Biology Reference
In-Depth Information
and cocaine but it acts on numerous other poisons such as sarin,
soman and tabun as well (Bencharit
et al.,
2003).
The multiple substrates of an enzyme are often also endogenous
substrates produced by cell metabolism in prokaryotes and eukary-
otes. This subject has already undergone analytical review (D'Ari and
Casadesus, 1998). We shall only mention a few of the elements here
as examples. In some cases the different activities of an enzyme are
useful to the cell, for example isoleucine and valine are synthesised by
the same enzymes, and, similarly, four transaminases which have
crossed specificities catalyse the formation of seven amino acids. But
in other cases multiple reactions do not seem to be useful to the cell.
One example is the oxygenase activity of the enzyme rubisco, which
wastes oxygen during oxidative hydrolysis of ribulose diphosphate.
4.1.2
Non-specificity in the immune reaction
The antigen-antibody reaction was considered as the perfect model
of specific interaction, permitting the immune system to effectively
resist infection or contamination. But this dogma has now been
demolished. The antibodies produced against a particular antigen
exhibit cross-reactions with other antigens. This non-specificity is
due to the flexibility of the antibody binding domains (Manivel
et al
., 2000; Mundorff
et al.,
2000; Garcia
et al
., 1998). Cross-reac-
tions have been observed, among other things, between ovalbumin,
bovine gamma globulin and bovine serum albumin antibodies
(Sperling
et al
., 1983) and between butyrophilin and myelin anti-
bodies (Guggenmos
et al.,
2004). In the same way, antibodies
against either (4-hydroxy-3-nitrophenyl)acetyl, p-azophenylarson-
ate or a synthetic peptide react equally with a multitude of ligands
present in banks of random peptides
19
(Manivel
et al
., 2002).
19
These banks are obtained by randomly synthesizing a peptide of a given size.
Each amino acid can be one of the 20 possible. This technique generates an enor-
mous population of different molecules.
