| DESATURATION OF FATTY ACIDS
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| Desaturation reactions require molecular oxygen
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| The body has a requirement for mono- and polyunsaturated fatty acids, in addition to saturated fatty acids. Some of these need to be supplied in the diet; these two unsaturated fatty acids, linoleic and linolenic, are known as the essential fatty acids (EFA; see below). The desaturation system requires molecular oxygen, NADH, and cytochrome b5. The process of desaturation, like that of chain elongation, occurs on the endoplasmic reticulum, and results in the oxidation of both the fatty acid and NADH (Fig. 15.6).
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| In man, the desaturase system is unable to introduce double bonds between carbon atoms beyond carbon-9 and the ω (terminal methyl) carbon atom. Most desaturations occur between carbon atoms 9 and 10 (annotated as Δ9 desaturations), e.g. those with palmitic acid producing palmitoleic acid (C-16:1 Δ9), and those with stearic acid producing oleic acid (C-18:1, Δ9).
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| CHANGES IN THE ENZYME EXPRESSION IN RESPONSE TO FOOD INTAKE ARE KEY REGULATORS OF THE STORAGE OF ENERGY SUBSTRATES |
| The fed state is associated with the induction of enzymes that increase fatty acid synthesis in the liver. A wide range of enzymes are induced, including those involved in glycolysis, e.g. glucokinase (the hepatic form of hexokinase) and pyruvate kinase, as well as enzymes linked to increased production of NADPH (Glc-6-P dehydrogenase, 6-phosphogluconate dehydrogenase, and malic enzyme). Further, there is an increased expression of citrate lyase, acetyl-CoA carboxylase, fatty acid synthase, and Δ9 desaturase. |
| Further, in the fed state there is a concomitant repression of the key enzymes involved in gluconeogenesis. Phosphoenolpyruvate carboxykinase, glucose-6-phosphatase (Glc-6-P-ase), and some aminotransferases are reduced in amount, either by reduction in synthesis, or by increased degradation. |
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