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Abstract: Glutathione transferase reaches 0.5–0.8mM concentration in the cell so it works in vivo under the unusual conditions of, [S]≪[E]. As glutathione transferase lowers the pK a of glutathione (GSH) bound to the active site, it increases the cytosolic concentration of deprotonated GSH about five times and speeds its conjugation with toxic compounds that are non-typical substrates of this enzyme. This acceleration becomes more efficient in case of GSH depletion and/or cell acidification. Interestingly, the enzymatic conjugation of GSH to these toxic compounds does not require the assumption of a substrate–enzyme complex; it can be explained by a simple bimolecular collision between enzyme and substrate. Even with typical substrates, the astonishing concentration of glutathione transferase present in hepatocytes, causes an unusual “inverted” kinetics whereby the classical trends of v versus E and v versus S are reversed. [ABSTRACT FROM AUTHOR]

Abstract: HIV integrase (IN) is an essential enzyme in HIV replication and an important target for drug design. IN has been shown to interact with a number of cellular and viral proteins during the integration process. Disruption of these important interactions could provide a mechanism for allosteric inhibition of IN. We present the highest resolution crystal structure of the IN core domain to date. We also present a crystal structure of the IN core domain in complex with sucrose which is bound at the dimer interface in a region that has previously been reported to bind integrase inhibitors. Structured summary: MINT-7713125: IN (uniprotkb:P04585) and IN (uniprotkb:P04585) bind (MI:0407) by X-ray crystallography (MI:0114) [Copyright &y& Elsevier]

Picrotoxin, an antagonist of structurally-rated GABAA receptors (GABAARs) and glycine receptors (GlyRs), is an equimolar mixture of picrotoxinin (PTXININ) and picrotin (PTN). These compounds share a common structure except that PTN contains a slightly larger dimethylmethanol in place of the PTXININ isopropenyl group. Although the homomeric α1 GlyR is equally sensitive to both compounds, we show here that homomeric α2 and α3 GlyRs, like most GABAARs, are selectively inhibited by PTXININ. As conservative mutations to pore-lining 6′ threonines equally affect the sensitivity of the α1 GlyR to both compounds, we conclude that PTXININ and PTN bind to 6′ threonines by hydrogen bonding with exocyclic oxygens common to both molecules. In contrast, substitution of the 2′ pore-lining glycine by serine selectively reduces PTN sensitivity, whereas the introduction of 2′ alanines selectively increases PTXININ sensitivity. These results define the orientation of PTXININ and PTN binding in the α1 GlyR pore and allow us to conclude that the relatively reduced sensitivity of PTN at GABAARs and α2 and α3 GlyRs is due predominantly to its larger size and reduced ability to form hydrophobic interactions with 2′ alanines. [ABSTRACT FROM AUTHOR]

The 5-hydroxytryptamine type-3 receptor antagonist tropisetron is in clinical use as an anti-emetic drug. This compound also exerts both potentiating and inhibitory effects on the glycine receptor chloride channel. The inhibitory effects occur at micromolar concentrations, whereas the potentiating effects are shown here to occur at femtomolar concentrations at the homomeric α1 receptor. Potentiation occurred only when tropisetron was applied in the presence of glycine. We also sought to identify molecular determinants of tropisetron inhibition at the α1 glycine receptor by serially mutating residues located in or near known ligand-binding sites. We discovered that conservative mutations to N102 ablated tropisetron inhibition without affecting the magnitude or sensitivity of tropisetron potentiation. Several lines of evidence, including a structure-activity analysis of tropisetron, atropine and SB203186, suggest that N102 may bind to the tropisetron tropane nitrogen via H-bonding. Mutation of the N125 residue in the β subunit, which corresponds to N102 in the α1 subunit, had little effect on tropisetron inhibitory potency. These results show that N102 is required for tropisetron inhibition but not potentiation and that inhibitory tropisetron binds in different orientations at different subunit interfaces. To our knowledge, tropisetron is the most exquisitely sensitive modulator yet identified for a cys-loop receptor. [ABSTRACT FROM AUTHOR]