Polyprenyl diphosphate synthases

The 5-epi-aristoIochene synthase of tobacco makes the 5-epimer of aristolochene (Fig. 22-4). It binds the diphosphate group within a central cavity using two Mg2+ ions held by carboxylates, some of which are in the DDXXD sequence found also in polyprenyl diphosphate synthases. The enzyme active sites of both the epi-aristolochene synthase and a pentale-nene synthase from Streptomyces96 are rich in polar groups that form hydrogen-bonded networks and which participate in proton abstraction and donation during the rearrangement reactions that must occur. 

Ognra, K., Koyama, T, and Sagami, H. (1997). Polyprenyl diphosphate synthases. Subcell Biochem 28 57-87. 

Figure 22-1 Outline of biosynthetic pathways for polyprenyl compounds. Steps e,f, and g and many subsequent steps are catalyzed by isoprenyl diphosphate synthases.

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