followed by esterification (Figure 2), or as a one-step process of transesterification
(Figure 3). In fact, the two approaches involve the same steps in the reaction se-
quence and provide the same net reaction (Figures 2 and 3). Division of the ex-
change reaction into two steps makes the process more laborious to perform. How-
ever, the moiety hydrolyzed off in the first step can be effectively removed in a
separation step and thus does not compete with the moiety to be incorporated;
hence, purer products can be obtained.
It should be noted that in the one-step approach, acyl donors other than free acids
can also be used, for example esters.
Before going into detail about different synthetic possibilities, we will discuss
spontaneous acyl migration in lipids. It is important to be aware of this reaction
as it is involved in most of the synthetic applications discussed later in the chapter.
14.3 Acyl migration
One of the main advantages of enzymes as catalysts is their selectivity. The enzymes
shown in Figure 1 are selective to act in the positions of the molecule indicated by the
arrows. In order fully to take advantage of this selectivity it is important that ex-
change reactions between the different positions do not occur in an uncontrolled
manner.
294 14 Enzymatic Conversions of Glycerophospholipids
Figure 3. One-step fatty acid exchange. The example shows the exchange of the fatty acid in thesn-1
position of glycerophospholipids using an acidolysis reaction. E-OH is the lipase with the hydroxyl group
of the active site serine.