Complex Lipids

Fatty acids are toxic to cells. One mechanism to sequester them is to form complex lipids.[1'2'5-7] Triacylglycerols (TAG; Fig. 3) are synthesized from FAs. The primary energy storage molecule in mammalian cells is TAG. Adipose tissue cells contain large amounts of TAG so that a large lipid droplet occupies the majority of the cell

Triacylglycerol (tripalmitin)

Elongation (add 2-carbons) precursor product c16:0 c18:1 (a9) c18:2 (a9'12)

Desaturation (insert double bonds) desaturase precursor product a9 a6 a5

c18:0

c18:1 (a9) c18:4 (a6'9'12'15) c20:4 (a5.».".14)

[oleic acid] [arachidonic acid]

Phospholipid (phosphatidylcholine) h o

Steroid (cholesterol)

Steroid (cholesterol)

Fig. 3 Complex mammalian lipids.

Fig. 2 Mammalian interconversion of fatty acids.

Fig. 3 Complex mammalian lipids.

Diet

TAG, PL, CE

Absorb

MAG, FA, C, lysoPL, glycerol, choline, etc

Lymph

chylomicrons, VLDL

Portal blood

glycerol, choline, short FA

Arterial & Venous blood

lipoproteins (chylomicrons, VLDL, LDL, HDL, albumin)

Abbreviations: C=cholesterol, CE = cholesterol ester, FA=fatty acid, HDL=high-density lipoprotein, LDL = low-density lipoprotein, lysoPL = lysophospholipid, MAG = monoacyglycerol, PL=phospholipid, TAG=triacylglycerol, VLDL=very low density lipoprotein,

Fig. 4 Metabolism and transport of lipids.

Fig. 4 Metabolism and transport of lipids.

because they contain ionic groups (a negative charge in the phosphate group and a positive charge in the nitrogenous base, e.g., choline).

The other major class of complex lipids is the steroids (Fig. 3) synthesized by plants and animals. The major animal steroid, cholesterol (C), is not made by plants but is obtained from animal feedstuffs or is synthesized de novo with the liver being the predominant site. Much of the C is stored as the ester (CE, i.e., C with an attached FA). Cholesterol is a component of cell membranes and a precursor to bile acids and steroid hormones (e.g., estrogen, testosterone, glucocorticoids, etc.).

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