Polyprenyl diphosphate synthases.

It is noteworthy that in spite of the similarity of the reactions catalyzed by these prenyltransferases, the modes of expression of catalytic function are surprisingly different, varying according to the chain length and stereochemistry of reaction products. These enzymes are summarized and classified into four groups, as shown in Figure 13. Short-chain prenyl diphosphates synthases such as FPP and GGPP synthases require no cofactor except divalent metal ions, Mg2+ or Mn2+, which are commonly required by all prenyl diphosphate synthases. Medium-chain prenyl diphosphate synthases, including the enzymes for the synthesis of all-E-HexPP and all-E-HepPP, are unusual because they each consist of two dissociable dissimilar protein components, neither of which has catalytic activity. The enzymes for the synthesis of long-chain all-E-prenyl diphosphates, including octaprenyl (C40), nonaprenyl-(C45), and decaprenyl (C50) diphosphates, require polyprenyl carrier proteins that remove polyprenyl products from the active sites of the enzymes to maintain efficient turnovers of catalysis. The enzymes responsible for Z-chain elongation include Z,E-nonaprenyl-(C45) and Z,E-undecaprenyl (C55) diphosphate synthases, which require a phospholipid. The classification of mammalian synthases seems to be fundamentally similar to that of bacterial synthases except that no medium-chain prenyl diphosphate synthases are included. The Z-prenyl diphosphate synthase in mammalian cells is dehydrodolichyl PP synthase, which catalyzes much longer chain elongations than do bacterial enzymes. Dehydrodolichyl PP synthase will be a major target of future studies in this field in view of its involvement in glycoprotein biosynthesis.