due to the large number of possible combinations and the small differences in the
properties of the different molecular species. When completely homogeneous pre-
parations (identical molecules) of glycerophospholipids are needed, synthesis is a
more convenient route than is isolation from biological material. The synthesis
can be started either from simple building blocks, such as glycerol, fatty acids,
etc., or from a mixture of natural glycerophospholipids. In the latter case, the
part of the molecule which is identical with the target substance is retained (for
example the glycerol backbone with the chiral center), while the other parts are
exchanged. Another important type of lipid modification is the introduction of fatty
acids which are not present in the natural lipid. There is for example considerable
interest in the incorporation of polyunsaturated fatty acids from fish oils and other
sources into glycerophospholipids.
This chapter will concentrate on the enzymatic methods for glycerophospholipid
synthesis. In addition, a number of chemical methods are available, but these are
beyond the scope of this text (for a review, see Paltauf and Hermetter, 1994). In
some cases, it is beneficial to use a combination of chemical and enzymatic steps.
When enzymes are used to exchange fatty acids in glycerophospholipids, the ex-
change reaction can be carried out either as a two-step process, which is hydrolysis
14.2 Synthetic strategies 293
Figure 2. Two-step fatty acid exchange. The example shows the exchange of the fatty acid in thesn-1
position of glycerophospholipids using hydrolysis followed by re-esterification with a new fatty acid.
Both steps are catalyzed by a lipase. Normally, there is a separation step between the two biocatalytic
steps. E-OH is the lipase with the hydroxyl group of the active site serine.