tina sui
(Tina Sui)
#1
12.2.3 Phospholipase C
The primary interest in PLCs originates from their great importance in cell function
and signaling. Besides PLC with the preference for PC as substrate, the highly spe-
cific PI-PLC has attracted much attention (Rhee and Bae, 1997).Bacillus cereus
proved to be an appropriate organism from which to obtain both types of enzymes
with similar properties as those of mammals. The nonspecific PLC can be overex-
pressed inE. coli(Tan et al., 1997). Biocatalytic studies also preferably use PLCs
fromBacillus cereus(Table 1). In addition,Clostridium perfringens(Krug and Kent,
1984; Tso and Siebel, 1989),Achromobacter xylosoxidans(Kostadinova et al.,
1991),Pseudomonas fluorescens(Ivanov et al., 1996) and other microorganisms
(reviewed in Titball, 1993) have been used as producers of PLC.
12.2.4 Phospholipase D
In contrast to the phospholipases mentioned so far, PLD was used as potent bioca-
talyst (see Section 12.5), long before its importance in mammalian cell signaling was
realized. Its occurrence not only in plants such as carrots, cabbage, peanuts, castor
beans, cotton-seeds, but also in microorganisms and mammals has long been known
(reviewed in Heller, 1978; Dennis, 1983; Waite, 1987). At present, a vehement pro-
gress in knowledge on the physiological function and regulation of PLD in plants,
microorganisms and above all human tissues can be observed (reviewed in Daniel et
al., 1999; Exton, 1998; 1999; Frohmann et al., 1999; Houle and Bourgoin, 1999;
Jones et al., 1999; Liscovitch et al., 1999; Pappan and Wang, 1999; Rudge and En-
gebrecht, 1999; Venable and Obeid, 1999). In biocatalysis, however, the traditional
sources of cabbage andStreptomycesstrains have maintained their dominant position
to date (Table 1). The great number of studies on cloning, sequencing and expression
of PLDs from different species (see Section 1.3.3) undoubtedly will promote the
utilization of PLDs from other sources. The optimization of enzymes for biotech-
nological purposes by site-directed mutagenesis or directed evolution (Arnold, 1998;
Bornscheuer, 1998) is expected.
12.3 Molecular structure and mechanism of
phospholipases used as biocatalysts
The state of research into phospholipases interesting for biocatalyses is rather dif-
ferent. While the tertiary structures and catalytic mechanisms of PLA 2 from several
extracellular sources have been thoroughly investigated and the crystal structures of
two PLCs have also been elucidated, no tertiary structure of a PLD has been made
available so far.
224 12 Phospholipases Used in Lipid Transformations