Principles of Biochemistry, Volume 2 - A. Lehninger 1985

Bioenergetics and Metabolism
Lipid Biosynthesis
Palmitic acid serves as a precursor for other long-chain fatty acids.

In animal Cells, palmitic acid, a common product generated in the fatty acid synthase cycle, serves as a precursor for other long-chain Fatty acids (Fig. 21-12). Through the action of enzyme systems catalyzing fatty acid chain elongation, it can be extended by The addition of acetyl groups to yield stearic acid (18 carbon atoms) or even longer saturated fatty acids; this process takes place in the Endoplasmic reticulum and Cell/35.html">Mitochondria. The elongation system in The endoplasmic reticulum, which exhibits higher activity, Functions in much the same way as palmitate synthesis: stearoyl-ACP is formed from palmitoyl-CoA through the addition of two-carbon units donated by malonyl-CoA.

Palmitic and stearic acids are precursors to the two most abundant monounsaturated fatty acids in animal Tissues (Fig. 21-12): palmitoleic acid (16 carbon atoms) and oleic acid (18 carbon atoms), each containing a single cis double bond at the ∆⁹ position (Section 12.1). The Introduction of a double bond into the fatty acid molecule occurs via an oxidation reaction catalyzed by acyl-CoA oxygenase

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These reactions serve as classic Examples of oxidations catalyzed by mixed-function oxygenases, as they involve the simultaneous oxidation of two distinct groups: a single bond of the Fatty acid and NADPH.

Fig. 21-12. Pathways of fatty acid synthesis. Palmitic acid serves as a precursor for stearic acid and other long-chain saturated fatty acids, as well as the monounsaturated fatty acids palmitoleic and oleic acids. In animals, oleic acid cannot be converted into linoleic acid, making linoleic acid an essential fatty acid that must be obtained from the diet. The figure illustrates The conversion of linoleic acid into other polyunsaturated Fatty Acids and Prostaglandins. The nomenclature of Unsaturated fatty acids indicates the number of carbon atoms, as well as the number and position of double bonds. For example, linoleic acid (C₁₈∆^9,12) contains 18 carbon atoms and two double bonds: one between carbon atoms 9 and 10, and another between 12 and 13.

In animal tissues, a double bond is readily introduced at the ∆⁹ position of a fatty acid molecule, whereas The formation of an additional double bond between the ∆⁹ double bond and the methyl end of the fatty acid is impossible. Mammals are unable to synthesize linoleic acid (containing two double bonds at the ∆⁹ and ∆¹² positions) and a-linolenic acid (C₁₈∆^9,12,15). Because these Fatty acids are utilized as precursors for the Synthesis of Other vital compounds, they must be supplied in the animal's diet from plant sources. Consequently, these compounds are referred to as Essential Fatty Acids. Dietary deficiency of linoleic acid causes scaly dermatitis in rats. Once ingested by mammals, linoleic acid acts as the sole precursor for other polyunsaturated fatty acids, such as y-linolenic and arachidonic acids (Section 12.1); (Fig. 21-12). Arachidonic acid contains 20 carbon atoms and four double bonds at the ∆⁵, ∆⁸, ∆¹¹, and ∆¹⁴ positions. This acid is of paramount importance because it serves as an indispensable precursor for most Prostaglandins and thromboxanes—hormone-like substances that regulate a wide range of cellular functions (Chapter 25).