Catalyzed hydrolysis of triacylglycerols

Effect of Surfactants on Pregastric Enzyme-Catalyzed Hydrolysis of Triacylglycerols and Esters... 

Hydrolysis of triacylglycerols is catalyzed by lipoprotein lipase, a membrane-bound enzyme located on the endothelium lining the capillary beds of the muscle and adipose tissue. 

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... 

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. 

Figure 12.8. The synthesis and hydrolysis of triacylglycerols are catalyzed by lipases.

During fasting or exercise and at times of stress, lipid is used as a source of energy. Hormone-sensitive lipase catalyzes the sequential hydrolysis of triacylglycerol to... 

One consequence of physical activity is the activation of the sympathetic nervous system, which in turn stimulates the adrenal gland to secrete epinephrine and norepinephrine. These hormones then activate the adipocyte enzyme hormone-sensitive lipase, which catalyzes the hydrolysis of triacylglycerol molecules to form glycerol and the fatty acids used to drive muscle contraction. 

The process of triacylglycerol hydrolysis is a complex phenomenon that involves at least three lipases, lipid droplet associated proteins, and FABPs, although other adipocyte lipases (i.e., triacylglycerol hydrolase) may play a role in basal lipolysis. The data at this time support the model that three lipases are the major contributors to adipocyte lipolysis. Complete hydrolysis of triacylglycerol involves the hydrolysis of three ester bonds to liberate three fatty acids and a glycerol moiety. ATGL catalyzes hydrolysis of the first... 

Lipoprotein lipase is an enzyme found in the capillaries that catalyzes the hydrolysis of triacylglycerols in chylomicrons to glycerol and fatty acids (Figure 18.6, Figure 18.7). Lipoprotein lipase is activated by apoprotein C-II, which is found in all of the lipoprotein complexes except LDLs. Apoprotein C-1 may also be involved in activation of lipoprotein lipase. 

Problem 19.16. Write the equation for the acid-catalyzed hydrolysis of the triacylglycerol containing equimolar amounts of palmitic, stearic, and oleic acids. 

Lipases are hydrolases that catalyze in vivo the hydrolysis of triacylglycerols to glycerol or glycerides and free fatty acids. However, in systems with low water content, such as in the case of reverse micelles, they can catalyze the synthesis of esters, either directly from acids and alcohols or through trans- and interesteriflcations. 

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. 

Triacylglycerol Upases [EC 3.1.1.3] (also known as triglyceride lipases, tributyrases, or simply as lipases) catalyze the hydrolysis of a triacylglycerol to produce a diac-ylglycerol and a fatty acid anion. The pancreatic enzyme acts only on an ester-water interface the outer ester links in the substrate are the ones which are preferentially... 

This enzyme [EC 3.1.1.34] (also called clearing factor lipase, diglyceride lipase, and diacylglycerol lipase) catalyzes the hydrolysis of a triacylglycerol to produce a diacylglycerol and a fatty acid anion. This enzyme hydrolyzes triacylglycerols in chylomicrons and in low-density lipoproteins and also acts on diacylglycerols. See also Lipases... 

Lipases are enzymes that catalyze the in vivo hydrolysis of lipids such as triacylglycerols. Lipases are not used in biological systems for ester synthesis, presumably because the large amounts of water present preclude ester formation due to the law of mass action which favors hydrolysis. A different pathway (using the coenzyme A thioester of a carboxylic acid and the enzyme synthase [Blei and Odian, 2000]) is present in biological systems for ester formation. However, lipases do catalyze the in vitro esterification reaction and have been used to synthesize polyesters. The reaction between alcohols and carboxylic acids occurs in organic solvents where the absence of water favors esterification. However, water is a by-product and must be removed efficiently to maximize conversions and molecular weights. 

Lipoprotein lipase (EC 3.1.1.34) is an enzyme or group of enzymes which catalyze the hydrolysis of the 1(3) ester bond(s) of triacylglycerols and the 1 ester bond of phospholipids. The enzyme plays a central role in lipoprotein metabolism, being responsible in particular for the hydrolysis of chylomicron and VLDL triglycerides and the formation of remnant particles from these lipoproteins. There have been reviews of this enzyme [e.g., (N9, Ql)] and lipoprotein lipase will not be discussed in detail in this review. Familial lipoprotein lipase deficiency and related disorders of chylomicron metabolism have also been reviewed (B58, N8) and will not be discussed in detail. 

Recent studies have attempted to improve the efficiency of epoxidation under milder conditions that minimize the formation of byproducts. Chemo-enzymatic epoxidation uses the immobilized lipase from Candida antartica (Novozym 435) (56) to catalyze conversion of fatty acids to peracids with 60% hydrogen peroxide. The fatty acid is then self-epoxidized in an intermolecular reaction. The lipase is remarkably stable under the reaction conditions and can be recovered and reused 15 times without loss of activity. Competitive lipolysis of triacylglycerols is inhibited by small amounts of fatty acid, allowing the reaction to be carried out on intact oils (57). Rapeseed oil with 5% of rapeseed fatty acids was converted to epoxidized rapeseed oil in 91% yield with no hydroxy byproducts. Linseed oil was epoxidized in 80% yield. Methyl esters are also epoxidized without hydrolysis under these conditions. 

Among enzymes, lipases proved to be the most efficient for the in vitro polyester synthesis. Lipases or triacylglycerol acylhydrolases are water-soluble enzymes that catalyze the hydrolysis of ester bonds in water-insoluble, lipid substrates, and therefore comprise a subclass of the esterases. 

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