C-terminal region amino acid substitutions contribute to catalytic differences between murine class alpha glutathione transferases mGSTA1-1 and mGSTA2-2 toward anti-diol epoxide isomers of benzo[c]phenanthrene

The intersubunit lock-and-key motif in human glutathione transferase A1-1: role of the key residues Met51 and Phe52 in function and dimer stability

The dimeric structure of certain cytosolic GSTs (glutathione S-transferases) is stabilized by a hydrophobic lock-and-key motif at their subunit interface. In hGSTA1-1 (human class Alpha GST with two type-1 subunits), the key consists of two residues, Met51 and Phe52, that fit into a hydrophobic cavity (lock) in the adjacent subunit. SEC (size-exclusion chromatography)-HPLC, far-UV CD and tryptophan fluorescence of the M51A and M51A/F52S mutants indicated the non-disruptive nature of these mutations on the global structure. While the M51A mutant retained 80% of wild-type activity, the activity of the M51A/F52S was markedly diminished, indicating the importance of Phe52 in maintaining the correct conformation at the active site. The M51A and M51A/F52S mutations altered the binding of ANS (8-anilinonaphthalene-l-sulphonic acid) at the H-site by destabilizing helix 9 in the C-terminal region. Data from urea unfolding studies show that the dimer is destabilized by both mutations and that the dimer dissociates to aggregation-prone monomers at low urea concentrations before global unfolding. Although not essential for the assembly of the dimeric structure of hGSTA1-1, both Met51 and Phe52 in the intersubunit lock-and-key motif play important structural roles in maintaining the catalytic and ligandin functions and stability of the GST dimer.

Tertiary Interactions Stabilise the C-terminal Region of Human Glutathione Transferase A1-1: a Crystallographic and Calorimetric Study

The C-terminal region in class Alpha glutathione transferase A1-1 (GSTA1-1), which forms an amphipathic alpha-helix (helix 9), is known to contribute to the catalytic and non-substrate ligand-binding functions of the enzyme. The region in the apo protein is proposed to be disordered which, upon ligand binding at the active-site, becomes structured and localised. Because Ile219 plays a pivotal role in the stability and localisation of the region, the role of tertiary interactions mediated by Ile219 in determining the conformation and dynamics of the C-terminal region were studied. Ligand-binding microcalorimetric and X-ray structural data were obtained to characterise ligand binding at the active-site and the associated localisation of the C-terminal region. In the crystal structure of the I219A hGSTA1-1.S-hexylglutathione complex, the C-terminal region of one chain is mobile and not observed (unresolved electron density), whereas the corresponding region of the other chain is localised and structured as a result of crystal packing interactions. In solution, the mutant C-terminal region of both chains in the complex is mobile and delocalised resulting in a hydrated, less hydrophobic active-site and a reduction in the affinity of the protein for S-hexylglutathione. Complete dehydration of the active-site, important for maintaining the highly reactive thiolate form of glutathione, requires the binding of ligands and the subsequent localisation of the C-terminal region. Thermodynamic data demonstrate that the mobile C-terminal region in apo hGSTA1-1 is structured and does not undergo ligand-induced folding. Its close proximity to the surface of the wild-type protein is indicated by the concurrence between the observed heat capacity change of complex formation and the type and amount of surface area that becomes buried at the ligand-protein interface when the C-terminal region in the apo protein assumes the same localised structure as that observed in the wild-type complex.

Reciprocal regulation of glutathione S-transferase spliceforms and the Drosophila c-Jun N-terminal kinase pathway components

In mammalian systems, detoxification enzymes of the GST (glutathione S-transferase) family regulate JNK (c-Jun N-terminal kinase) signal transduction by interaction with JNK itself or other proteins upstream in the JNK pathway. In the present study, we have studied GSTs and their interaction with components of the JNK pathway from Diptera. We have evaluated the effects of four Delta class Anopheles dirus GSTs, GSTD1-1, GSTD2-2, GSTD3-3 and GSTD4-4, on the activity of full-length recombinant Drosophila HEP (mitogen-activated protein kinase kinase 7; where HEP stands for hemipterous) and the Drosophila JNK, as well as the reciprocal effect of these kinases on GST activity. Interestingly, even though these four GSTs are alternatively spliced products of the same gene and share >60% identity, they exerted different effects on JNK activity. GSTD1-1 inhibited JNK activity, whereas the other three GST isoforms activated JNK. GSTD2-2, GSTD3-3 and GSTD4-4 were inhibited 50-80% by HEP or JNK but GSTD1-1 was not inhibited by JNK. However, there were some similarities in the actions of HEP and JNK on these GSTs. For example, binding constants for HEP or JNK inhibiting a GST were similar (20-70 nM). Furthermore, after incubation of the GSTs with JNK, both JNK and the GSTs changed catalytic properties. The substrate specificities of both GSTs and JNK were also altered after their co-incubation. In addition, glutathione modulated the effects of JNK on GST activity. These results emphasize that different GST spliceforms possess different properties, both in their catalytic function and in their regulation of signalling through the JNK pathway.

A conserved N-capping motif contributes significantly to the stabilization and dynamics of the C-terminal region of class Alpha glutathione S-transferases

Helix 9, the major structural element in the C-terminal region of class Alpha glutathione transferases, forms part of the active site of these enzymes where its dynamic properties modulate both catalytic and ligandin functions. A conserved aspartic acid N-capping motif for helix 9 was identified by sequence alignments of the C-terminal regions of class Alpha glutathione S-transferases (GSTs) and an analysis by the helix-coil algorithm AGADIR. The contribution of the N-capping motif to the stability and dynamics of the region was investigated by replacing the N-cap residue Asp-209 with a glycine in human glutathione S-transferase A1-1 (hGST A1-1) and in a peptide corresponding to its C-terminal region. Far-UV circular dichroism and AGADIR analyses indicate that, in the absence of tertiary interactions, the wild-type peptide displays a low intrinsic tendency to form a helix and that this tendency is reduced significantly by the Asp-to-Gly mutation. Disruption of the N-capping motif of helix 9 in hGST A1-1 alters the conformational dynamics of the C-terminal region and, consequently, the features of the H-site to which hydrophobic substrates (e.g. 1-chloro-2,4-dinitrobenzene (CDNB)) and nonsubstrates (e.g. 8-anilino-1-naphthalene sulfonate (ANS)) bind. Isothermal calorimetric and fluorescence data for complex formation between ANS and protein suggest that the D209G-induced perturbation in the C-terminal region prevents normal ligand-induced localization of the region at the active site, resulting in a less hydrophobic and more solvent-exposed H-site. Therefore, the catalytic efficiency of the enzyme with CDNB is diminished due to a lowered affinity for the electrophilic substrate and a lower stabilization of the transition state.