Front Matter

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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
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