Biology Reference
In-Depth Information
by its four aldo-keto reductase (AKR) enzymes named YafB, YqhE, YeaE,
andYghZ, which are dispensable for
E. coli
growth of
Ko et al. (2005)
. These
AKR enzymes belong to a large superfamily of NADPH-dependent oxi-
doreductases that also occur in eukaryotes, including humans (
Mindnich &
Penning, 2009
) and plants (
Turoczy et al., 2011
).
All cyanobacteria, except
Synechococcus JA-2-3B'a(2-13)
, have at least one
presumptive pathway for MG detoxification, mostly the glyoxalase enzymes
(
Table 5.3
), which are dispensable for the normal growth of
Synechocystis
PCC 6803 (Narainsamy, Chauvat and Cassier-Chauvat unpublished results).
In addition, many strains also have AKR enzymes (
Table 5.3
). In
Synechococcus
PCC 7002, a rare strain capable of growing heterotrophically at the expense
of glycerol, one of the four presumptive AKR enzymes (A1474) was found
to be a genuine, but dispensable, AKR enzyme. The A1474-deleted mutant is
sensitive to glycerol, which triggers the accumulation of MG (
Xu, Liu, Guo,
& Zhao, 2006
).
3.2. Glutathione Operates in the Detoxification
of Formaldehyde
Formaldehyde is extremely reactive in producing covalent cross-linked
complexes with proteins and nucleic acids. It is not only a common envi-
ronmental pollutant but also an endogenous compound present in all living
organisms as the result of the catabolism of methionine, methanol and
glyoxylate; or the oxidative demethylation of DNA and RNA (
Gonzalez
et al., 2006
) and references therein. The GSH-dependent defence system
found in most prokaryotes and all eukaryotes (
Fig. 5.1
) proceeds as follows.
Formaldehyde spontaneously reacts with GSH to produce S-hydroxymeth-
ylglutathione, which is oxidized by formaldehyde dehydrogenase (Fdh1) to
S-formylglutathione, which is hydrolysed by S-formylglutathione hydro-
lase (Fdh2) to generate formate and GSH. The Fdh2 enzymes from yeasts
and
Arabidopsis thaliana
are not strictly specific to S-formylglutathione, in
showing significant carboxylesterase activity against the model substrates
α-naphthyl acetate and p-nitrophenyl acetate. It has also been proposed that
Fdh2 are cysteinyl hydrolases because the
A. thaliana
enzyme is highly sensi-
tive to the inhibition by
N
-ethylmaleimide.
The two functionally related Fdh1 and Fdh2 enzymes also show a
genetic linkage since their genes are adjacent in the genomes of many bac-
teria and some cyanobacteria (
Table 5.3
) or even fused in some eukaryotes.
Also interestingly, these genes are absent in the marine Prochlorococcales
cyanobacteria (
Table 5.3
).
