Lipases, a group of enzymes which catalyze hydrolysis of insoluble oil droplets into soluble products, are ubiquitous in nature. Lipases were first identified in 1856 by Claude Bernard. They are found in organisms ranging from bacteria and fungi to plants and animals. These enzymes aid in processes such as digestion, membrane phospholipid metabolism, and inflammatory reactions. These hydrolytic reactions can be reversed under micro aqueous environment, such as in organic solvents, leading to esterification and transesterification. Lipase research has largely been focused on investigating the broad substrate specificity and regio-, chemo- and chiral-selectivity. Moreover, these enzymes are commercially important in industries such as detergent, oil and fat, baking, organic synthesis, paper and cocoa butter. Lipases are well known industrial biocatalyst due to their ability to carry out multitude of bioconversion reactions.
Lipases bind to the water-organic interface and catalyze hydrolysis at this interface. Most lipases are poor catalyst in the absence of an interface. The increase in lipase activity at the lipid-water interface suggests that lipases might undergo conformational change at the oil-water interface before substrate binding takes place. It has been observed that lipases usually are in ‘closed’ conformation where a lid or flap (a helical segment) blocks the active site. Upon binding to a hydrophobic interface such as lipid droplet, the lid opens and the catalytic activity of the lipase increases. This conformational change at the interface is supported by the X-ray structures of human, Mucor miehei lipases, phospholipase A29. Some lipases, for example, lipase from Pseudomonas aeruginosa and CAL-B, do not show interfacial activation even though they contain a lid.
Structural of lipases
The lipases from two fungi—Aspergillus niger and Geotrichum candidum were first to be crystallized. These crystals were unstable and of poor quality, likely a result of heterogeneity in the enzyme preparations. Since then, many lipases have been crystallized in a form suitable for high resolution X-ray diffraction studies.
Triacylglycerol lipases are α/β proteins, with a central β-sheet with the active site serine placed in a loop, in which the nucleophilic residue is essential for catalysis. This putative hydrolytic site is covered by a surface loop and is therefore, inaccessible to solvent. Interfacial activation, a property of lipolytic enzymes acting on water-insoluble substrates at water-lipid interfaces, probably involves the movement of this flap region in lipoprotein lipases. This movement changes the surface at the entrance of the active site, making it more hydrophobic and changing the lipid-binding properties.
The activities of these enzymes rely on a catalytic triad usually formed by three residues following the order Ser-Asp/Glu-His. The active site serine is found in the consensus pentapeptide Gly-X-Ser-X-Gly (X being any amino acid). This sole presence of this motif has identified many serine hydrolases. The active site serine is embedded in a secondary structure element: β-strand-turn--helix. It was found not only in lipases, but also in other hydrolytic enzymes.
The two glycine residues of the Gly-X-Ser-X-Gly consensus sequence are critical in maintaining the tight bend between the P-strand and the a-helix. These two residues face each other; the distance between their C atoms is very short. Not only should the Gly residue be conserved, but also right after this Gly, there is always a small amino acid such as alanine or another glycine. This keeps the outlook or nature of the β-strand and -helix unchanged. This conserved motif is seen in all known esterase structures: acetyl cholinesterase, cutinase and strepotomyces scabies esterase.
Industrial and Biological applications
Lipases have evolved to be efficient catalysts for lipolytic reactions concerning the hydrolysis of ester linkages of mono-, di- and triglycerides in aqueous emulsions. The hydrolysis of these bonds is important in industrial applications: generation of fatty acids from natural oils for the production of soaps; removal of oils and fats from fabrics, machinery, hides and waste water; production of mono- and diglycerides for food emulsifiers.
Lipases are also used to conduct transesterification reactions in commercial applications, such as the production of cocoa butter substitutes. Since lipases have the unique ability to act at oil-water interfaces, their structures are of particular interest. The selectivity of lipases towards the length of the fatty acids or the number and location of unsaturation in the fatty acids is employed to produce high-value fats or oils. The high stereoselectivity of lipases is also exploited to synthesize specific compounds--in particular, enantiomerically pure compounds containing ester bonds. For thermodynamic reasons, these synthesis reactions must be conducted in environments containing little water. There have been many reports of lipases in organic solvents synthesizing a variety of compounds, including precursors for biologically active therapeutics, herbicides, and pesticides.
Sources of lipases
Lipases are ubiquitous in nature and are found in multiple unicellular and multicellular organisms. However, yeast and fungi are one of the most important sources of lipases for industrial applications. For example, fungi produce various families of lipolytic enzymes including true lipases (EC 22.214.171.124), carboxylesterases (EC 126.96.36.199), secretory lipases (EC 188.8.131.52), and a variety of phospholipases, namely, phospholipase Al (EC 184.108.40.206), phospholipase A2 (EC 220.127.116.11), lysophospholipase (EC 18.104.22.168), phospholipase C or 1 -phosphatidylinositol-4, 5- bisphosphate phosphodiesterase 1 (EC 22.214.171.124), and phospholipase D (EC 126.96.36.199). The first three classes of enzymes belong to the structure-based superfamily of α/β-hydrolase’s, a variety of enzymes whose activities rely mainly on a catalytic triad usually formed by serine, histidine and aspartic acid residues.
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