Isopentenyl pyrophosphate (IPP) is one of two five-carbon building blocks of the isoprenoids. Dimethylallyl pyrophosphate (IPP) is the other. IPP is synthesized from 3-phospho-5-pyrophosphomevalonate, as shown in Figure 19.19. The first step in isoprenoid synthesis is as follows:

DPP + IPP <=> Geranyl Pyrophosphate + PPi (Figure 19.20)

Subsequently, in cholesterol biosynthesis, another molecule of IPP reacts with geranyl pyrophosphate (Figure 19.20) to form farnesyl pyrophosphate, as follows:

Geranyl Pyrophosphate + IPP <=> Farnesyl Pyrophosphate + PPi (Figure 19.20)