Monoterpene Biosynthesis

In 1937 it was shown that labelled acetate gave labelled terpenes, but evidently by a different route from the fatty acids and acetogenins. In 1956 it was accidentally found that a substance called mevalonic acid (Figure 6.2) was an intermediate between acetate and terpenes, and this gave the clue needed to study their biosynthesis.

In the first step, acetate (as a coenzyme A thioester) plus malonate gives acetoacetate, just as for the fatty acids. The next step is a different kind of reaction. Another acetate unit (as malonate) is added via an aldol condensation, using a HMG-CoA synthase enzyme, to give p-hydroxy-p-methylglutaryl coenzyme A (HMG-CoA). This is reduced by NADPH with a HMG-CoA reductase to mevalonic acid (MVA) in the rate-determining step for the whole sequence of reactions that builds up the h?c"

¿VCH2 h isoprene

(3-myrcene from myrrh and bay leaves a monoterpene frans-ß-ocimene widepread in plants epecially basil geraniol from oil of roses

OH linalool in cinnamon, orange flowers, and bergamot citronellol in lemon peel and rose oil nerol .

from orange blossom oir and usually with geraniol

(+)-limonene in lemon oil

Hi oh menthol, in peppermint oil

>f carvone in spearmint oil zingiberene from oil of ginger, a sesquiterpene

Figure 6.1

-oh farnesol, from lily-of-the-valley, a sesquiterpene

Isoprene and some examples of simple terpenes, showing the linkage between their isoprene units. Note that some of these substances have chiral centres

II Claisen aldol HQ

condensation n3y 0 condensation h+ "OOC^^

hq ,ch30

,coa

(rate-determining step)

hq ch3

HMG-CoA

0 0

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