Although fatty acid synthesis involves the formation of an unsaturated intermediate, the next step, reduction to the saturated fatty acid derivative, is an obligatory part of the reaction sequence and cannot be omitted. The product of the fatty acid synthase multienzyme complex is always saturated. Some unsaturated fatty acids can be synthesized from saturated fatty acids, by dehydrogenation to yield a carbon—carbon double bond.
Mammalian tissues have a A9-desaturase, which can introduce a carbon—carbon double bond between carbons 9 and 10 of the fatty acid (counting from the carboxyl group). This will yield oleic acid (C18:1 W9) from stearic acid (C18:0). Other mammalian desaturases permit insertion of double bonds between A9-10 and the carboxyl group, but not between A9-10 and the methyl group.
This means that fatty acids with double bonds between A9-10 and the methyl group must be provided in the diet. Linoleic acid (C18:2 w6) is A9-10, 12-13, and a-linolenic acid (C18:3 W3) is A9-10, 12-13, 15-15. Both of these are dietary essentials and undergo co.
s—CoA acetyl CoA
_ ATP , acetyl CoA carboxylase
s—CoA malonyl CoA
S—ACP fatty acyl ACP
c=0 /3-keto-acyIA CP synthase i—acp
malonyl acyl carrier protein
S—ACP keto-acyl ACP
jB-keto-acyl A CP —u fi-hydroxy-acyl A CP reductase ^_ACp dehydratase hydroxy-acyl ACP
S—ACP enoyl ACP
enoyl ACP reductase
Figure 5.28 The synthesis of fatty acids.
chain elongation and further desaturation (between A9-10 and the carboxyl group) to yield the precursors of the eicosanoids.
The same series of enzymes catalyses elongation and desaturation of the W3 and W6 families of fatty acids, and they compete with each other. Although, as discussed in section 18.104.22.168, it is considered desirable to increase intakes of polyunsaturated fatty acids, there is some concern that an imbalance between intakes of w6 polyunsaturated fatty acids (which come mainly from vegetable oils) and the W3 series (which come mainly from fish oils) may lead to an undesirable imbalance in the synthesis of the two groups of eicosanoids.
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