Computer-aided analysis of the interactions of glutamine synthetase with its inhibitors

Identification of Concomitant Inhibitors against Glutamine Synthetase and Isocitrate Lyase in Mycobacterium tuberculosis from Natural Sources

Tuberculosis (T.B.) is a disease that occurs due to infection by the bacterium, Mycobacterium tuberculosis (Mtb), which is responsible for millions of deaths every year. Due to the emergence of multidrug and extensive drug-resistant Mtb strains, there is an urgent need to develop more powerful drugs for inclusion in the current tuberculosis treatment regime. In this study, 1778 molecules from four medicinal plants, Azadirachta indica, Camellia sinensis, Adhatoda vasica, and Ginkgo biloba, were selected and docked against two chosen drug targets, namely, Glutamine Synthetase (G.S.) and Isocitrate Lyase (I.C.L.). Molecular Docking was performed using the Glide module of the Schrӧdinger suite to identify the best-performing ligands; the complexes formed by the best-performing ligands were further investigated for their binding stability via Molecular Dynamics Simulation of 100 ns. The present study suggests that Azadiradione from Azadirachta indica possesses the potential to inhibit Glutamine Synthetase and Isocitrate Lyase of M. tuberculosis concomitantly. The excellent docking score of the ligand and the stability of receptor-ligand complexes, coupled with the complete pharmacokinetic profile of Azadiradione, support the proposal of the small molecule, Azadiradione as a novel antitubercular agent. Further, wet lab analysis of Azadiradione may lead to the possible discovery of a novel antitubercular drug.

Glutamine Synthetase Drugability beyond Its Active Site: Exploring Oligomerization Interfaces and Pockets

BACKGROUND

Glutamine synthetase (GS) is a crucial enzyme to the nitrogen cycle with great commercial and pharmaceutical value. Current inhibitors target the active site, affecting GS activity indiscriminately in all organisms. As the active site is located at the interface between two monomers, the protein-protein interface (PPI) of GSs gains a new role, by providing new targets for enzyme inhibition. Exploring GSs PPI could allow for the development of inhibitors selective for specific organisms. Here we map the PPI of three GSs-human (hsGS), maize (zmGS) and Mycobacterium tuberculosis (mtGS)-and unravel new drugable pockets.

METHODS

The PPI binding free energy coming from key residues on three GSs from different organisms were mapped by computational alanine scan mutagenesis, applying a multiple dielectric constant MM-PBSA methodology. The most relevant residues for binding are referred as hot-spots. Drugable pockets on GS were detected with the Fpocket software.

RESULTS AND CONCLUSIONS

A total of 23, 19 and 30 hot-spots were identified on hsGS, zmGS and mtGS PPI. Even possessing differences in the hot-spots, hsGS and zmGS PPI are overall very similar. On the other hand, mtGS PPI differs greatly from hsGS and zmGS PPI. A novel drugable pocket was detected on the mtGS PPI. It seems particularly promising for the development of selective anti-tuberculosis drugs given its location on a PPI region that is highly populated with hot-spots and is completely different from the hsGS and zmGS PPIs. Drugs targeting this pockets should be inactive on eukaryotic GS II enzymes.

Inhibition of glutamine synthetase: a potential drug target in Mycobacterium tuberculosis

Tuberculosis is an infectious disease caused by Mycobacterium tuberculosis. Globally, tuberculosis is second only to AIDS in mortality and the disease is responsible for over 1.3 million deaths each year. The impractically long treatment schedules (generally 6-9 months) and unpleasant side effects of the current drugs often lead to poor patient compliance, which in turn has resulted in the emergence of multi-, extensively- and totally-drug resistant strains. The development of new classes of anti-tuberculosis drugs and new drug targets is of global importance, since attacking the bacterium using multiple strategies provides the best means to prevent resistance. This review presents an overview of the various strategies and compounds utilized to inhibit glutamine synthetase, a promising target for the development of drugs for TB therapy.

