Lipid Structure and Function

What are the structures and functions of the most common fatty acids?

● The most abundant fatty acids in nature are unbranched hydrocarbon chains that contain either reduced methylene groups (CH2), called saturated fatty acids, or oxidized C=C bonds, called unsaturated fatty acids. Fatty acids with multiple C=C bonds are called polyunsaturated fatty acids.

● The common names of fatty acids indicate how or where the fatty acid was discovered. For example, palmitate was first isolated from palm oil and linoleate was isolated from flax seed. Humans require three essential fatty acids in their diet: the polyunsaturated fatty acids linoleate, α-linolenate, and arachidonate.

● The number and configuration of C=C bonds in unsaturated fatty acids affect the melting points of fatty acid mixtures, with long-chain saturated fatty acids having a higher melting point than long-chain unsaturated fatty acids because of differences in intermolecular interactions.

● Carbons in fatty acids are numbered from the carboxylic acid end, with the carboxyl carbon being C-1. Any C=C bonds present are denoted as superscript numerals associated with the symbol “D.” For example, palmitate is a saturated C16 fatty acid (16:0), whereas α-linolenate is an unsaturated C18 fatty acid with three cis C=C bonds at C-9, C-12, and C-15, written as cis 18:3(delta 9,12,15).

● Fatty acids can also be numbered from the methyl carbon (the ω-carbon). Polyunsaturated fatty acids with the most distal C=C bond from the carboxyl group positioned three carbons away from the ω-carbon are called ω-3 fatty acids, such as eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA). The same nomenclature can be used to describe the ω-6 fatty acids linoleate and arachidonate.

How are triacylglycerols used in energy storage processes in animals?

● Triacylglycerols are either obtained from the diet, primarily from digesting animal fat and nuts in the small intestine, or synthesized in the liver, using glucose and amino acids as a source of acetyl-CoA.

● Triacylglycerols are transported through the blood as components of lipoprotein complexes, which can be very large chylomicrons produced in intestinal epithelial cells or small very-low-density lipoprotein (VLDL) particles synthesized and exported by liver cells.

● Adipocytes hydrolyze stored triacylglycerols in response to hormone signaling (glucagon and epinephrine) and release free fatty acids and glycerol into the circulatory system, where the fatty acids are transported throughout the body by a carrier protein called albumin.

● Dietary triacylglycerols are hydrolyzed in the small intestine by lipase enzymes that release the three fatty acids and glycerol. Free fatty acids and glycerol enter the intestinal epithelial cells, where they are resynthesized into triacylglycerols and then exported to the lymphatic system as components of chylomicron lipoproteins.

● The biosynthesis of triacylglycerols in liver cells uses acetyl-CoA produced by the degradation of carbohydrates and proteins to generate palmitic acid in the cytosol, which is then converted to triacylglycerols and exported as VLDL particles.

● Lipoprotein particles contain proteins on the surface that facilitate fatty acid delivery to peripheral tissues through binding and activation of lipoprotein lipase on the surface of endothelial cells. The free fatty acids diffuse into nearby adipose and muscle cells, whereas the glycerol travels through the blood to the liver.

What are the major types of lipids in cell membranes?

● The most abundant lipids in membranes are phospholipids—including both glycerophospholipids and sphingomyelin—followed by cholesterol and sphingoglycolipids. Glycerophospholipids represent about half of all membrane lipids in eukaryotic cell membranes, with sphingolipids and cholesterol making up the other half.

● Glycerophospholipids are derived from phosphatidate and are represented by the membrane phospholipids phosphatidylcholine, phosphatidylserine, phosphatidylethanolamine, and phosphatidylinosito

● Sphingolipids are derived from the biomolecule sphingosine (which is synthesized by linking serine to the carboxyl group of palmitate) and one fatty acid. There are two types of sphingolipids: the sphingophospholipids, represented by sphingomyelin, and the sphingoglycolipids, which are called cerebrosides and gangliosides.

● Cholesterol contains a rigid, four-ring structure that disrupts the packing of glycerophospholipids and sphingolipids when inserted into membranes. Cholesterol also contributes to the formation of lipid rafts in the plasma membrane of eukaryotic cells.

How do lipids function in cell signaling processes?

● Cholesterol is the precursor to steroid hormones (mineralocorticoids, glucocorticoids, progesterones, androgens, and estrogens) and vitamin D. Eicosanoids are derived from the ω-6 fatty acid arachidonate, a polyunsaturated fatty acid that is converted into prostaglandins, prostacyclins, thromboxanes, and leukotrienes.

● Many of the enzymes involved in steroid biosynthesis (steroidogenesis) are hydroxylases, of which one type is the cytochrome P450 monooxygenases. Steroidogenesis begins with removal of the side chain attached to the D ring of cholesterol to generate pregnenolone, the biosynthetic precursor to all animal steroids.

● Eicosanoids are a group of signaling molecules derived from C20 polyunsaturated fatty acids such as arachidonate, which are released from the membrane by phospholipases and modified by mitochondrial enzymes.

● Eicosanoids are produced by cells at their sites of action (paracrine) and have half-lives of only a few minutes. The four major classes of arachidonate-derived eicosanoids are prostaglandins, prostacyclins, thromboxanes, and leukotrienes, which mediate cell signaling by activating G protein–coupled receptors on target cells.

● The synthesis of prostaglandins, prostacyclins, and thromboxanes requires cyclooxygenation of arachidonate by either cyclooxygenase-1 (COX-1) or cyclooxygenase-2 (COX- 2) to generate the precursor prostaglandin H2. The enzyme lipoxygenase generates leukotrienes directly from arachidonate.

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