Triacylglycerols functions

Hydrolysis of the phosphate ester function of the phosphatidic acid gives a diacylglycerol which then reacts with a third acyl coenzyme A molecule to produce a triacylglycerol... 

The melting points of even-numbered-carbon fatty acids increase with chain length and decrease according to unsaturation. A triacylglycerol containing three sam-rated fatty acids of 12 carbons or more is solid at body temperature, whereas if the fatty acid residues are 18 2, it is liquid to below 0 °C. In practice, natural acylglyc-erols contain a mixture of fatty acids tailored to suit their functional roles. The membrane lipids, which must be fluid at all environmental temperatures, are... 

Triacylglycerols are the major energy-storing lipids, whereas phosphoglycerols, sphingomyelin, and gly-cosphingolipids are amphipathic and have structural functions in cell membranes as well as other specialized roles. 

For a variety of reasons, lipid—mainly as triacylglycerol—can accumulate in the hver (Figure 25—6). Extensive accumulation is regarded as a pathologic condition. When accumulation of lipid in the Ever becomes chronic, fibrotic changes occur in the cells that progress to cirrhosis and impaired liver function. 

Saponification (see Section 7.4) is carried out to extract more recalcitrant lipids, and the yields are higher than for conventional solvent extraction (Stern et al. 2000). 3 ml of 0.5 M methanolic NaOH is added to 0.1 g of the shard powder and heated at 70°C for 3 hours in a sealed glass vial. After cooling, the supernatant is acidified with HC1 and extracted with three aliquots of 3 ml //-hexane. The hexane will not mix with the methanolic solution (unlike the DCM MeOH used above), but will absorb the lipids and can be transferred into a new clean vial. The removal of excess hexane is carried out as above. Saponification will hydrolyze and methylate any ester functionalities, which removes the requirement to derivatize the samples (Section 7.4) unless other molecules are suspected of being present. However, any wax esters or triacylglycerols will also be hydrolyzed to their fatty acid methyl esters and alcohols therefore, if information on their composition is required, then conventional solvent extraction is recommended as a first step. For subsequent characterization of the lipid extract, see Chapter 7. 

Triacylglycerols, commonly refered to as fats and oils, consist of three fatty acids linked to a molecule of glycerol, a three-carbon alcohol. Fatty acids are long-carbon-chain molecules, each with a single carboxyl functional group. Common examples are stearic acid and palmitic acid, shown in Figure 16.3. 

The function of the apparently futile triacylglycerol cycle (futile cycles are discussed in Chapter 15) is not well understood. However, as we learn more about how the triacylglycerol cycle is sustained via metabolism in two separate organs and is coordinately regulated, some possibilities emerge. For example, the excess capacity in the triacylglycerol cycle (the fatty acid that is even-... 

Modification of nascent chylomicron particles The particle released by the intestinal mucosal cell is called a "nascent" chylomicron because it is functionally incomplete. When it reaches the plasma, the particle is rapidly modified, receiving apo E (which is recognized by hepatic receptors) and C apolipoproteins, The latter include apo C-ll, which is necessary for the activation of lipoprotein lipase, the enzyme that degrades the triacylglycerol contained in the chylomicron (see below). The source of these apolipoproteins is circulating HDL (see Figure 18.16). 

VLDLs are produced in the liver (Figure 18.17). They are composed predominantly of triacylglycerol, and their function is to carry this lipid from the liver to the peripheral tissues. There, the triacylglycerol is degraded by lipoprotein lipase, as discussed for chylomicrons (see p. 226). [Note "Fatty liver" (hepatic steatosis) occurs in conditions in which there is an imbalance between hepatic triacylglycerol synthesis and the secretion of VLDL. Such conditions include obesity, uncontrolled diabetes mellitus, and chronic ethanol ingestion.]... 

The plasma lipoproteins include chylomicrons, very-low-density lipoproteins (VLDL), low-density lipoproteins (LDL), and high-density lipoproteins (HDL). They function to keep lipids (primarily triacylglyc-erol and cholesteryl esters) soluble as they transport them between tissues. Lipoproteins are composed of a neutral lipid core (containing triacylglycerol, cholesteryl esters, or both) surrounded by a shell of amphipathic apolipoproteins, phospholipid, and nonesterified cholesterol. Chylomicrons are assembled in intestinal mucosal cells from dietary lipids (primarily, triacylglycerol) plus additional lipids synthesized in these cells. Each nascent chylomicron particle has one molecule of apolipoprotein B-48 (apo B-48). They are released from the cells into the lymphatic system and travel to the blood, where they receive apo C-ll and apo E from HDLs, thus making the chylomicrons functional. Apo C-ll activates lipoprotein lipase, which degrades the... 

Chylomicrons are assembled in intestinal mucosal cells from dietary lipids (primarily triacylglycerol), plus additional lipids synthesized in these cells. Each nascent chylomicron particle has one molecule of apolipoprotein B-48 (apo B-48). They are released from the cells into the lymphatic system and travel to the blood, where they receive apo C-ll and apo E from HDLs. This makes the chylomicrons functional. 

Triacylglycerols and the ether lipids described in the previous section are classified as neutral lipids. Other neutral lipids are alcohols, waxes, aldehydes, and hydrocarbons derived from fatty acids. These sometimes have specific biological functions. For example, fatty aldehydes are important in the bioluminescence of bacteria (Eq. 23-47). 

Each apolipoprotein has one or more distinct functions. The apoB proteins probably stabilize the lipoprotein micelles. In addition, apoB-100 is essential to recognition of LDL by its receptors. The 79-residue apoC-II has a specific function of activating the lipoprotein lipase that hydrolyses the triacylglycerols of chylomicrons and VLDL. Lack of either C-II or the lipase results in a very high level of triacylglycerols in the blood.11... 

Outline of pathways for the biosynthesis of major cellular lipids (other than cholesterol) in a mammalian cell. Most of the metabolism of these lipids occurs on membrane surfaces because of the insoluble nature of the substrates and products. These lipids play three major roles (l) they act as a storehouse of chemical energy, as with triacylglycerols (2) they are structural components of membranes (boxed compounds) and (3) they act as regulatory compounds (underlined), either as eicosanoids, which act as local hormones, or as phosphorylated inositols and diacylglycerols, which function as second messengers. 

Chylomicrons transport dietary triacylglycerol and cholesteryl ester from the intestine to other tissues in the body. Very-low-density lipoprotein functions in a manner similar to the transport of endogenously made lipid from the liver to other tissues. These two types of triacylglycerol-rich particles are initially degraded by the action of lipoprotein lipase, an extracellular enzyme that is most active within the capillaries of adipose tissue, cardiac and skeletal muscle, and the lactating mammary gland. Lipoprotein lipase catalyzes the hydrolysis of triacylglycerols (see fig. 18.3). The enzyme is specifically activated by apoprotein C-II, which... 

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