A homology model for rat mu class glutathione S-transferase 4-4

Binding of GSH conjugates to π-GST: a cross-docking approach

The high degree of flexibility characterizing the members of the GST protein family is supposed to be an evolution-resolved feature related to the detoxifying role of these enzymes. Many evidences suggest that overexpression of these enzymes may be implicated in the development of acquired resistance to antitumor agents. Among the most effective GST inhibitors, GSH conjugates have been found to be particularly promising because of their low toxicity. Here, we used a cross docking approach based on an ensemble of X-ray structures of GST bound complexes to model the effects of protein flexibility on the binding of GSH conjugates. We showed that our multitarget approach, allows to analyze the impact of protein flexibility and induced fit effects in GSH conjugate docking to GST. Moreover, the inclusion of conserved water molecules in the model allowed to include a further source of target variability and improve the performances in the docking of GSH conjugates through an enhanced description of the GSH moiety interactions. Therefore, a map of ligand-protein interactions reflecting the target variability included in the docking model was retraced and used to gain a thorough insight about the way GSH conjugates bind to GST.

Glutathione transferases from parasites: a biochemical view

The glutathione transferase (GST) system of parasites represents the main detoxification mechanism of hydrophobic and electrophilic compounds. Parasites lack the CYP450 activity, hence part of its function has been taken over by other enzymes including GSTs. Cytosolic GSTs (cGSTs) are found in this system and constitute a versatile and numerous group that in parasites display many peculiarities in contrast to mammalian cGSTs. This review summarizes aspects of the biochemistry of parasite cGSTs such as substrate specificities, inhibitor sensitivities, classification, kinetics and catalysis, as well as some aspects of their protective role.

In vitro metabolism of the epoxide substructure of cryptophycins by cytosolic glutathione S-transferase: species differences and stereoselectivity

The enzyme kinetics of the glutathione (GSH) conjugation of cryptophycin 52 (C52, R-stereoisomer) and cryptophycin 53 (C53, S-stereoisomer) by cytosolic glutathione S-transferases (cGSTs) from human, rat, mouse, dog and monkey liver were studied. Vmax, Km, and CLint values for glutathione conjugation of C52 (R-stereoisomer) were 0.10 +/- 0.01 nmol min-1 mg-1, 3.24 +/- 0.23 microM, and (3.15 +/- 0.09) x 10(-2) ml min-1 mg-1, respectively, in human cytosol. Due to limited solubility relative to the Km, only CLint values were determined in rat ((7.76 +/- 0.10) x 10-2 ml min-1 mg-1) and mouse ((7.61 +/- 0.50) x 10(-2) ml min-1 mg-1) cytosol. Enzyme kinetic parameters could not be determined for C53 (S-stereoisomer). Microsomal GSH conjugation in human, rat, and mouse was attributed to cytosolic contamination. No GSH conjugation was seen in any biological matrix from dog or monkey. There was little GSH conjugation of C53 by cytosol or microsomes from any species. The metabolism of C52 and C53 by epoxide hydrolase was also investigated. No diol product was observed in any biological matrix from any species. Thus, cGSTs are primarily responsible for C52 metabolism.

Catalytically active monomer of class mu glutathione transferase from rat

Although rat glutathione transferase M1-1 is crystallized as a homodimer (GST M1-1), we have generated monomers (GST M1) of the enzyme by adding potassium bromide to buffer solutions containing the wild-type enzyme and by introducing point mutations in the electrostatic region of the subunit interface. The wild-type enzyme was evaluated in 0.05 M MES (pH 6.5) containing up to 3 M KBr. We report that the addition of KBr greatly influences the monomer-dimer equilibrium of the wild-type enzyme and that at 3 M KBr GST M1 has a specific activity close to that of GST M1-1. Since the effect of KBr is likely due to charge screening at the subunit interface, the influence on the monomer-dimer equilibrium exerted by the amino acid residues in the electrostatic region of the interface (Arg77, Asp97, Glu100, Asn101) was investigated. Mutations introduced at positions 97, 100, and 101 promote monomerization, resulting in enzymes that exhibit a decreased weight average molecular weight in comparison to that of the wild-type enzyme. However, only mutations at position 97 result in enzymes that have catalytic activity in the monomeric form. The mutations introduced at positions 100 or 101 result in enzymes whose activity can be accounted for by the amount of dimeric enzyme present. Our results indicate that the electrostatic region of the interface is important in the monomer-dimer equilibrium of glutathione transferase and that, although GST M1-1 may be more active than GST M1, the dimer is not required for catalytic function.

Analysis of the activating mutations within the activation loop of leukemia targets Flt-3 and c-Kit based on protein homology modeling

Molecular modeling provides a mechanistic hypothesis at the molecular level for the constitutive activation recently observed and reported for tyrosine protein kinases Flt-3 and c-Kit. Three-dimensional homology models for the active and inactive forms of these two kinases were made. Comparison of these models at the molecular level reveals that mutations of specific residues located in the activation loop (D835X and 836-deletion in Flt-3; D816V in c-Kit) as well as a 6-base pair (6-bp) insertion at residue 840 in Flt-3 operate in a similar way. Each mutation tends to weaken the forces that maintain the activation-loop folded inwards. None of the mutations are found to particularly stabilize the active state directly. The reason why the equilibrium is shifted towards the gate-open conformation of the protein is because, at least in these models, the mutations are found to critically destabilize the inactive conformational state of the kinase.

Structure-activity relationships for chemical and glutathione S-transferase-catalysed glutathione conjugation reactions of a series of 2-substituted 1-chloro-4-nitrobenzenes

Glutathione S-transferases (GSTs) constitute an important class of phase II (de)toxifying enzymes, catalysing the conjugation of glutathione (GSH) with electrophilic compounds. In the present study, Km, kcat and kcat/Km values for the rat GST 1-1-, 3-3-, 4-4- and 7-7-catalysed conjugation reactions between GSH and a series of 10 different 2-substituted 1-chloro-4-nitrobenzenes, and the second-order rate constants (ks) of the corresponding base-catalysed reactions, were correlated with nine classical physicochemical parameters (electronic, steric and lipophilic) of the substituents and with 16 computer-calculated molecular parameters of the substrates and of the corresponding Meisenheimer complexes with MeS- as a model nucleophile for GS- (charge distributions and several energy values), giving structure-activity relationships. On the basis of an identical dependence of the base-catalysed as well as the GST 1-1- and GST 7-7-catalysed reactions on electronic parameters (among others, Hammett substituent constant sigma p and charge on p-nitro substituents), and the finding that the corresponding reactions catalysed by GSTs 3-3 and 4-4 depend to a significantly lesser extent on these parameters, it was concluded that the Mu-class GST isoenzymes have a rate-determining transition state in the conjugation reaction between 2-substituted 1-chloro-4-nitrobenzenes and GSH which is different from that of the other two GSTs. Several alternative rate-limiting transition states for GST 3-3 and 4-4 are discussed. Furthermore, based on the obtained structure-activity relationships, it was possible to predict the kcat/Km values of the four GST isoenzymes and the ks of the base-catalysed GSH conjugation of 1-chloro-4-nitrobenzene.