The primary end product of FAS activity is palmitate, a fully saturated LCFA with 16 carbons (16:0). However, palmitate can be further elongated in the smooth endoplasmic reticulum (SER) by adding two-carbon units to the carboxylate end. This process requires separate enzymes instead of a multifunctional enzyme. The two-carbon donor is Malonyl CoA, and the electrons are supplied by NADPH. The brain has additional elongation capabilities, allowing it to produce very-long-chain fatty acids (VLCFA) with over 22 carbons, which are needed for the synthesis of brain lipids.

Desaturation of LCFA, which involves adding cis double bonds, is also carried out by enzymes (fatty acyl CoA desaturases) present in the SER. This process requires oxygen (O2), NADH, cytochrome b5, and its flavin adenine dinucleotide (FAD)-linked reductase. As the O2 gets reduced to H2O, the fatty acid and NADH get oxidized. The first double bond is typically inserted between carbons 9 and 10, resulting in the production of primarily oleic acid, 18:1(9), and small amounts of palmitoleic acid, 16:1(9). Additional desaturation combined with elongation can produce various polyunsaturated fatty acids.

Humans have desaturases for carbon 9, 6, 5, and 4, but lack the ability to introduce double bonds from carbon 10 to the ω end of the chain. This is why polyunsaturated ω-6 linoleic acid and ω-3 linolenic acid are nutritionally essential.

Mono-, di-, and triacylglycerols consist of one, two, or three molecules of fatty acid esterified to a molecule of glycerol. The esterification of fatty acids occurs through their carboxyl groups, resulting in the formation of neutral fat and a loss of negative charge. The arrangement of fatty acids in a triacylglycerol (TAG) molecule is typically such that the fatty acid on carbon 1 is saturated, the one on carbon 2 is unsaturated, and the one on carbon 3 can be either. The presence of unsaturated fatty acids decreases the melting point (Tm) of the lipid.

TAG molecules, due to their low solubility in water, coalesce within white adipocytes to form large oily droplets that are almost free of water. These cytosolic lipid droplets serve as the major energy reserve of the body. TAG stored in brown adipocytes, on the other hand, function as a source of heat through nonshivering thermogenesis.
tricarboxylic acid; PC = pyruvate carboxylase; PDH = pyruvate dehydrogenase.
E. Further elongation
Although palmitate, a 16-carbon, fully saturated LCFA (16:0), is the primary end product of FAS activity, it can be further elongated by the addition of two-carbon units to the carboxylate end primarily in the smooth endoplasmic reticulum (SER). Elongation requires a system of separate enzymes rather than a multifunctional enzyme. Malonyl CoA is the two-carbon donor, and NADPH supplies the electrons. The brain has additional elongation capabilities, allowing it to produce the very-long-chain fatty acids ([VLCFA] over 22 carbons) that are required for synthesis of brain lipids.
F. Chain desaturation
Enzymes (fatty acyl CoA desaturases) also present in the SER are responsible for desaturating LCFA (that is, adding cis double bonds). The desaturation reactions require oxygen (O2), NADH, cytochrome b5, and its flavin adenine dinucleotide (FAD)-linked reductase. The fatty acid and the
NADH get oxidized as the O2 gets reduced to H2O. The first double bond is typically inserted between carbons 9 and 10, producing primarily oleic acid, 18:1(9), and small amounts of palmitoleic acid, 16:1(9). A variety of polyunsaturated fatty acids can be made through additional desaturation combined with elongation.
Humans have carbon 9, 6, 5, and 4 desaturases but lack the ability to introduce double bonds from carbon 10 to the ω end of the chain. This is the basis for the nutritional essentiality of the polyunsaturated ω-6 linoleic acid and ω-3 linolenic acid.
G. Storage as triacylglycerol components
Mono-, di-, and triacylglycerols consist of one, two, or three molecules of fatty acid esterified to a molecule of glycerol. Fatty acids are esterified through their carboxyl groups, resulting in a loss of negative charge and formation of neutral fat. [Note: An acylglycerol that is solid at room temperature is called a fat. If liquid, it is an oil.] 1. Arrangement: The three fatty acids esterified to a glycerol molecule to form a TAG are usually not of the same type. The fatty acid on carbon 1 is typically saturated, that on carbon 2 is typically unsaturated, and that on carbon 3 can be either. Recall that the presence of the unsaturated fatty acid(s) decrease(s) the Tm of the lipid. An example of a TAG molecule is shown in Figure 16.12.
2.
Triacylglycerol storage and function: Because TAG are only slightly soluble in water and cannot form stable micelles by themselves, they coalesce within white adipocytes to form large oily droplets that are nearly anhydrous. These cytosolic lipid droplets are the major energy reserve of the body. [Note: TAG stored in brown adipocytes serve as a source of heat through nonshivering thermogenesis (see p. 79).] 3.