| 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).
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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].
Alpha-class GSTs show substrate specificity for cumene hydroperoxide (CuOOH)
and 7-chloro-4-nitrobenz-2-oxa-1,3-diazole (NBD-C1), amongst others. In
addition, this class exhibits a number of differences from the
characteristic GST structure: within domain II, there is a short 3-residue
beta-strand near the C-terminal segment and a longer alpha-7 helix (due to
insertions at the N-terminus and near to the middle of this helix); domain I
is formed from two separate segments of the sequence. This occurs because
an extra helix (alpha-11) formed via folding of the C-terminal region of the
polypeptide chain is also part of this domain [3]. This helix covers the
substrate bound in the H subsite, which is thought to explain the preference
of alpha class GSTs for more hydrophobic compounds [1].
GSTRNSFRASEA is a 4-element fingerprint that provides a signature for alpha-
class glutathione S-transferases. The fingerprint was derived from an
initial alignment of 10 sequences: the motifs were drawn from conserved
regions spanning the full alignment length - motif 1 spans alpha-helix 1;
motif 2 includes the N-terminal region of helix 4 and the preceding loop;
motif 3 spans helix 7 and the N-terminal region of the following loop; and
motif 4 includes helix 11 and the C-terminal loop. Two iterations on
SPTR37_10f were required to reach convergence, at which point a true set
comprising 24 sequences was identified.
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