Triacylglycerol lipase, reaction catalyzed

The hydrolytic reaction catalyzed by lipases generally takes place at the oil-water interface. The hydrolytic activity in the presence of triacylglycerols is the basis characteristic of lipases (Table 4). However, hydrolytic activity of different lipases may not always give a good indication of the potential synthesis activity (Table 4). 

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. 

Energy production from triacylglycerols starts with their hydrolysis into free fatty acids and glycerol. Enzymes called lipases, which catalyze the reaction, carry out this hydrolysis. 

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. 

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. 

Yamane, T. Lipase-catalyzed synthesis of structured triacylglycerols containing polyunsaturated fatty acids monitoring the reaction and increasing the yield. In Enzymes in Lipid Modification Bornscheuer, U.T., Ed. Wiley-VCH Weinheim, Germany, 2000 148-169. 

Pancreatic lipase is an enzyme found in intestinal mucosal cells that digests triacylglycerols to a mixture of free fatty acids, glycerol, monoacylglycerols, and diacylglycerols. The enzyme requires calcium and is unusual in catalyzing its reaction at an oil-water interface (Figure 18.6). 

Lipases (triacylglycerol hydrolases, EC 3.1.1.3) are enzymes that catalyze reactions such as hydrolysis, interesterification, esterification, alcoholysis, acidolysis, and aminolysis [1]. There is an increasing interest in the development of lipase applications to oleochemical transformations to obtain esters of long-chain fatty acids, as monoalkyl esters of fatty acids [2]. Utilization of lipase as a catalyst for the production of biodiesel, defined as a mixture of monoalkyl esters, is a clean technology due to its nontoxic and environmental fnendly nature, requiring mild operating conditions compared with chemical method [3]. 

FIGURE 13.1 Lipase-catalyzed reactions and production of MAG using the reactions. FA, fatty acid MAG, monoacylglycerol TAG, triacylglycerol FAEE, fatty acid ethyl ester EtOH, ethanol. 

Xu, X. 2000. Production of specific-structured triacylglycerols by lipase-catalyzed reactions a review, Eur. J. Lipid Sci. Technol, 102, 287-303. 

Contrary to these processes, enzyme-catalyzed reactions using lipases have proven to be an interesting alternative for industrial-scale biodiesel production in order to reduce production costs. Lipases (triacylglycerol acyl-hydrolases, E.C. 3.1.1.3.) are powerful tools that can catalyze not only hydrolysis but also various synthetic reactions including esterification and transesterification. 

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