Triacylglycerols binding

Fate of the remaining chylomicron components After most of tt triacylglycerol has been removed, the chylomicron remnan (which contain cholesteryl esters, phospholipids, apolipoprotein and some triacylglycerol) bind to receptors on the liver (seej 228) and are then endocytosed. The remnants are the hydrolyzed to their component parts. Cholesterol and the nitrogf nous bases of phopholipids (for example, choline) can be req cled by the body. [Note If removal of chylomicron remnants by th liver is defective, they accumulate in the plasma. This is seen i type III hyperlipoproteinemia (also called familial dysbetalipopro teinemia, see p. 229). 

Using diffuse reflectance Fourier transform infrared spectroscopy, Adhikari et al. studied the binding of oleic acid (88), triacylglycerol (89), and phosphatidylcholine (90) on silica gel at room temperature. Their interpretation was that oleic acid and triacylglycerols bind to silica surface hydroxyls via hydrogen bonding interactions between the carboxylate and ester carbonyls of these molecules, respectively. In contrast, phospholipids hydrogen bond to silica surface hydroxyls via the phosphate... 

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. 

The free fatty acids formed by lipolysis can be reconverted in the tissue to acyl-CoA by acyl-CoA synthetase and reesterified with glycerol 3-phosphate to form triacylglycerol. Thus, there is a continuous cycle of lipolysis and reesterification within the tissue. However, when the rate of reesterification is not sufficient to match the rate of lipolysis, free fatty acids accumulate and diffuse into the plasma, where they bind to albumin and raise the concentration of plasma free fatty acids. 

Silver nitrate may be incorporated in the adsorbent slurry (25 g l-1) giving a final concentration of about 5% in the dry plate. The silver ions bind reversibly with the double bonds in the unsaturated compounds, resulting in selective retardation, and the lipids are separated according to the number and configuration (cis or trans) of their double bonds. This technique is extremely useful in fatty acids, mono-, di- and particularly triacylglycerol analyses when even positional isomers may be resolved. Borate ions may also be incorporated in the silica gel and these plates are used to separate compounds with adjacent free hydroxyl groups. 

VLDLs, IDLs, and LDLs are closely related to one another. VLDLs formed in the liver (see p. 312) transport triacylglycerols, cholesterol, and phospholipids to other tissues. Like chylomicrons, they are gradually converted into IDL and LDL under the influence of lipoprotein lipase [1]. This process is also stimulated by HDL. Cells that have a demand for cholesterol bind LDL through an interaction between their LDL receptor and ApoB-100, and then take up the complete particle through receptor-mediated endocytosis. This type of transport is mediated by depressions in the membrane ( coated pits"), the interior of which is lined with the protein clathrin. After LDL binding, clathrin promotes invagination of the pits and pinching off of vesicles ( coated vesicles"). The clathrin then dissociates off and is reused. After fusion of the vesicle with ly-sosomes, the LDL particles are broken down (see p. 234), and cholesterol and other lipids are used by the cells. 

Chylomicrons deliver tiiacylglycerols to tissues, where lipoprotein lipase releases free fatty acids for entry into cells. Triacylglycerols stored in adipose tissue are mobilized by a hormone-sensitive triacylglycerol lipase. The released fatty acids bind to serum albumin and are carried in the blood to the heart, skeletal muscle, and other tissues that use fatty acids for fuel. 

When certain hormones (e.g., epinephrine) bind to their receptors in adipose tissue, adenylate cyclase is activated. The cAMP that is formed activates protein kinase A, which phosphorylates triacylglycerol lipase. The phosphorylated form of this enzyme is the active species, and triacylglycerols are degraded to fatty acids. 

The breakdown of fatty acids in (3-oxidation (see Topic K2) is controlled mainly by the concentration of free fatty acids in the blood, which is, in turn, controlled by the hydrolysis rate of triacylglycerols in adipose tissue by hormone-sensitive triacylglycerol lipase. This enzyme is regulated by phosphorylation and dephosphorylation (Fig. 5) in response to hormonally controlled levels of the intracellular second messenger cAMP (see Topic E5). The catabolic hormones glucagon, epinephrine and norepinephrine bind to receptor proteins on the cell surface and increase the levels of cAMP in adipose cells through activation of adenylate cyclase (see Topic E5). The cAMP allosterically activates... 

