Carbocationic intermediates seem to be rare in enzymatic reactions, but this may be illusory. The best known examples are the glycosyl cations in the actions of glycosyltrans-ferases. However, most of the many enzymes engaged in the biosynthesis of more than 23,000 terpenes remain to be studied, and many of their reactions are likely to proceed through carbocationic intermediates. The substrates generally include allylic pyrophosphates that can form allylic carbocations on elimination of pyrophosphate (Sacchettini and Poulter, 1997), The cations can react with alkenes to form new carbon-carbon bonds. The basic chemistry of this process is outlined in fig. 14-1C, and allylic pyrophosphates such as dimethylallyl pyrophosphate (DMAPP) as sources of allylic carbocations are subjects of the present discussion. The prenyl pyrophosphates exist as complexes with Mg2+ in cells.
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