Biology Reference
In-Depth Information
(
Marteyn, Domain, Legrain, Chauvat, & Cassier-Chauvat, 2009
). The GSH/
GSSG molecular ratio is about 200 in
E. coli
cells growing in the rich stan-
dard-medium LB (
Masip et al., 2006
), and even higher in plants (
Zaffagnini,
Bedhomme, Marchand et al., 2012
). Because the NAD(P)H/NAD(P) and
GSH/GSSG systems do not exchange electrons directly at any appreciable
rate, the two redox couples can be maintained within the cell at different
redox potentials as required for cellular processes (
Masip et al., 2006
).
Four unicellular cyanobacteria apparently lack a glutathione reductase
encoding gene, namely the two euryhaline strains
Synechococcus
PCC 7020
and
Synechocystis
PCC 6803, and the two
Synechococcus
strains isolated in
Yellowstone (
Table 5.2
). In the filamentous cyanobacterium
Anabaena
PCC
7120, the glutathione reductase gene (
all4968
in CyanoBase) is expressed
from two
E. coli
-like sigma70-type promoters, which are active alternatively
or in combination, depending on the nitrogen source (
Jiang, Hellman,
Sroga, Bergman, & Mannervik, 1995
). The
all4968
gene, which has been
cloned and expressed in a glutathione reductase-deficient
E. coli
strain, pro-
duce an enzyme carrying the GXGXXG fingerprint motif (amino acids
173-178) normally present in NADH-dependent enzymes, instead of the
GXGXXA motif occurring in NAD(P)H-dependent enzymes (
Danielson,
Jiang, Hansson, & Mannervik, 1999
;
Jiang et al., 1995
). It will be very inter-
esting to study and compare the pleiotropic roles of GSH in the responses
to oxidative and metal stresses in the cyanobacteria
Synechococcus
PCC 7942
and
Synechocystis
PCC 6803, which possess or lack the glutathione reductase
enzyme, respectively (
Marteyn et al., 2009
).
3. GLUTATHIONE OPERATES IN THE DEFENCE
AGAINST TOXIC ENDOGENOUS METABOLITES
3.1. Glutathione Operates in the Detoxification
of Methylglyoxal
Methylglyoxal (MG), also called pyruvaldehyde or 2-oxopropanal (CH
3
-
CO-CH
]
O or C
3
H
4
O
2
), is the aldehyde form of the pyruvic acid. Since
MG possesses two carbonyl groups, it is more active than glucose in protein
glycation (the nonenzymatic glycosylation), which is the result of, typi-
cally covalent, bonding of a sugar molecule with a protein (the free amino
groups of lysine and arginine and the thiol group of cysteine). The for-
mation and accumulation of the so-called advanced glycation endproducts
(AGEs), which impair protein functions, has been implicated in ageing and
the progression of age-related diseases such as diabetes, Parkinson's (
Lee
et al., 2012
) and Alzheimer's (
Xue, Rabbani, & Thornalley, 2011
) diseases.
