Food Lipids: Chemistry, Nutrition, and Biotechnology

Figure 12 The Ping-Pong Bi-Bi mechanism for lipase-catalyzed transesterification, with
the transfer of an acyl group from one triacylglycerol (TAG 1) to a diacylglycerol (DAG 2)to
form a new triacylglycerol (TAG 4).
1. Nonspecific Lipases
Certain lipases show no positional or fatty acid specificity during interesterification.
Interesterification with these lipases after extended reaction times gives complete
randomization of all fatty acids in all positions and gives the same products as
chemical interesterification (Fig. 13). Examples of nonspecific lipases include lipases
derived from Candida cylindraceae, Corynebacterium acnes, and Staphylococcus
aureus (9,10).
2. Positional Specificity
Positional specificity, i.e., specificity toward ester bonds in positions sn-1,3 of the
triacylglycerol, results from an inability of lipases to act on position sn-2 on the
triacylglycerol, due to steric hindrance (Fig. 14). Steric hindrance prevents the fatty
acid in position sn-2 from entering the active site (9,55). An interesterification reaction
using a 1,3-specific lipase will initially produce a mixture of triacylglycerols,
1,2- and 2,3-diacylglycerols, and free fatty acids (55). After prolonged reaction periods,
acyl migration can occur, with the formation of 1,3-diacylglycerols, which
allows some randomization of the fatty acids existing at the middle position of the
triacylglycerols. In comparison with chemical interesterification, 1,3-specific lipasecatalyzed
interesterification of oils with a high degree of unsaturation in the sn-2
position of the triacylglycerols will decrease the saturated to unsaturated fatty acid
level (56). Lipases that are 1,3-specific include those from Aspergillus niger, Mucor
miehei, Rhizopus arrhizus, and Rhizopus delemar (9). The specificity of individual
lipases can change due to microenvironmental effects on the reactivity of functional
groups or substrate molecules (57). For example, lipase from Pseudomonas fragi is
known to be 1,3-specific but has also produced random interesterification, possibly
due to a microemulsion environment. As of yet, lipases that are specific toward fatty
acids in the sn-2 position have been difficult to identify. Under aqueous conditions,
one such lipase from Candida parapsilosis hydrolyzes the sn-2 position more rapidly
than either of the sn-1 and sn-3 positions, and is also specific toward long chain
polyunsaturated fatty acids (58).
The differences in the nutrition of chemically interesterified fats and oils compared
to enzymatically interesterified samples can be linked to the positional specificity
exhibited by some lipases. In fish oils and some vegetables oils that contain
high degrees of essential PUFAs, these fatty acids are usually found in greater quantities
in the sn-2 position. In the intestines, 2-monoacylglycerols are more easily
absorbed than sn-1 or sn-3 monoacylglycerols. Using a 1,3-specific lipase, the fatty
Copyright 2002 by Marcel Dekker, Inc. All Rights Reserved.