VLDLs are synthesized in the liver and transport triacylglycerols, cholesterol and phospholipids to other tissues, where lipoprotein lipase hydrolyzes the triacylglycerols and releases the fatty acids for uptake. The VLDL remnants are transformed first to IDLs and then to LDLs as all of their apoproteins other than apoB-100 are removed and their cholesterol esterified. The LDLs bind to the LDL receptor protein on the surface of target cells and are internalized by receptor-mediated endocytosis. The cholesterol, which is released from the lipoproteins by the action of lysosomal lipases, is either incorporated into the cell membrane or re-esterified for storage. High levels of intracellular cholesterol decrease the synthesis of the LDL receptor, reducing the rate of uptake of cholesterol, and inhibit HMG CoA reductase, preventing the cellular synthesis of cholesterol. 

The release of fatty acids from adipose tissue is regulated by the rate of hydrolysis of triacylglycerol and the rate of esterification of acyl-CoA with glycerol 3-phosphate. The rate of hydrolysis is stimulated by hormones that bind to cell-surface receptors and stimulate adenylate cyclase (which catalyzes the production of cAMP from ATP). Hormone-sensitive lipase (Sec. 13.4) can exist in two forms, one of which exhibits very low activity and a second which is phosphorylated and has high activity. Before hormonal stimulation of adenylate cyclase, the low-activity lipase predominates in the fat cell. Stimulation of protein kinase by an increase in cAMP concentration leads to phosphorylation of the low-activity lipase. An increase in the rate of hydrolysis of triacylglycerol and the release of fatty acids from the fat cell follows. This leads to a greater utilization of fatty acids by tissues such as heart, skeletal muscle, and liver. 

In response to energy demands, the fatty acids of stored triacylglycerols must be mobilised for use by peripheral tissues. This release is controUed by a complex series of interrelated cascades that result in the activation of hormone-sensitive lipase. In adipocytes this stimulus can come from glucagon, adrenaline (epinephrine) or 8-corticotropin. These hormones bind cell-surface receptors that are coupled to the activation of adenyl cyclase the resultant increase in intracellular cAMP leads to activation of cAMP-dependent protein kinase, which in turn phosphorylates and activates hormone-sensitive lipase (Figure 5.5). 

Triacylglycerols are highly concentrated stores of metaholic energy because they are reduced and anhydrous. The yield from the complete oxidation of fatty acids is about 9 kcal g" (38 kJ g- ), in contrast with about 4 kcal g l (17 kJ g-i) for carbohydrates and proteins. The basis of this large difference in caloric yield is that fatty acids are much more reduced. Furthermore, triacylglycerols are nonpolar, and so they are stored in a nearly anhydrous form, whereas much more polar proteins and carbohydrates are more highly hydrated. In fact, 1 g of dry glycogen binds about 2 g of water. 

The chylomicrons are released into the lymph system and then into the blood. These particles bind to membrane-bound lipoprotein lipases, primarily at adipose tissue and muscle, where the triacylglycerols are once again degraded into free... 

The initial event in the utilization of fat as an energy source is the hydrolysis of triacylglycerols by lipases, an event referred to as lipolysis. The lipase of adipose tissue are activated on treatment of these cells with the hormones epinephrine, norepinephrine, glucagon, and adrenocorticotropic hormone. In adipose cells, these hormones trigger 7TM receptors that activate adenylate cyclase (Section 15,1.3 ). The increased level of cyclic AMP then stimulates protein kinase A, -which activates the lipases by phosphorylating them. Thus, epinephrine, norepinephrine, glucagon, and adrenocorticotropic hormone induce lipolysis (Figure 22.6). In contrast, insulin inhibits lipolysis. The released fatty acids are not soluble in blood plasma, and so, on release, serum albumin binds the fatty acids and serves as a carrier. By these means, free fatty acids are made accessible as a fuel in other tissues. 

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