In addition to their catalytic functions, GSTs (glutathione S-transferases) bind a wide variety of structurally diverse non-substrate ligands. This ligandin function is known to result in the inhibition of catalytic function. The interaction between hGSTA1-1 (human class Alpha GST with two type 1 subunits) and a non-substrate anionic ligand, BSP (bromosulphophthalein), was studied by isothermal titration calorimetry and inhibition kinetics. The binding isotherm is biphasic, best described by a set of two independent sites: a high-affinity site and a low-affinity site(s). The binding stoichiometries for these sites are 1 and 3 molecules of BSP respectively. BSP binds to the high-affinity site 80 times more tightly (Kd=0.12 μM) than it does to the low-affinity site(s) (Kd=9.1 μM). Binding at these sites is enthalpically and entropically favourable, with no linkage to protonation events. Temperature- and salt-dependent studies indicate the significance of hydrophobic interactions in the binding of BSP, and that the low-affinity site(s) displays low specificity towards the anion. Binding of BSP results in the release of ordered water molecules at these hydrophobic sites, which more than offsets unfavourable entropic changes during binding. BSP inhibition studies show that the binding of BSP to its high-affinity site does not inhibit hGSTA1-1. This site, located near Trp-20, may be related to the buffer-binding site observed in GSTP1-1. The low-affinity-binding site(s) for BSP is most probably located at or near the active site of hGSTA1-1. Binding to this site(s) results in non-competitive inhibition with respect to CDNB (1-chloro-2,4-dinitrobenzene) (KiBSP=16.8±1.9 μM). Given the properties of the H site and the relatively small size of the electrophilic substrate CDNB, it is plausible that the active site of the enzyme can simultaneously accommodate both BSP and CDNB. This would explain the non-competitive behaviour of certain inhibitors that bind the active site (e.g. BSP).
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Research Article|
August 24 2004
Characterization of bromosulphophthalein binding to human glutathione S-transferase A1-1: thermodynamics and inhibition kinetics
Doris KOLOBE;
Doris KOLOBE
1Protein Structure–Function Research Programme, School of Molecular and Cell Biology, University of the Witwatersrand, Johannesburg 2050, South Africa
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Yasien SAYED;
Yasien SAYED
1Protein Structure–Function Research Programme, School of Molecular and Cell Biology, University of the Witwatersrand, Johannesburg 2050, South Africa
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Heini W. DIRR
Heini W. DIRR
1
1Protein Structure–Function Research Programme, School of Molecular and Cell Biology, University of the Witwatersrand, Johannesburg 2050, South Africa
1To whom correspondence should be addressed (email heinid@gecko.biol.wits.ac.za).
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Publisher: Portland Press Ltd
Received:
January 08 2004
Revision Received:
May 12 2004
Accepted:
May 18 2004
Accepted Manuscript online:
May 18 2004
Online ISSN: 1470-8728
Print ISSN: 0264-6021
The Biochemical Society, London
2004
Biochem J (2004) 382 (2): 703–709.
Article history
Received:
January 08 2004
Revision Received:
May 12 2004
Accepted:
May 18 2004
Accepted Manuscript online:
May 18 2004
Citation
Doris KOLOBE, Yasien SAYED, Heini W. DIRR; Characterization of bromosulphophthalein binding to human glutathione S-transferase A1-1: thermodynamics and inhibition kinetics. Biochem J 1 September 2004; 382 (2): 703–709. doi: https://doi.org/10.1042/BJ20040056
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