A lipase is a water-soluble enzyme that catalyzes the hydrolysis of ester bonds in water–insoluble, lipid substrates. Lipases thus comprise a subclass of the esterases.

Lipases perform essential roles in the digestion, transport and processing of dietary lipids (e.g. triglycerides, fats, oils) in most- if not all- living organisms. Genes encoding lipases are even present in certain viruses.

Function

Most lipases act at a specific position on the glycerol backbone of a lipid substrate (A₁, A₂ or A₃).

In the example of human pancreatic lipase (HPL), which is the main enzyme responsible for breaking down fats in the human digestive system, a lipase acts to convert triglyceride substrates found in oils from food to monoglycerides and free fatty acids.

Myriad of other lipase activities exist in nature, especially when the phospholipases and sphingomyelinases are considered.

Structure

While a diverse array of genetically distinct lipase enzymes are found in nature, and represent several types of protein folds and catalytic mechanisms, most are built on an alpha/beta hydrolase fold (see image) and employ a chymotrypsin-like hydrolysis mechanism involving a serine nucleophile, an acid residue (usually aspartic acid), and a histidine.

Physiological distribution

Lipases are involved in diverse biological processes ranging from routine metabolism of dietary triglycerides to cell signaling and inflammation. Thus, some lipase activities are confined to specific compartments within cells while others work in extracellular spaces.

• In the example of lysosomal lipase, the enzyme is confined within an organelle called the lysosome.

• Other lipase enzymes, such as pancreatic lipases, are secreted into extracellular spaces where they serve to process dietary lipids into more simple forms that can be more easily absorbed and transported throughout the body.

• Fungi and bacteria may secrete lipases to facilitate nutrient absorption from the external medium (or in examples of pathogenic microbes, to promote invasion of a new host).

• Certain wasp and bee venoms contain phospholipases that enhance the "biological payload" of injury and inflammation delivered by a sting.

• As biological membranes are integral to living cells and are largely composed of phospholipids, lipases play important roles in cell biology.

Lipases of humans

The main lipases of the human digestive system are human pancreatic lipase (HPL) and pancreatic lipase related protein 2 (PLRP2), which are secreted by the pancreas. Humans also have several other related enzymes, including hepatic lipase (HL), endothelial lipase, and lipoprotein lipase. Not all of these lipases function in the gut (see table).