12.3.1 Phospholipase A 2
In contrast to intracellular PLA 2 s–the structural information on which is poor (re-
viewed in Ackermann and Dennis, 1995) – amino acid sequences of more than 150
PLA 2 enzymes of the extracellular type are known (reviewed in Danse et al., 1997).
PLA 2 s from pancreas and snake venoms are small proteins of high homology and
occur in multiple forms. All secretory PLA 2 s have molecular masses between 13 and
15 kDa, require Ca2+ions in the millimolar concentration range for optimum ac-
tivity and possess – apart from very few exceptions – seven disulfide bridges, this
probably being the reason for their high stability towards denaturing influences.
Reduction of the disulfide bridges results in loss of enzyme activity which can
be restored after reoxidation (Van Scharrenburg et al., 1980). Despite its remarkable
stability against denaturation, the structure of PLA 2 seems to be very sensitive to
point mutations, as revealed in a study of PLA 2 from bovine pancreas (Yuan et
al., 1999b). Based on their primary structures, PLA 2 s can be classified into four
groups (reviewed in Kini, 1997). For instance, group I and group II enzymes, differ
in the position of a disulfide bridge and in the length of the C-terminal chain. Mam-
malian pancreatic PLA 2 s belong to the group I consisting of 115–120 amino acid
residues and seven disulfide bridges. They are synthesized as zymogens and are
activated by proteolysis. Figure 3 shows the crystal structure of PLA 2 from bovine
pancreas, elucidated by Dijkstra et al. (1981), as an example. It contains three major
and two minora-helical segments (approximately 50 %a-helix content), a double-
stranded antiparallelb-sheet (approximately 30 %b-sheet content), and a calcium-
binding loop. The crystal structures of PLA 2 s from porcine pancreas (Dijkstra et al.,
12.3 Molecular structure and mechanism of phospholipases used as biocatalysts 225
Figure 3. Tertiary structure of PLA 2 from bovine pancreas with seven disulfide bridges and one Ca2+
ion. The structure was taken from the Brookhaven Protein Data Bank, file 1BP2, and drawn by RasMol.
(See color plate, page XIX).