Effectiveness and mode of action of phosphonate inhibitors of plant glutamine synthetase

BACKGROUND

Aiming at the rational design of new herbicides, the availability of the three-dimensional structure of the target enzyme greatly enhances the optimisation of lead compounds and the design of derivatives with increased activity. Among the most widely exploited herbicide targets is glutamine synthetase. Recently, the structure of a cytosolic form of the maize enzyme has been described, making it possible to verify whether steric, electronic and hydrophobic features of a compound are in agreement with inhibitor-protein interaction geometry.

RESULTS

Three series of compounds (aminophosphonates, hydroxyphosphonates and aminomethylenebisphosphonates) were evaluated as possible inhibitors of maize glutamine synthetase. Aminomethylenebisphosphonate derivatives substituted in the phenyl ring retained the inhibitory potential, whereas variations in the scaffold, i.e. the replacement of the second phosphonate moiety with a hydroxyl or an amino residue, resulted in a significant loss of activity. A kinetic characterisation showed a non-competitive mechanism against glutamate and an uncompetitive mechanism against ATP. A docking analysis suggested the mode of bisphosphonate binding to the active site.

CONCLUSION

Results made it possible to define the features required to maintain or enhance the biological activity of these compounds, which represent lead structures to be further exploited for the design of new substances endowed with herbicidal activity.

Glutamine synthetase becomes nitrated and its activity is reduced during repetitive seizure activity in the pentylentetrazole model of epilepsy

PURPOSE

The astrocyte-specific glutamine synthetase (GS) plays a key role in glutamate recycling and Gamma-aminobutyric acid (GABA) metabolism. Changes in the expression or activity of GS have been proposed to contribute to epileptogenesis. The mechanisms or how and where GS may contribute to epilepsy is still a matter of discussion. Here we asked the question whether brain regions, which show an astrocytic stress response respond with alterations of GS.

METHODS

Biochemical and histological alterations of GS, HSP-27, and GFAP were studied after pentylenetetrazole-induced repetitive epileptic seizures (PIRS) in rats using a topographical quantification of the GS-immunoreactivity (GSIR) in relation to the focal heat shock response (HSR). Saline-treated rats served as controls and rats treated by the GS-inhibitor, L-methionine-sulfoximine (MSO) served as a positive control.

RESULTS

No changes in the amount of GSIR and GS-protein occurred during PIRS. A significant reduction of GSIR was observed by histochemistry (in situ) and in native (nonheated) protein extracts of MSO-treated rats. In rats affected by PIRS, GS-activity showed a significant, region-specific reduction in association with a nitration of the enzyme.

DISCUSSION

These results show that neither PIRS nor GS-inhibition reduced the amount of GS protein, but that MSO interferes with antibody binding to native GS. PIRS resulted in a focal increase of astrocytic stress response, whereas MSO caused a widespread, homogeneous astrocytic HSR independent from quantitative changes of GS content. In rats with PIRS the regions showing a strong glial HSR, respond with reduced GS-activity and GS-nitration, which all together are clear indicators of a nitrosative stress response.

l-(+)-2-Amino-4-thiophosphonobutyric Acid (l-thioAP4), a New Potent Agonist of Group III Metabotropic Glutamate Receptors: Increased Distal Acidity Affords Enhanced Potency

L-2-Amino-4-phosphonobutyric acid (l-AP4), l-2-amino-4-thiophosphonobutyric acid (l-thioAP4), and l-2-amino-4-(hydroxy)phosphinylbutyric acid (desmethylphosphinothricin, DMPT) were synthesized from protected vinylglycine. They were tested as agonists at group III metabotropic glutamate receptors (mGluR) along with phosphinothricin (PT). DMPT and PT display a much lower potency at mGlu4 receptor (EC50 = 4.0 and 1100 microM, respectively) in comparison to l-AP4 (EC50 = 0.08 microM), whereas l-thioAP4 has a 2-fold higher potency (EC50 = 0.039 microM). Similar rank orders of potency were observed at mGlu6,7 and mGlu8 receptors. The higher potency of l-thioAP4 is due to its stronger second acidity compared to l-AP4. These pKa values of 5.56 and 6.88, respectively, were determined using 31P NMR chemical shift variations. The second distal negative charge of l-AP4/l-thioAP4 probably provides stronger binding to specific basic residues of the binding sites of group III mGluRs, which stabilizes the active conformation of the receptor.