Food Lipids: Chemistry, Nutrition, and Biotechnology

Figure 2 Three-dimensional representation of the extracellular lipase from the fungus
Rhizopus delemar showing the lid partially open (A) and closed (B) (97b). (Plates courtesy
of Dr. Michael Haas, U.S. Department of Agriculture.)
B. Interfacial Activation and the Hydrolytic Reaction
In 1958 Sarda and Desnuelle (101) showed that pancreatic lipase does not exhibit
normal Michaelis–Menten kinetics with respect to substrate concentration. Lipases
are inactive in aqueous media with the substrate present in its monomeric form, but
there is a sharp increase in activity when the substrate exceeds the critical micelle
concentration. The inactive enzyme must first adsorb to the surface of the bulk
substrate, which initiates interfacial activation (Fig. 3). Interfacial enzyme kinetics
in lipolysis have been reviewed by Verger and Haas (102a).
Solving the crystal structures of lipases has given insight into the mechanism
of interfacial activation (102b). It is believed that the preferred conformation in
aqueous solution is with the lid covering the active site, thus denying access to the
substrate. Adsorption of the lipase to the interface involves a conformational change
in the enzyme whereby the lid, which covers a cavity containing the active site and
is held in place by mostly hydrophobic and some hydrogen bonds, undergoes reorientation.
This is accompanied by additional conformational changes that expose
Copyright 2002 by Marcel Dekker, Inc. All Rights Reserved.