Front Matter

Besides the catalytic motifs, PLDs show some other typical structural features.

PLDs from plants possess a C2 domain in theN-terminal region (Ponting and Par-

ker, 1996; reviewed in Rizo and Su ̈dhof, 1998). This domain occurs in more than 100

proteins, as well as in PLCs and protein kinases C. In some of these proteins the C2

domain possesses Ca2+-binding properties and mediates phospholipid binding. The

C2 domain comprises approximately 130 residues forming structures of an eight-

stranded antiparallelb-sandwich (reviewed in Nalefski and Falke, 1996). A part

of the C 2 domains assigned to theb3- andb5-sheets of selected PLDs is shown

in Figure 6. A phosphoinositide binding motif mediating activation of these en-

zymes has been identified in some mammalian and yeast PLD isoenzymes (Sciorra

et al., 1999).

12.4 Kinetic particularities of phospholipases and their

consequences in lipid transformation

Although a number of membrane-bound phospholipases exist, those enzymes com-

monly used in biocatalysis are water-soluble. Their substrates, however, are mostly

water-insoluble and tend to form supramolecular structures.

12.4.1 Suprastructures of phospholipids

Above their critical micellar concentration (cmc), which is in the range of 10–10M,

phospholipids aggregate into a number of polymorphic phases (reviewed in Walde et

al., 1990; Chopineau et al., 1998). Some of the most important aggregates are shown

schematically in Figure 8. The monolayer is formed at the water – air interface, while

micelles, bilayers, uni- or multilamellar vesicles (liposomes) and hexagonal HII-

phases arise in the aqueous phase. The type of suprastructure arising depends on

the physico-chemical structure of the phospholipid, the medium including ions

and additional surfactants, as well as the conditions of preparation (e.g., ultrasonic

treatment). An important parameter for the type of aggregation is the ratio between

polar and nonpolar moieties in the phospholipid molecule. Cylindrically shaped

molecules (Figure 9A) such as in PC tend to form bilayers, while cone-shaped mo-

lecules with a dominating hydrophilic moiety (Figure 9B) (such as lyso-PC) form

micelles, and molecules shaped like an inverted truncated cone and with a dominat-

ing hydrophobic region (such as phosphatidylethanolamine) (Figure 9C) prefer to

form hexagonal structures. In organic solvents, reverse micelles can be produced

(Figure 8) which, in their interior, are able to host water and hydrophilic compounds

such as salts and even proteins. The phospholipid aggregates change as their sur-

roundings are modified. In particular, additional surfactants such as SDS or Triton

X-100 belong to effective solubilizers of phospholipids (reviewed in Lichtenberg et

al., 1983; Lasch, 1995). They promote the formation of micellar structures, where

they are incorporated to give mixed micelles (or mixed reverse micelles). In addition

to these lyotropic phase transitions, which are mediated by the medium, phospho-

12.4 Kinetic particularities of phospholipases and their consequences 233