secondary metabolites are those that occur in some species and not others and they are usually classified into terpenoids, shikimates, polyketides, and alkaloids. The most important as far as essential oils are concerned are the terpenoids and the shikimates are the second. There are a number of polyketides of importance in essential oils but very few alkaloids. Terpenoids, shikimates, and polyketides will therefore be the main focus of this chapter.
The general scheme of biosynthetic reactions (Bu'Lock, 1965; Mann et al., 1994) is shown in Figure 5.1. Through photosynthesis, green plants convert carbon dioxide and water into glucose. Cleavage of glucose produces phosphoenolpyruvate (1), which is a key building block for the shiki-mate family of natural products. Decarboxylation of phosphoenolpyruvate gives the two-carbon unit of acetate and this is esterified with coenzyme-A to give acetyl CoA (2). Self-condensation of this species leads to the polyketides and lipids. Acetyl CoA is also a starting point for synthesis of mevalonic acid (3), which is the key starting material for the terpenoids. In all of these reactions, and indeed all the natural chemistry described in this chapter, Nature uses the same reactions that chemists do (Sell, 2003). However, nature's reactions tend to be faster and more selective because of the catalysts it uses. These catalysts are called enzymes and they are globular proteins in which an active site holds the reacting species together. This molecular organization in the active site lowers the activation energy of the reaction and directs its stereochemical course (Lehninger, 1993; Matthews and van Holde, 1990).
Many enzymes need cofactors as reagents or energy providers. Coenzyme-A has already been mentioned above. It is a thiol and is used to form thioesters with carboxylic acids. This has two effects on the acid in question. Firstly, the thiolate anion is a better leaving group than alkoxide and so the carbonyl carbon of the thioester is reactive toward nucleophiles. Secondly, the thioester group increases the acidity of the protons adjacent to the carbonyl group and therefore promotes the formation of the corresponding carbanions. In biosynthesis, a key role of adenosine triphosphate (ATP) is to make phosphate esters of alcohols (phosphorylation). One of the phosphate groups of ATP is added to the alcohol to give the corresponding phosphate ester and adenosine diphosphate (ADP). Another group of cofactors of importance to biosynthesis includes pairs such as NADP/NADPH, TPN/TPNH, and DPN/DPNH. These cofactors contain an ^-alkylated pyridine ring. In each pair, one form comprises an ^-alkylated pyridinium salt and the other the corresponding ^-alkyl-1,4-dihydropyridine. The two forms in each pair are interconverted by gain or loss of a hydride anion and therefore constitute redox reagents. In all of the cofactors mentioned here, the reactive part of
Carbon dioxide -
+ Green plants
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