Food Lipids: Chemistry, Nutrition, and Biotechnology

T c than animal fats. The T c for vegetable oils ranges from 5�C to10�C, whereas it
is between 30�C and 46�C for animal fats. By reducing the temperature below T c,
yields of monoacylglycerols can be increased from 30% up to yields as high as 90%
(35,36). Water content can also have an effect on glycerolysis since the reaction is
an esterification. McNeill et al. (36) found that increasing the water content from
0.5% to 5.7% increased the production of monoacylglycerols, whereas higher levels
of water did not increase the rate of reaction further. The main problem with lipasecatalyzed
glycerolysis is the long reaction time in the order of 4–5 days required to
produce high yields (36).
II. LIPASES
A. Three-Dimensional Structure
While lipases can be derived from animal, bacterial, and fungal sources, they all
tend to have similar three-dimensional structures. In the period from 1990 to 1995,
crystallographers solved the high resolution structures of 11 different lipases and
esterases including 4 fungal lipases, 1 bacterial lipase, and human pancreatic lipase
(12). Comparison of the amino acid sequences has shown large differences between
most lipases, yet all have been found to fold in similar ways and have similar catalytic
sites. The characteristic patterns found in all lipases studied so far have included
�/� structures with a mixed central � sheet containing the catalytic residues.
In general, a lipase is a polypeptide chain folded into two domains: the C-terminal
domain and the N-terminal domain. The N-terminal domain contains the active site
with a hydrophobic tunnel from the catalytic serine to the surface that can accommodate
a long fatty acid chain.
In solution, a helical segment covers the active site of lipase, but in the presence
of lipids or organic solvent, there is a conformational change in which the lid opens,
exposing the hydrophobic core containing the active site. The structure of the lid
differs for lipases in the number and position of the surface loops. For example,
human pancreatic lipase has one � helix (residues 237–261) in the loop covering
the active site pocket (37,38). The fact that the � helix in the lid is amphipathic is
very important in terms of the ability of the lipase to bind to lipid at the interface.
If the amphiphilic properties of the loop are reduced, the activity of the enzyme is
decreased (39). The outside of the loop is relatively hydrophilic whereas the side
facing the catalytic site is hydrophobic. Upon association with the interface, the lid
folds back, revealing its hydrophobic side which leads to increased interactions with
the lipid at the interface (40). The substrate can then enter the hydrophobic tunnel
containing the active site.
B. The Active Site
Koshland’s modern induced fit hypothesis states that the active site does not have
to be a preexisting rigid cavity but instead can be a precise spacial arrangement of
several amino acid residues that are held in the correct orientation by the other amino
acids in the enzyme molecule (41). The main component of the catalytic site is an
�/�-hydrolase fold that contains a core of predominantly parallel � sheets surrounded
by � helices. The folding determines the positioning of the catalytic triad composed
of serine, histidine, and either glutamic acid or aspartic acid along with several
Copyright 2002 by Marcel Dekker, Inc. All Rights Reserved.