Deepsikha Kar

Unlike humans, Mycobacterium tuberculosis ( Mtb ), the causative agent of human tuberculosis, has a de novo histidine-biosynthesis pathway. The enzyme imidazole glycerol phosphate dehydratase (IGPD), which catalyses the conversion of imidazole glycerol phosphate to imidazole acetol phosphate, has been studied extensively from various organisms and has become a major target for the development of antibacterial, antiweed and antifungal small molecules. In our previous studies, we have shown that in crystals IGPD forms a 24-mer oligomeric state in which the monomers are arranged in 432 symmetry. In order to gain insights into the oligomeric state of Mtb IGPD in solution, we determined cryogenic sample electron microscopy (cryo-EM) structures of apo IGPD at 2.2 and 3.1 Å resolution. In addition, we also determined the cryo-EM structure of IGPD in the presence of 3-amino-1,2,4-triazole (ATZ) to 2.8 Å resolution. The results of this work, which corroborate those from the crystallographic studies, indicate that IGPD forms a homo-oligomeric structure in solution comprising of 24 subunits. ATZ binds in the active-site pocket of the enzyme, which is located at the interface of three monomers and tethers 24 ATZ molecules. The results of this study suggest that cryo-EM, in addition to being a rapidly evolving and complementary imaging technology for elucidating 3D structures of biological macromolecules, can be useful in pinpointing the mode of binding small molecules of low mass (here ∼85 Da) and mapping protein-ligand interactions, which could assist in the design of accurate (high-potency) inhibitors.

Enolase, a ubiquitous enzyme, catalyzes the reversible conversion of 2-phosphoglycerate (2PG) to phosphoenolpyruvate (PEP) in the glycolytic pathway of organisms of all three domains of life. The underlying mechanism of the 2PG to PEP conversion has been studied in great detail in previous work, however that of the reverse reaction remains to be explored. Here we present structural snapshots of Mycobacterium tuberculosis (Mtb) enolase in apo, PEP-bound and two 2PG-bound forms as it catalyzes the conversion of PEP to 2PG. The two 2PG-bound complex structures differed in the conformation of the bound product (2PG) viz the widely reported canonical conformation and a novel binding pose, which we refer to here as the alternate conformation. Notably, we observed two major differences compared with the forward reaction: the presence of Mg_B is non-obligatory for the reaction and 2PG assumes an alternate conformation that is likely to facilitate its dissociation from the active site. Molecular dynamics studies and binding free energy calculations further substantiate that the alternate conformation of 2PG causes distortions in both metal ion coordination and hydrogen-bonding interactions, resulting in an increased flexibility of the active-site loops and aiding product release. Taken together, this study presents a probable mechanism involved in PEP to 2PG catalysis that is likely to be mediated by the conformational change of 2PG at the active site.

Apart from the canonical fingers, palm and thumb domains, the RNA dependent RNA polymerases (RdRp) from the viral orderNidoviralespossess two additional domains. Of these, the function of the Nidovirus RdRp associated nucleotidyl transferase domain (NiRAN) remains unanswered. The elucidation of the 3D structure of RdRp from the severe acute respiratory syndrome coronavirus-2 (SARS-CoV-2), provided the first ever insights into the domain organisation and possible functional characteristics of the NiRAN domain. Usingin silicotools, we predict that the NiRAN domain assumes a kinase or phosphotransferase like fold and binds nucleoside triphosphates at its proposed active site. Additionally, using molecular docking we have predicted the binding of three widely used kinase inhibitors and five well characterized anti-microbial compounds at the NiRAN domain active site along with their drug-likeliness. For the first time ever, using basic biochemical tools, this study shows the presence of a kinase like activity exhibited by the SARS-CoV-2 RdRp. Interestingly, a well-known kinase inhibitor- Sorafenib showed a significant inhibition and dampened viral load in SARS-CoV-2 infected cells. In line with the current global COVID-19 pandemic urgency and the emergence of newer strains with significantly higher infectivity, this study provides a new anti-SARS-CoV-2 drug target and potential lead compounds for drug repurposing against SARS-CoV-2.