Free fatty acids lipase-catalyzed esterification

Surprisingly, esterification of fatty acids with simple sugars, such as glucose and mannitol, in AOT-based microemulsions did not take place at all [82]. No reaction was seen with either of two different lipases. This is probably due to poor phase contact between the very hydrophilic sugar molecule in the water pool and the fatty acid that resides in the hydrocarbon domain. Sugar monoesters can be produced in high yields by lipase-catalyzed esterification in a water-free medium [90]. 

Lipase-catalyzed esterification of free fatty acids can also be used for the deacidification of hyperacid vegetable oils. This is useful in the refining of oils to minimize the production of soaps leading to stable emulsions (53). 

Based on the substrates involved in the lipase-catalyzed reactions, they can be classified into different categories esterification, hydrolysis, acidolysis, alcoholysis and interesterification (1). Direct esterification reaction may be enqjloyed for the preparation of stmctured lipids by reacting free fatty acids with glycerol. However, this process is not commonly used in stmctured lipid production. The major problem is that the water molecules are formed as a result of the esterification reaction. The water molecules so produced need to be removed in order to prevent the hydrolysis of the product. Hydrolysis is the... 

Properties such as large interfacial area and an ability to solubilize both oil-soluble and water-soluble reactants in a single phase system makes microemulsions ideal as reaction media (Flanagan and Singh, 2006 Gaonkar and Bagwe, 2002). For example, Morgado and co-workers (1996) nsed a continnons reversed micellar system to synthesize lysophospholipids and free fatty acids from lecithin hydrolysis, with applications to the food, pharmaceutical and chemical industries. Hydrolysis was catalyzed by porcine pancreatic phospholipase A. Carvalho and Cabral (2000) reviewed the use of reversed micellar systems as reactors to carry out lipase-catalyzed esterification, biocatalysis, transesterificadon, and hydrolysis reactions. 

Lipase is known to catalyze esterification through an acyl-intermediate formed between the fatty acid substrate and the enzyme. Free enzyme can bind fatty acid to produce either this intermediate or the ester product. With a high concentration of alcohol, the acyl-intermediate will be consumed, and the enzyme may then start to bind product and catalyze its hydrolysis, thereby reversing the reaction. When present in an excess of fatty acid, however, most of the enzyme is found in the acylated form, preventing it from binding the product (15,16). 

LDL binds specifically to lipoprotein receptors on the cell surface. The resulting complexes become clustered in regions of the plasma membrane called coated pits. Endocytosis follows (see Fig. 13-3). The clathrin coat dissociates from the endocytic vesicles, which may recycle the receptors to the plasma membrane or fuse with lysosomes. The lysosomal proteases and lipases then catalyze the hydrolysis of the LDL-receptor complexes the protein is degraded completely to amino acids, and cholesteryl esters are hydrolyzed to free cholesterol and fatty acid. New LDL receptors are synthesized on the endoplasmic reticulum (ER) membrane and are subsequently reintroduced into the plasma membrane. The cholesterol is incorporated in small amounts into the endoplasmic reticulum membrane or may be stored after esterification as cholesteryl ester in the cytosol this occurs if the supply of cholesterol exceeds its utilization in membranes. Normally, only very small amounts of cholesteryl ester reside inside cells, and the majority of the free cholesterol is in the plasma membrane. 

Non-specific esterification of wood sterols can be performed chemically (www. freshpatents.com/Phytosterol-esterification-product-and-method-of-make-same-dt-20070628ptan20070148311.php) however, enzymatic esterification with lipases has the potential advantages of higher specificity and mild reaction conditions which are desirable, both from process and environmental perspectives. More than 20 lipases were previously screened for their ability to catalyze the transesterification of wood sterols and fatty acid esters (Martinez et al. 2004). The goal was now to screen among them those specific for stanol esterification, so as to obtain a product consisting in mostly esterified stanols and mostly free sterols (see Fig. 6.3.4) amenable for separation through short-path distillation. 

There are various reports in the literature concerning kinetic studies of the Upase-catalyzed hydrolysis or synthesis of esters in microemulsions [8,9,49,83,84]. A simple MichaeUs-Menten kinetic model was proposed for the hydrolysis of triglycerides [85,86], while the esterifications of aliphatic alcohols with fatty acids follow a ping-pong bi-bi mechanism [87]. According to this mechanism the lipase reacts with the fatty acid to form a noncovalent enzyme-fatty acid complex, which is then transformed to an acyl-enzyme intermediate, while water, the first product, is released this is followed by a nucleophile attack (by the alcoholic substrate) to form another binary complex that finally yields the ester and the free enzyme. The kinetic parameters and determined in these studies represent apparent... 

---