Food Lipids: Chemistry, Nutrition, and Biotechnology

in the oxyanion hole is the amino acid following the catalytic serine, whereas the
other one comes from a separate loop (37).
C. Activation by Interfaces
As previously stated, an advantage of enzyme-catalyzed interesterification in comparison
with chemical methods is that it can operate effectively under relatively mild
conditions. Enzyme-catalyzed reactions can increase the rate of a reaction by 10 6 –
10 15 times even at 25�C (41). The kinetics of lipase-catalyzed interesterification can
get complicated due to the many factors that can affect the reaction. Activation by
interfaces as well as participation of multiple substrates in the interesterification
reaction must all be considered when describing the action of lipases at interfaces.
The natural substrates of lipases, long chain triacylglycerols, are uncharged and
insoluble in water and as such form two phases in aqueous solutions. The property
of being active at lipid–water interfaces is unique to lipases. At low concentrations
of lipids, termed monomeric solutions, the lipids are dissolved in aqueous phase. The
maximal concentration of monomers in aqueous solution is the solubility limit or
critical micelle concentration, after which triacylglycerols form emulsions. For example,
the critical micelle concentration for triacetin in aqueous solution is 0.33 M,
whereas for long chain triacylglycerols, it can be as low as 1.0 �M (12,45). It has
been shown that lipases display almost no activity toward monomeric solutions of
lipids, whereas the lipids are dissolved and do not form interfaces. Once the level
of lipids exceeds the critical micellar concentration, the reaction rate increases dramatically,
by a factor of 10 3 –10 4 in some cases depending on the quality of the
interface (Fig. 8). Lipases have been found to act at several interfaces, including
emulsions, bilayers, and micelles (46). Action of lipases at the lipid–water interface
is believed to follow two successive equilibria involving penetration of lipase into
the interface, followed by the formation of the enzyme substrate complex (Fig. 9).
Initially, the enzyme penetrates the interface and undergoes a conformational
change, folding back the lid and thereby increasing the hydrophobic surface area of
Figure 8 Comparison of the effect of substrate concentration on lipase and esterase activity
at monomeric and saturation levels (beyond vertical dashed lines). (From Ref. 45.)
Copyright 2002 by Marcel Dekker, Inc. All Rights Reserved.