X-ray structure methods for glutathione binding

Energetics of Glutathione Binding to Human Eukaryotic Elongation Factor 1 Gamma: Isothermal Titration Calorimetry and Molecular Dynamics Studies

The energetics of ligand binding to human eukaryotic elongation factor 1 gamma (heEF1γ) was investigated using reduced glutathione (GSH), oxidised glutathione (GSSG), glutathione sulfonate and S-hexylglutathione as ligands. The experiments were conducted using isothermal titration calorimetry, and the findings were supported using computational studies. The data show that the binding of these ligands to heEF1γ is enthalpically favourable and entropically driven (except for the binding of GSSG). The full length heEF1γ binds GSSG with lower affinity (K _d = 115 μM), with more hydrogen-bond contacts (ΔH = -73.8 kJ/mol) and unfavourable entropy (-TΔS = 51.7 kJ/mol) compared to the glutathione transferase-like N-terminus domain of heEF1γ, which did not show preference to any specific ligand. Computational free binding energy calculations from the 10 ligand poses show that GSSG and GSH consistently bind heEF1γ, and that both ligands bind at the same site with a folded bioactive conformation. This study reveals the possibility that heEF1γ is a glutathione-binding protein.

Interaction of the antitumor antibiotic chromomycin A3 with glutathione, a sulfhydryl agent, and the effect upon its DNA binding properties

Chromomycin A3 (CHR), an anticancer antibiotic, blocks macromolecular synthesis via reversible interaction with DNA only in the presence of divalent cations like Mg2+. In the absence of DNA, the antibiotic forms a dimer: Mg2+ complex [(CHR)2Mg2+]. It is the DNA-binding ligand. The antibiotic has potential reactive centers that could interact with GSH, the most abundant non-protein thiol in eukaryotic cells and a putative cofactor involved in the activation of many antibiotics in vivo. To understand the mode of action of CHR in vivo, we studied the interactions of CHR and the (CHR)2Mg2+ complex with GSH and the association of the resultant complexes with DNA by means of absorption, fluorescence, and circular dichroism spectroscopy. The novel finding was that GSH interacts non-covalently with CHR without a chemical modification of the antibiotic. The interaction was reversible in nature. The results are reported in two parts: the interaction of CHR with GSH in the absence and presence of Mg2+, and the effect of this interaction on the DNA-binding properties of the antibiotic. CHR forms a single type of complex with GSH. In contrast, (CHR)2Mg2+ forms two different types of complexes with GSH: a low GSH complex at approximately 12 mM GSH and a high GSH complex at > or = 16 mM GSH. Binding and thermodynamic parameters for the reversible association of the complexes with DNA demonstrated that they bind differently to the same DNA. The thermodynamic parameters indicate that the presence of GSH alters the mode of binding of the (CHR)2Mg2+ complex with DNA. The (CHR)2Mg2+ complex binds to DNA via an entropy-driven process, whereas in the presence of GSH the association is enthalpy-driven. The significance of these results in the understanding of the molecular basis of action of the antibiotic is discussed.

Phosphono analogues of glutathione as new inhibitors of glutathione S-transferases

Phosphono-analogues of glutathione containing the O = P(OR)2 moiety in place of the cysteinyl residue CH2SH 1a-1d were prepared by solution phase peptide synthesis. Benzyl, benzyloxy-carbonyl, and tert-butyl protecting groups were used to mask the individual amino acid functional groups. The formation of peptide bonds was achieved by the usual peptide synthesis via activation of carboxylic functions with cyclohexylcarbodiimide and subsequent reaction with free amino groups. The thus obtained, fully-protected peptides were each purified by normal phase column chromatography. Deprotection was accomplished by hydrogenolysis and by treatment with HBr/acetic acid yielding the desired phosphonic acid diester 1a-1d. The inhibition of the glutathione conjugation of 1-chloro-2,4-dinitrobenzene by human placental glutathione S-transferase was studied by determining the IC50 values of the new glutathione analogues. The IC50 values were 291 microM, 139 microM, 64 microM, and 21 microM for the dimethyl, diethyl, diisopropyl, and di-n-butyl esters, respectively. The results clearly show that the formal substitution of the glutathione thiol function by phosphonic acid esters leads to a new class of glutathione S-transferase inhibitors. Further investigations directed at the question of whether or not these glutathione analogues are suitable for a modulation in chemotherapy are in progress.

Effect of glutathione, glutathione sulphonate and S-hexylglutathione on the conformational stability of class pi glutathione S-transferase

The glutathione S-transferases (GST) are a supergene family of phase II detoxification enzymes which catalyse the S-conjugation between glutathione and an electrophilic substrate. The active site can be divided into two adjacent functional regions, a highly specific G-site for binding the physiological substrate glutathione and a nonspecific H-site for binding nonpolar electrophilic substrates. Equilibrium and kinetic unfolding experiments employing tryptophan fluorescence and enzyme activity measurements were preformed to study the effect of ligand binding to the G-site on the unfolding and stability of the porcine class pi glutathione S-transferase against urea. The presence of glutathione caused a shift in the equilibrium-unfolding curves towards lower urea concentrations and enhanced the first-order rate constant for unfolding suggesting a destabilisation of the pGSTP1-1 structure against urea. The presence of either glutathione sulphonate or S-hexylglutathione, however, produced the opposite effect in that their binding to the G-site appeared to exet a stablising effect against urea. The binding of these glutathione analogues also reduced significantly the degree of cooperativity of unfolding indicating a possible change in the protein's unfolding pathway.