| 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.BIOCHEMISTRY 220 645-661 (1994).
4. TAKAHASHI, Y., CAMBELL, E.A., HIRATA, Y., TAKAYAMA, T. AND LISTOWSKY, I.
A basis for differentiating among the multiple human mu-glutathione
S-transferases and molecular cloning of brain GSTM5.
J.BIOL.CHEM. 268(12) 8893-8 (1993).
5. HANSSON, L.O., BOLTON-GROB, R., MASSOUD, T. AND MANNERVIK, B.
Evolution of differential substrate specificities in mu class glutathione
transferases probed by DNA shuffling.
J.MOL.BIOL. 287 265-276 (1999).
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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 phenomenom 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].
At present, human mu-class GSTs can be subdivided into 5 isoforms based on
differing substrate specificities [4]. Mu-class GSTs are thought to be
involved in the detoxification of reactive oxygen species (cyclised
o-quinones) produced via oxidative metabolism of catecholamines. These
toxins are thought to be involved in neurological disorders of the
nigrostriatal and mesolimbic systems (Parkinsons and Schizophrenia,
respectively). Indeed, mu-class GSTs are expressed in the substantia nigra
and have preferential substrate specificity for the cyclised o-quinones
formed by catecholamine metabolism [5]. Mu-class GSTs possess the so-called
"mu-loop", which occurs between strand beta-2 and helix alpha-3. This is a
consequence of an insertion in the primary sequence and the loop allows the
overall domain I topology to remain [3].
GSTRNSFRASEM is a 4-element fingerprint that provides a signature for mu-
class glutathione S-transferases. The fingerprint was derived from an
initial alignment of 12 sequences: the motifs were drawn from conserved
regions spanning virtually the full alignment length - motif 1 includes the
C-terminal region of beta-strand 2 and the loop between strand 2 and
alpha-helix 3; motif 2 spans helix 2 and the following loop; motif 3
includes the N-terminal region of helix 5; and motif 4 includes the
C-terminal region of helix 6 and the following loop. Three iterations on
SPTR37_10f were required to reach convergence, at which point a true set
comprising 22 sequences was identified. Three partial matches were also
found: GTM1_DERPT and O16058 are mu-class glutathione transferases from
Dermatophagoides pteronyssinus (house-dust mite) and Echinococcus
granulosus, respectively, which match motifs 1 and 2; and Q27653 is
an unclassified GST that again matches motifs 1 and 2.
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