerized into equivalent amounts of the electrophilic dimethylallyl pyrophosphate

(2). (B) Proposed mechanism for the key "head-to-tail" joining of the nucleophilic isopentenyl pyrophosphate to the allylic electrophilic "acceptor," which may be either dimethylallyl pyrophosphate (2) or geranyl pyrophosphate (53A). An encircled P indicates phosphate. (C) Pathway of reactions leading from isopentenyl pyrophosphate + dimethylallyl pyrophosphate to squalene (56). A key reaction is the "nose-to-nose" reaction of trans, fnms-farnesyl pyrophosphate (54) to yield presqualene pyrophosphate (55), which, after reduction by NADPH, produces squalene (56).

2. Steroid Hormones

trans-Geranyl pyrophosphate (53A)

trans-trans-Farnesyl pyrophosphate (54)

Presqualene pyrophosphate (55) NADPH + H+
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