with the hydrolysis of the substrate molecules to PA. Furthermore, the product ob-
tained by the transfer reaction principally is also a suitable substrate for PLD, so that
the product formation is kinetically controlled. To summarize, the catalytic potential
of phospholipases allows several different strategies for the synthesis of new phos-
pholipids or phospholipid mixtures with altered composition (Table 1; Figure 1).
Starting from the inexpensive crude lecithin, PLA 2 is used to manufacture lyso-phos-
pholipids on an industrial scale. In the presence of PLD, either PA or phospholipids
with modified head groups are obtained. PLA 1 and PLA 2 convert natural PC to gly-
cerophosphorylcholine, from which PCs with defined acyl residues are prepared by
chemical reacylation. Further treatment with PLD in the presence of an alcohol ac-
ceptor again considerably enhances the number of interesting and high-value com-
pounds available, due to the wide substrate specificity of PLD. Examples have re-
cently been published by Ulbrich-Hofman et al. (1998), Hirche et al. (1997) and
Pisch et al. (1997).
Thereaction behaviorof phospholipases and their catalysis mechanisms – as
understood so far – is quite complex. Depending on their concentration, phospho-
lipids form different types of aggregates in water. One consequence is an uncertainty
with regard to the full substrate specificity of phospholipases. Furthermore, the ki-
netics of these enzymes is markedly affected by the aggregation state. Normally, the
rate of phospholipase-catalyzed reactions decreases significantly when the substrate
concentration falls below the critical micellar concentration (CMC) value where the
phospholipid molecules exist in the reaction mixture as monomers (Pieterson et al.,
1974). A model for the binding mechanism of PLA 2 to micellar structures (with the
consequence of interfacial activation) and the chemistry of catalysis was developed
by Scott et al. (1991) on the basis of results from high-resolution X-ray structures of
PLA 2 /inhibitor complexes (Chinese cobraNaja najavenom and bee-venom PLA 2 ).
The catalytic activity of many phospholipases increases after the addition of sur-
factants such as sodium dodecylsulfate (SDS) or Triton X-100 to the reaction mix-
ture, as these form mixed micelles with the substrate, though this may not be valid for
all enzymes. In the case of PLA 1 for example, Triton X-100 enhances the activity
towards all substrates with the exception of PE, which clearly forms mixed micelles
whose physical structure has an inhibitory effect. Most phospholipases require metal
ions (mainly Ca2+) in order to develop activity, but again differences exist with
respect to the amount and type of this cofactor, according to the enzyme source.
13.2 Phospholipids and phospholipases 265
Table 1.Some examples for well-known, current applications of native and immobilized phospholipases.
Enzyme Reaction Product/properties
Phospholipase A 2 Hydrolysis Lysophospholipids/enhanced
emulsifying capacity
Phospholipase D Hydrolysis
TransesterificationPhosphatidic acid/masking taste of
bitter food ingredients
Products such as PG, PS/enhanced
emulsion capacity and temperature
stability, therapeutic applications
(PS)