| Literature References | 1. ALLARDYCE, C.S., MCDONAGH, P.D., LIAN, L-Y., WOLF, R. AND ROBERTS, G.C.K.
The role of tyrosine-9 and the C-terminal helix in the catalytic mechanism
of alpha-class glutathione S-transferases.
BIOCHEM.J. 343 525-531 (1999).
2. NUCCETELLI, M.N., MAZZETTI, A.P., ROSSJOHN, J., PARKER, M.W., BOARD, P.,
CACCURI, A.M., FEDERICI, G., RICCI, G. AND LO BELLO, M.
Shifting substrate specificity of human glutathione transferase (from class
pi to class alpha) by a single point mutation.
BIOCHEM.BIOPHYS.RES.COMMUN. 252(1) 184-189 (1998).
3. DIRR, H., REINEMER, P. AND HUBER, R.
X-ray crystal structures of cytosolic glutathione S-transferases.
Implications for protein architecture, substrate recognition and catalytic
function.
EUR.J.BIOCHEM. 220 645-661 (1994).
4. BOARD, P.G., COGGAN, M., CHELVANAYAGAM, G., EASTEAL, S., JERMIIN, L.S.,
SCHULTE, G.K., DANLEY, D.E., HOTH, L.R., GRIFFOR, M.C., KAMATH, A.V.,
ROSNER, M.H., CHRUNYK, B.A., PERREGAUX, D.E., GABEL, C.A., GEOGHEGAN, K.F.
AND PANDIT, J.
Identification, characterization and crystal structure of the omega class
glutathione transferases.
J.BIOL.CHEM. 275(32) 24798-24806 (2000).
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| Documentation | Glutathione S-transferases (GSTs) are a range of dimeric proteins that
catalyse the conjugation of glutathione to a wide range of hydrophobic
compounds through the formation of a thioether bond with their
electrophilic centre. Based on amino acid sequence identity, there are at
least seven major classes of GST (designated alpha, kappa, mu, pi, sigma,
theta and zeta). Pi-, mu-, alpha- and theta-class crystal structures have
been elucidated; all possess a similar GSH-binding site (G subsite), but
the hydrophobic substrate-binding site (H subsite) is subject to variation
across the classes [1]. Whilst most of the GSTs share common substrates,
there are distinct differences in substrate preference between subfamilies.
Sequence similarity between classes is rather low, ranging between 20-30%.
However, a single point mutation in the H-subsite region is enough to shift
substrate specificity from class pi to alpha [2].
These enzymes have evolved as a cellular protection system against a range
of xenobiotics, oxidative metabolism by-products, and in particular are
known to metabolise a number of environmental carcinogens. The wide range
of GST isoforms present in the various subfamilies provides cells with an
efficient way of scavenging the huge number of potentially toxic compounds
encountered. Genetic differences in GST expression have been implicated in
individual susceptibility to certain types of cancer. Conversely, over-
expression of GSTs is thought to be involved in the phenomenon of multi-drug
resistance to cancer chemotherapy.
In spite of relatively low sequence identity, the GSTs exhibit a high degree
of structural similarity. The structure comprises 2 domains: domain I is the
smaller of the two and is formed from the N-terminal region of the sequence
- it possesses an alpha/beta-type core structure comprising a central
4-stranded beta-pleated sheet, flanked on one side by two alpha-helices and
on the other by a single helix; domain II is the larger of the domains and
occurs towards the C-terminal region of the sequence - it contains a
predominantly all-alpha-type core comprising 5 amphipathic alpha-helices,
arranged in a right-handed spiral. The active site is situated near the
subunit interface. G-subsite molecular recognition is attributable mostly
to residues in domain I of one subunit and 1 or 2 residues in domain II of
the other subunit. Residues contributing to H-subsite specificity are found
within domains I and II of the same subunit [3].
Recently, a new class of GST has been discovered by analysis of expressed
sequence tag databases, termed the omega-class. Recombinant human omega-
class GST shows glutathione-dependent thiol transferase and dehydroascorbate
reduction activity. This sort of activity has not been observed in any other
class of GSTs, but is associated with the glutaredoxins (thioltransferases).
Members of this class of GST have a novel unique N-terminal extension, and a
cysteine residue in the active site, which is different from the tyrosine
and serine residues found at the active sites of other eukaryotic GSTs [4].
GSTRNSFRASEO is a 3-element fingerprint that provides a signature for
omega-class glutathione S-transferases. The fingerprint was derived from an
initial alignment of 4 sequences: the motifs were drawn from conserved
regions spanning virtually the full alignment length - motif 1 includes
strand 1; motif 2 encompasses the loop between helices 3 and 4, and the
N-terminal portion of helix 4; and motif 3 encodes the loop between helices
6 and 7, and the N-terminal half of helix 7. Two iterations on SPTR40_18f
were required to reach convergence, at which point a true set comprising 15
sequences was identified. A single partial match was also found, YKJ3_CAEEL,
a C.elegans chromosome III hypothetical protein that exhibits a high degree
of similarity to omega-class GSTs, but fails to make a significant match
with motif 1.
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