considered an acyl-esterase, a thio-esterase activity has also been demonstrated for
PLA 2 from porcine pancreas if compounds with an acyl-thioester bond in thesn- 2
position of glycerophospholipids were used as substrate (Yu and Dennis, 1991b). A
certain activity has also been reported towards nonphospholipid thioesters such as
palmitoyl-CoA or palmitoyl-acyl carrier protein (Nocito et al., 1996). Compounds
where thesn-2 ester has been replaced by an amide, a phosphonate, a difluoromethyl
ketone or a sulfonamide, respectively, act as inhibitors of PLA 2 (references in Gelb et
al., 1995). In addition, 2,3-diacyl-sn-glycero-3-phosphocholines (Bonsen et al.,
1972), fatty acid amides, phosphate diesters, substitutedN,N-bis-carboxymethyl an-
ilines and other compounds (references in Gelb et al., 1995) are inhibitors of PLA 2.
More information on the molecular structure and mechanism of PLA 2 s can be found
in recent reviews (Dennis, 1994; 1997; Scott and Sigler, 1994; Verheij and Dijkstra,
1994; Gelb et al., 1995; Roberts, 1996; Scott, 1997; Tischfield, 1997; Yuan and Tsai,
1999).
12.3.2 Phospholipase C
Bacterial PLCs are usually small metalloenzymes. PLC fromBacillus cereus(see
Table 1) is a monomeric extracellular enzyme containing 245 amino acid resi-
dues. Cloning and sequencing of its gene have revealed a 24-residue signal peptide
and a 14-residue propeptide (Johansen et al., 1988). The crystal structure (Hough et
al., 1989) indicates that 66 % of the amino acid residues form tena-helical regions
folded into a single, tightly packed domain, while the remaining amino acid residues
are present as loops (Figure 5). The enzyme contains three zinc ions forming a metal
12.3 Molecular structure and mechanism of phospholipases used as biocatalysts 227
Figure 5. Tertiary structure of PLC fromBacillus cereuswith three Zn2+ions. The structure was taken
from the Brookhaven Protein Data Bank, file 1AH7, and drawn by RasMol. (See color plate, page XIX).