Food Lipids: Chemistry, Nutrition, and Biotechnology

oxyanion-stabilizing residues. The nucleophilic serine rests between a � strand and
an � helix, whereas histidine and aspartic acid or glutamic acid rest on one side of
the serine (12).
The importance of the serine residue for the catalytic activity of lipase has been
demonstrated using site-directed mutagenesis. Substitution of Ser153 in human pancreatic
lipase produces a drastic decrease in the catalytic activity of the enzyme, but
has no effect on the ability of the enzyme to bind to micelles. As well, the presence
of a highly hydrophobic sequence of amino acid residues has been verified in the
vicinity of the active site, which is important in the interaction of the enzyme with
the interface (42). The chemical properties of the groups within the catalytic triad
are consistent with a hydrophobic environment (11). The process of opening the lid
covering the active site causes the oxyanion hole to move into proper positioning
for interaction with the substrate. For example, lipase for Rhizomucor miehei has a
serine side chain at position 82 that assumes a favorable conformation for oxyanion
interactions only after the lid has moved away from the active site (43). During
binding of the substrate with the enzyme, an ester binds in the active site, so that
the alcohol portion of the substrate rests on a floor formed by the end of the � strand
while the acyl chain arranges itself in the hydrophobic pocket and tunnel region (42)
(Fig. 7).
In lipase from Mucor miehei, the substrate binding region is seven carbons
long. When longer chains are encountered, the rest of the carbons in the chain hang
outside the hydrophobic tunnel (44). When the lipase approaches the interface and
the lid is folded back, an oxyanion-stabilizing residue is placed in proper orientation
(12). During hydrolysis the tetrahedral intermediate is stabilized by hydrogen bonds
with backbone amide groups of oxyanion-stabilizing residues. One stabilizing residue
Figure 7 Crystal structure and location of catalytic residues of the active site of Candida
rugosa lipase. (Adapted from Ref. 37.)
Copyright 2002 by Marcel Dekker, Inc. All Rights Reserved.