Food Lipids: Chemistry, Nutrition, and Biotechnology

Figure 9 Model for activation and action of lipases at interfaces.
the lipase making contact with the interface. The enzyme adsorbs to the interface
following a Langmuir adsorption isotherm. Once adsorption has taken place, the
enzyme is in its catalytically active form, meaning that interfacial activation has
taken place. The lipid substrate can then fit into the active site and be transformed
into product. The product is believed to be water-soluble and leave the interface
rapidly by diffusion into the surrounding solution. Several mechanisms have been
proposed to explain interfacial activation of lipases. The first theory relates interfacial
activation to a conformational change of the enzyme, where the lid moves to make
the active site available to substrate molecules at the interface. The second theory
points to changes in the concentration and organization of substrate molecules at the
interface to cause activation of the lipase. In the presence of a non-substrate lipid
interface, a lipase will not be active, but once the concentration of substrate in the
interface exceeds that of nonsubstrate lipids to become the continuous phase, lipase
activity increases. There are several other theories as to why lipase activity is increased
at an interface. One theory states that the higher substrate concentration at
the interface produces more frequent collisions between the lipase and substrate than
in monomeric solutions. Other theories involve decreased energy of activation induced
by substrate aggregation, reduced hydration of the substrate, and progressive
lipid-induced lipase aggregation at the interface (46).
In considering the action of lipases at interfaces, several factors have to be
considered, including the reversibility of adsorption, the possibility of inactivation,
and the quality of the interface. In general, lipases are considered to be reversibly
adsorbed at interfaces, since by increasing surface pressure, lipases have been found
to desorb from the interface (46). The quality of the interface can affect the activity
of lipases. Any factor that affects the affinity of the enzyme for the interface as well
as packing and orientation of the molecules at the interface can affect activity (11).
D. The Problem of Substrate Concentration
Since long chain triacylglycerols are insoluble in water and form aggregates, lipasecatalyzed
interesterification cannot be strictly governed by the Henri–Michaelis rule
relating the rate of the reaction to the molar concentration of substrate in solution.
In interesterification reactions, the insoluble substrate is in large excess as the continuous
solvent phase, making it difficult to define its concentration in the reaction
mixture. Since the substrate is insoluble, only the concentration of the substrate
present at the interface, which is available to the lipase, is considered. Lipase activity
is controlled by the concentration of micellar substrates at the interface and is independent
of the molar concentration of the substrate (47). In contrast, esterases in
acting only on water-soluble substrates, have a Michaelis–Menten dependence on
substate concentration (42,48). The dependence of lipase activity on the surface area
Copyright 2002 by Marcel Dekker, Inc. All Rights Reserved.