Lipase-catalyzed esterification

Lipases are versatile catalysts. In addition to their natural reaction of f at hydrolysis, lipases catalyze a plethora of other reactions such as ester ification, amidation, and transesterification of esters as well as organic carbonates. Moreover, lipases accept a wide variety of substrates while mai ntaining their regioselectivity and stereoselectivity. Lipases are highly s table even under adverse conditions such as organic solvents, high temperat ures, and so forth. Applications of lipases include production of food addi tives, chiral intermediates, and pharmaceutical products. Among these, synt hesis of various chiral intermediates in pharmaceutical industry and cocoa butter substitutes is being commercially exploited currently. Lipase-catalyzed esterification and transesterification in anhydrous media (e.g., organic solvents and supercritical fluids) has been an area of major research activity in the past decade or so,Absence of water eliminates the competing hydrolysis reaction. Moreover, substrate specificity, regioselec tivity, and stereoselectivity of the enzyme can be controlled by varying th e reaction medium. Although organic solvents, which are generally used for lipase-catalyzed reactions, are nearly anhydrous; they contain water in tra ce quantities. This water content can be controlled over a range and has a profound effect on the activity of lipases. Water not only affects the enzy me but also acts as a competing nucleophile. Enzyme activity has been corre lated with thermodynamic activity of water in the medium rather than with t he concentration of water. Because lipases are not soluble in most organic solvents, the method of preparation of the enzyme has a strong influence on the enzymatic activity. The major factors are the pH of the aqueous soluti on in which the enzyme last existed, additives used during preparation, and method of removal of water (e.g., freeze-drying, evaporation, extraction o f enzyme into solvent, etc.). Immobilization of lipases allows easy recover y and reuse of the enzyme. Various immobilization techniques have been stud ied for lipases and some of them have been shown to enhance the activity an d stability of the enzyme. Enzyme stability is an important parameter deter mining the commercial feasibility of the enzymatic process. Various factors , such as temperature, reaction medium, water concentration, as well as the method of:preparation, affect the stability of the lipases. This review deals with fundamental as well as practical aspects of lipase c atalysis. A discussion has been presented on various factors affecting lipa se activity and stability. Moreover, a brief account of current and potenti al applications of lipases has been given.