Lipase, also known as glycerol ester hydrolase, is a carboxyl ester hydrolase that gradually hydrolyzes triglycerides to glycerol and fatty acids. Lipases are present in fat-containing tissues of animals, plants, and microorganisms (such as molds, bacteria, etc.). Lipases contain phosphatase, sterolase and carboxylesterase. Fatty acids are widely used in the industries of food, medicine, leather, daily-use chemicals and so on.
Sources
Lipases are widely found in animals, plants and microorganisms. In plants, seeds of oil crops contain more lipases, such as castor beans and rapeseed. When oil seeds germinate, lipases can work synergistically with other enzymes to catalyze the breakdown of oils and fats, generate sugar, and provide seeds nutrients and energy for germination. In animals, pancreas and adipose tissue contain more lipases. A small amount of lipase is exist in the intestinal fluid, which is used to supplement the deficiency of pancreatic lipase on the digestion of fat in meat animals. In the animals, various lipases control the processes of digestion, absorption, fat reconstitution and lipoprotein metabolism. The lipases in bacteria, fungi and yeasts are more abundant. Microbial-derived lipases are generally belongs to secreted extracellular enzymes. Due to the variety, rapid propagation, prone to genetic variation, wide pH range, wide temperature range, and substrate specificity, microorganisms are suitable for industrialized production and high-purity samples. Therefore, microbial lipases are the important sources of industrial lipases. The main fermentation microorganisms are Aspergillus niger and Candida. The characteristics of lipases from different sources are also different, and they have important implications in theoretical research.
Classification
The specificity of lipase on substrates can be divided into three categories: fatty acid specificity, position specificity, and stereospecificity. According to the different sources of lipases, lipases can also be divided into animal lipase, vegetable lipase and microbial lipase. Different sources of lipase can catalyze the same reaction, but when the reaction conditions are the same, the react rate and specificity of the enzymatic reaction are not the same.
Properties
Lipases are a class of enzymes with various catalytic abilities that can catalyze the hydrolysis, alcoholysis, esterification, transesterification of triacylglycerides and other water insoluble esters, and can catalyze the reverse synthesis of esters. It also shows the activity of other enzymes, such as phospholipase, lysophospholipase, cholesterol esterase, and acyl-peptidase. The different activities of lipase depends on the characteristics of the reaction system, such as ester hydrolysis in the oil-water interface, and enzymatic synthesis and transesterification in the organic phase.
The properties of lipases mainly include the optimum temperature and pH, temperature and pH stability, and substrate specificity. To date, a large number of microbial lipases have been isolated and purified, and their properties have been studied. They differ in molecular weight, optimum pH, optimum temperature, pH and thermal stability, isoelectric point and other biochemical properties. In general, microbial lipases have a wider action pH and temperature range, higher stability and activity, higher specificity for substrates than animal and plant lipases.
The catalytic properties of lipases are that their catalytic activity at the oil-water interface is the greatest. As early as 1958, Sarda and Desnnelv discovered this phenomenon. The water-soluble enzyme acts on the water-insoluble substrate and the reaction proceeds at the interface of two completely different phases that are separated from one another. This is a feature that distinguishes lipase from esterase. The substrate for esterase is water-soluble.
Catalytic Mechanism
Lipases have an affinity at the oil-water interface and can catalyze the hydrolysis of water-insoluble lipids at the oil-water interface at high rates. Different lipases from different sources may have large differences in amino acid sequence, but their tertiary structures are very similar. The residue of the active site of lipase consists of serine, aspartic acid, and histidine and belongs to serine proteases. The catalytic site of the lipase is buried in the molecule and the surface is covered by a screw cap structure formed by relatively hydrophobic amino acid residues (also called “lid”), which protects the triplet catalytic site. The amphipathic nature of the alpha-helix in the "lid" affects the ability of the lipase to bind to the substrate at the oil-water interface, and the diminished amphiphilicity will result in decreased lipase activity. The outer surface of the "lid" is relatively hydrophilic, while the inner surface facing the inside is relatively hydrophobic. Due to the association of the lipase with the oil-water interface, the “lid” is opened and the active site is exposed, so that the binding ability of the substrate to the lipase is enhanced, and the substrate easily enters the hydrophobic channel and binds to the active site. Interface activation can increase the hydrophobicity near the catalytic site, leading to α-helix reorientation, thereby exposing the catalytic site. The presence of the interface also allows the enzyme to form an incomplete hydration layer, which facilitates the folding of the aliphatic side chains to the surface of the enzyme molecule, making enzyme catalysis easy to perform.
Applications
Microbial-derived lipases can be used to enhance the flavor of cheese products. The limited hydrolysis of fat in milk can be used for the production of chocolate milk. Lipases can make foods generate special milk flavors. Lipases can prevent the flavor reversion of baked goods through the release of mono- and di-glycerides. Degreasing of bone during gelatin production needs to be carried out under mild conditions. Lipase-catalyzed hydrolysis can accelerate the degreasing process.
