Biology Reference
In-Depth Information
4. GLUTATHIONE PROTECTS CELLS AGAINST
CHEMICAL AND OXIDATIVE STRESSES VIA
THE GLUTATHIONE TRANSFERASE SYSTEM
Glutathione transferases (GSTs) constitute a protein superfamily that
plays a key role in the detoxification of chemical and oxidative stresses in aero-
bic prokaryotes and eukaryotes, but not in anaerobic bacteria or in Archaea
lacking GSH (
Allocati, Federici, Masulli, & Di Ilio, 2009
). The GST enzymes
catalyse nucleophilic attack by GSH on the electrophilic groups of a wide range
of hydrophobic toxic compounds, thus promoting their biodegradation and/
or excretion from the cell. All known crystallized cytosolic GSTs are dimeric
proteins, where each subunit is composed of two domains. The N-terminal
domain binds GSH, whereas the C-terminal part makes most contacts with
the electrophilic substrate. Eukaryotic and bacterial organisms have multiple
GST genes of widely divergent sequences and in some cases unknown func-
tion. Four different classes of canonical GSTs, beta, chi, theta and zeta, have
been identified in aerobic bacteria. In addition to the well-known beta-class
GST, characterized by the presence of a cysteine residue at the GSH-binding
site and a well-conserved overall structure,
E. coli
possesses six GST homo-
logues. Two of them, YfcF and YfcG, exhibit GST- and GSH-dependent per-
oxidase activities and are involved in the defence against oxidative stress.
Cyanobacteria might have been the first organism to harbour GSTs
(
Wiktelius & Stenberg, 2007
) because GSH and GSH-dependent enzymes
are regarded as dating back to the evolution of an oxygen-containing
atmosphere generated by cyanobacteria (
Schopf, 2011
). Consistently, all
known cyanobacterial genomes do contain various numbers (2-16) of
genes annotated as GSTs. The N
2
-fixing filamentous cyanobacteria and
the symbiotic
Acaryochloris
strain being the ones with the larger number of
GST genes (more than 10). Two cyanobacterial GSTs, originating from the
strains
Thermosynechococcus elongatus
BP-1 and
Synechococcus elongatus
PCC
6301, were studied and found to be belonged to the chi class of GSTs,
though they lack cysteines completely (
Wiktelius & Stenberg, 2007
).
5. GLUTATHIONE MAINTAINS THE REDOX
HOMEOSTASIS OF PROTEIN THIOLS VIA
THE GLUTAREDOXIN SYSTEM
ROS can generate two types of disulfides depending whether they
link two cysteinyl residues from the same or different proteins (protein-
S-S-protein) or from a protein and a GSH molecule (protein-S-SG mix
