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Figure 3.7 The proposed arrangement of ligands (including histidine and glutamic acid residues) at the di-iron cluster in a desaturase enzyme

Figure 3.8 The removal of the pro-HR from C-9 and the formation of a double bond in a fatty acid, illustrated by the conversion of stearic acid (as a Co A thioester J to oleic acid by a A9-desaturase enzyme. It is a free radical reaction with the removal of two syn-oriented vicinyl hydrogen atoms

enzyme

Fe"K_,Felv enzyme

Fe"K_,Felv enzyme H OH

Figure 3.9 By variation of the protein structure, hydroxylase enzymes, similar to desaturases, are produced which can introduce hydroxyl groups into a fatty acid at the same position hydroxylases, which introduce hydroxyl groups into the fatty acid at C-9 (Figure 3.9), and epoxidases. Variations produce A-7 or A-ll desaturases, and different conformations of the alkyl chain in the pocket of the enzyme can give trans double bonds as found in some of the lepidopteran pheromones (see later). Acetylenases, methyl oxidases (converting methane to methanol) and other enzymes have similar di-iron active sites.

Linoleic and linolenic acids (Figure 3.10) are made by plants by further desaturation of oleic acid. It is generally accepted that animals cannot make these acids, in other words, they are essential in their diets, but recent experiments have shown that a cricket and a cockroach, as well as a slug and a garden snail can make linoleic acid. It is possible then that all insects can make linoleic acid. There is no indication that linolenic acid can be made by animals. Arachidonic acid (5,8,11,14-eicosatetraenoic acid) is made by animals only, by chain extension and desaturation from linoleic acid obtained in their food.

oleic acid oleic acid

linoleic acid in animals

2, desaturase

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