Isopentenyl Pyrophosphate The Biological Isoprene Unit

Isopentenyl pyrophosphate is the biological isoprene unit it contains five carbon atoms connected m the same order as m isoprene... 

Biological isoprene unit (Section 26.8) Isopentenyl pyrophosphate, the biological precursor to terpenes and steroids ... 

Formation of the biological isoprene unit from mevalonic acid has been shown to proceed by stepwise phosphorylation of both alcohol groups, then elimination and decarboxylation to yield 3-methyl-3-butenyl pyrophosphate, 9 (often called A3-isopentenyl pyrophosphate) ... 

Essential oils of many plants and flowers are obtained by distilling the plant with water. The water-insoluble oil that separates usually has an odor characteristic of the particular plant (rose oil, geranium oil, and others). Compounds isolated from these oils contain multiples of five carbon atoms (that is, 5, 10, 15, and so on) and are called terpenes (some compounds of this type were described on page 222). They are synthesized in the plant from acetate by way of an important biochemical intermediate, isopentenyl pyrophosphate. The five-carbon unit with a four-carbon chain and a one-carbon branch at C-2 is called an isoprene unit (see A Word About... Biologically Important Alcohols and Phenols, in Chapter 7). 

There is a common biological isoprene unit which is isopentenyl pyrophosphate (IPP) which, by means of an isomerase, is in equilibrium with its isomer dimethylallyl pyrophosphate. These two compounds can undergo condensation to give the CIO compound geranyl pyrophosphate which is the precursor of all monoterpenes. 

Earlier work by Nancy Bucher showed an ATP requirement for cholesterol biosynthesis. The involvement of phosphorylated intermediates was established by Comforth, Popjak, and their associates in the early 1960s with the discovery of kinases which successively phosphorylated MVA to MVA-P and MVA-P to MVA-PP. MVA-PP was decar-boxylated and dehydrated to give the biological C5 isoprene unit, isopentenyl pyrophosphate. This undergoes successive head-to-tail condensations to give the linear 15C terpenoid, famesyl pyrophosphate. 

A fascinating area of research linking organic chemistry and biology is the study of the biogenesis of natural products the detailed sequence of reactions by which a compound is formed in living systems, plant or animal. All the isoprene units in nature, it appears, originate from the same compound, isopentenyl pyrophosphate. 

The isoprene rule is a convenient formalism, but isoprene itself is not the biological precursor of terpenes. Nature instead uses two isoprene equivalents —isopentenyl pyrophosphate and dimethylallyl pyrophosphate. These five-carbon molecules are themselves made from condensation of three acetyl CoA units (Section 21.9). 

The above results strongly suggest that in the biological formation of the furanoquinoline alkaloids an isoprene unit (probably as isopentenyl pyrophosphate) is introduced into 2,4-dihydroxyquinoline to give (35). Cyclisation of this intermediate leads to isopropylidene-furanoquinolines e.g. platydesminium salt (36). Then elimination of the side chain gives the dictamnine (37) type. 

Lynen subsequently showed that the active C5 unit from mevalonic acid was isopentenyl pyrophosphate (IPP, 6) and it was at once recognized as the long-sought biological isoprene imit. Later investigations, mostly by Lynen, Cornforth,... 

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