Biomedical Engineering Reference
In-Depth Information
hydrolytic enzymes can utilize such 'unusual' substrates as alcohols, amines and tiols
with the formation of corresponding products. In such conditions syntheses of esters
from acids and alcohols becomes thermodynamically allowed. Drastic changes have also
been observed in enantiomeric, prochiral, regio- and chemoselectivities.
Here we confine ourselves to a few typical examples of enzymatic systems in organic
solvents (Klibanov, 2001; and references therein). is stable in unhydrous
conditions for several hours at 100°C. The hydrophilic peptide substrate
is transformed in organic solvent three times faster than hydrophobic substrate, while the
latter in water is found to be non-reactive In water solution the dominant product of the
conversion of prochyral 2-(3,5-dimetoxybenzyl)1,3-propandiol by this enzyme is the S-
monoester, whereas in acetonitril R-enantiomer is formed.
The activity of enzymes in organic solvents is often dramatically low compared to
that in water. This limitation can be largely overcome by crown ether treatment of
enzymes. The marked activation (from 333 to 2480-fold) of subtilisin Carsberg in
ethanol and acetone in the presence of salts (sodium iodide and sodium acetate) has been
observed (Ru et al., 2000). Combination of co-immobilization of penicillin G acylase
with polyethyleneimine and its chemical modification by polyaldehyde dextran allowed
to increase of the enzyme activity in organic solvents (Fernandez-Lafuente et al., 1998).
It was shown that activity of enzymes in organic solvents is greatly increased by crown
ether treatment of enzymes. The complexation of 18-crown-6 with lysine ammonium
groups of enzymes leads to violation of inter- and intra molecular salt bridges and,
consequently, to improving thermodynamical and catalytical properties of the enzymes
in new conditions (Van Unen et al., 2002)
5.3. Enzymes in synthetic chemistry
Isolation and investigation of over 3000 enzymes have established a powerful basis for
synthesizing of myriad chemical compounds. The number of catalytic chemical
processes can be infinitely expanded by the use genetic engineering, chemical
modification, and a variety of media. A large body of publications exists on this subject
(see for example Silversman, 2000; Dordick, 1991); Fersht, 1999; Jones, 1989; Drauz
and Waldmann, 1995; Tramper,. (1996); Faber, 1997; Roberts, 1999; Adam et al., 1999;
Klibanov, 2001; Koeller and Wong, 2001; Walsh, 2001; Arnold, 2001; and references
therein). Recently it was shown that RNA and DNA possess catalytic activity as well
(Narlikar and Hershlag, 1997; Sheppard et al., 2000). This Section is restricted with a
brief over review on the use enzymes in synthetic chemistry and considering of several
specific examples.
Among enzymes commonly used in organic synthesis in research laboratories, and
pharmaceutical and biothechnological industry are the following: esterases (including
lipases), amidases, proteases and acylases, dehydrogenases, mono-and dioxidases,
peroxidases, kinases, aldolases, glycosidases, phosphorylases, phosphotases,
transaminases, hydrolases, and isomerases, lyases, hydrases and sulphotransferases.
Enzymes are also effective tools for protecting amino, tiol, carboxyl, and hydroxyl
groups (Kadereit and Waldmann 2001). The growing application of biocatalysis takes
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