Replacement of the Hydroxyl -OH Functional Group by Carbon An Example from Nature

The intermediate, geranyl diphosphate, is a C-10 hydrocarbon, which is formed by the enzyme-catalyzed (the enzyme is named prenyltransferase) reaction between the C-5 isomers dimethylallyl diphosphate and isopentenyl diphosphate (C-5 -I- C-5 = C-10). The combination of these fragments apparently involves an SnI 

in an analogous process, when phosphomevalonate is acted upon by the enzyme phosphormeva-lonate kinase (Chapter 12), a second equivalent of ATP is used and a second phosphate residue is transferred. The result is the formation of 5-pyrophosphomevalonate (and another equivalent of ADP). 

Action of the enzyme pyrophosphomevalonate decarboxylase on 5-pyrophosphomevalonate results in the consumption of a third equivalent of ATP and the simultaneous dehydration and decarboxylation (loss of carbon dioxide, CO2) of the pyrophosphomevalonate. Isopentenyl diphosphate results 

In a subsequent step, the allylic carbocation condenses with isopentenyl diphosphate, forming a new carbocation, which then loses the Hr (this chapter) proton to form the product. Alternatively, geranyl diphosphate can form if (as shown in the figure) proton loss is concomitant with condensation. 

Isopentenyl diphosphate (4-hydroxy-2-methyl-1 -pentene diphosphate) 

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