Pancreatic lipase and colipase: an example of heterogeneous biocatalysis.

The hydrolytic reactions catalyzed by pancreatic lipase represent a good example of heterogeneous catalysis. The particularity of this enzyme is provided by its preferential action on emulsified substrates. The first step of catalysis resides in a reversible adsorption of the enzyme to the oil-water interface. In fact, the formation of this adsorption complex is an obligatory step for the enzyme to display its full activity. Two principal but not necessarily exclusive hypotheses have been proposed to explain the observed interfacial activation: Either the interface confers new properties on the substrate which allow its subsequent hydrolysis, or the enzyme itself is modified by adsorption at the interface. Different approaches have recently been developed to clarify this point further. The results obtained by chemical modifications of lipase are consistent with the following hypothesis. The active site preexists in solution and becomes fully functional only by interaction of the interface with an additional site on the enzyme molecule which can be tentatively called the "interfacial activation site." Finally, a protein of low molecular weight, colipase, seems necessary for lipase to express its activity under physiological conditions. This protein enters specific interactions with bile salts micelles and is responsible for the reversal of the inhibition of lipolysis brought about by these detergents.