Of triacylglycerols

Fats and oils are naturally occurring mixtures of triacylglycerols also called triglyc erides They differ m that fats are solids at room temperature and oils are liquids We generally ignore this distinction and refer to both groups as fats... 

Section 26 4 Phospholipids are intermediates in the biosynthesis of triacylglycerols from fatty acids and are the principal constituents of the lipid bilayer component of cell membranes... 

A synthesis of triacylglycerols has been described that begins with the substance shown... 

Phosphatidic acids not only are intennediates in the biosynthesis of triacylglycerols but also are biosynthetic precursors of other members of a group of compounds called phosphoglycerides or glycerol phosphatides. Phosphorus-containing derivatives of lipids are known as phospholipids, and phosphoglycerides are one type of phospholipid. 

FIGURE 24.1 Scanning electron micrograph of an adipose cell (fat cell). Globules of triacylglycerols occupy most of the volume of such cells. (Prof. P. Motta, Dept, of Anatomy, University La Sapienza, Rome/Science Photo Library/Photo Researchers, Inc.)... 

FIGURE 24.3 (a) A duct at the junction of the pancreas and duodenum secretes pancreatic juice into the duodenum, the first portion of the small intestine, (b) Hydrolysis of triacylglycerols by pancreatic and intestinal lipases. Pancreatic lipases cleave fatty acids at the C-1 and C-3 positions. Resulting monoacylglycerols with fatty acids at C-2 are hydrolyzed by intestinal lipases. Fatty acids and monoacylglycerols are absorbed through the intestinal wall and assembled into lipoprotein aggregates termed chylomicrons (discussed in Chapter 25). 

Calculate the volume of metabolic water available to a camel through fatty acid oxidation if it carries 30 lb of triacylglycerol in its hump. 

Levels of free fatty acids are very low in the typical cell. The palmitate made in this process is rapidly converted to CoA esters in preparation for the formation of triacylglycerols and phospholipids.)... 

FIGURE 25.17 Hormonal signals regulate fatty acid synthesis, primarily through actions on acetyl-CoA carboxylase. Availability of fatty acids also depends upon hormonal activation of triacylglycerol lipase. 

The metabolic breakdown of triacylglycerols begins with their hydrolysis to yield glycerol plus fatty acids. The reaction is catalyzed by a lipase, whose mechanism of action is shown in Figure 29.2. The active site of the enzyme contains a catalytic triad of aspartic acid, histidine, and serine residues, which act cooperatively to provide the necessary acid and base catalysis for the individual steps. Hydrolysis is accomplished by two sequential nucleophilic acyl substitution reactions, one that covalently binds an acyl group to the side chain -OH of a serine residue on the enzyme and a second that frees the fatty acid from the enzyme. 

Diacylglycerol is glycerol esterified to two fatty acids at the sn-1 and sn-2 positions. It is a membrane-embedded product of phospholipase C action and an activator of protein kinase C. It is also an intermediate in the biosynthesis of triacylglycerol, phosphatidyletha-nolamine and phosphatidylcholine. 

Increased lipid synthesis/inhibi-tion of lipolysis Activation of lipoprotein lipase (LPL)/induc-tion of fatty acid synthase (FAS)/inactivation of hormone sensitive lipase (HSL) Facilitated uptake of fatty acids by LPL-dependent hydrolysis of triacylglycerol from circulating lipoproteins. Increased lipid synthesis through Akt-mediated FAS-expression. Inhibition of lipolysis by preventing cAMP-dependent activation of HSL (insulin-dependent activation of phosphodiesterases )... 

The triacylglycerols (Figure 14—6) are esters of the tri-hydric alcohol glycerol and fatty acids. Mono- and di-acylglycerols wherein one or two fatty acids are esteri-fied with glycerol are also found in the tissues. These are of particular significance in the synthesis and hydrolysis of triacylglycerols. 

Phospholipids may be regarded as derivatives of phos-phatidic acid (Figure 14-8), in which the phosphate is esterified with the —OH of a suitable alcohol. Phos-phatidic acid is important as an intermediate in the synthesis of triacylglycerols as well as phosphoglycerols but is not found in any great quantity in tissues. 

Phosphatidate Is the Common Precursor in the Biosynthesis of Triacylglycerols, Many Phosphoglycerols, Cardiolipin... 

The regulation of triacylglycerol, phosphatidylcholine, and phosphatidylethanolamine biosynthesis is driven by the availability of free fatty acids. Those that escape oxidation are preferentiaUy converted to phos-phohpids, and when this requirement is satisfied they are used for triacylglycerol synthesis. 

Tijburg LBM, Geelen MJH, van Golde LMG Regulation of the biosynthesis of triacylglycerol, phosphatidylcholine and phos-phatidylethanolamine in the liver. Biochim Biophys Acta 1989 1004 1. 

Plasma lipids consist of triacylglycerols (16%), phospholipids (30%), cholesterol (14%), and cholesteryl esters (36%) and a much smaller fraction of unesteri-fied long-chain fatty acids (free fatty acids) (4%). This latter fraction, the free fatty acids (FFA), is metaboh-cally the most active of the plasma hpids. 

The free fatty acids (FFA, nonesterified fatty acids, im-esterified fatty acids) arise in the plasma from hpolysis of triacylglycerol in adipose tissue or as a result of the action of hpoprotein hpase during uptake of plasma tri-acylglycerols into tissues. They are found in combination with albumin, a very effective solubilizer, in concentrations varying between 0.1 and 2.0 ieq/mL of plasma. Levels are low in the ftiUy fed condition and rise to 0.7-0.8 leq/mL in the starved state. In uncontrolled diabetes mellitus, the level may rise to as much as 2 Ieq/mL. 

Figure 25-2. The formation and secretion of (A) chylomicrons by an intestinal cell and (B) very low density lipoproteins by a hepatic cell. (RER, rough endoplasmic reticulum SER, smooth endoplasmic reticulum G, Golgi apparatus N, nucleus C, chylomicrons VLDL, very low density lipoproteins E, endothelium SD, space of Disse, containing blood plasma.) Apolipoprotein B, synthesized in the RER, is incorporated into lipoproteins in the SER, the main site of synthesis of triacylglycerol. After addition of carbohydrate residues in G, they are released from the cell by reverse pinocytosis. Chylomicrons pass into the lymphatic system. VLDL are secreted into the space of Disse and then into the hepatic sinusoids through fenestrae in the endothelial lining.

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