Imaging active site chemistry and protonation states: NMR crystallography of the tryptophan synthase α-aminoacrylate intermediate

NMR-assisted crystallography—the integrated application of solid-state NMR, X-ray crystallography, and first-principles computational chemistry—holds significant promise for mechanistic enzymology: by providing atomic-resolution characterization of stable intermediates in enzyme active sites, including hydrogen atom locations and tautomeric equilibria, NMR crystallography offers insight into both structure and chemical dynamics. Here, this integrated approach is used to characterize the tryptophan synthase α-aminoacrylate intermediate, a defining species for pyridoxal-5′-phosphate–dependent enzymes that catalyze β-elimination and replacement reactions. For this intermediate, NMR-assisted crystallography is able to identify the protonation states of the ionizable sites on the cofactor, substrate, and catalytic side chains as well as the location and orientation of crystallographic waters within the active site. Most notable is the water molecule immediately adjacent to the substrate β-carbon, which serves as a hydrogen bond donor to the ε-amino group of the acid–base catalytic residue βLys87. From this analysis, a detailed three-dimensional picture of structure and reactivity emerges, highlighting the fate of the L-serine hydroxyl leaving group and the reaction pathway back to the preceding transition state. Reaction of the α-aminoacrylate intermediate with benzimidazole, an isostere of the natural substrate indole, shows benzimidazole bound in the active site and poised for, but unable to initiate, the subsequent bond formation step. When modeled into the benzimidazole position, indole is positioned with C3 in contact with the α-aminoacrylate Cβ and aligned for nucleophilic attack. Here, the chemically detailed, three-dimensional structure from NMR-assisted crystallography is key to understanding why benzimidazole does not react, while indole does.

Solid-State 17O NMR Study of α-D-Glucose: Exploring New Frontiers in Isotopic Labeling, Sensitivity Enhancement, and NMR Crystallography

We report synthesis and solid-state 17O NMR characterization of α-D-glucose for which all six oxygen atoms are site-specifically 17O-labeled. Solid-state 17O NMR spectra were recorded for α-D-glucose/NaCl/H2O (2/1/1) cocrystals under...

Exploring the SARS-CoV-2 main protease (Mpro) and RdRp targets by updating current structure-based drug design utilizing co-crystals to combat COVID-19

: The unprecedented pandemic of COVID-19 caused by the novel strain of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) engulfs millions of death worldwide. It has directly hit the socio-economic status of the affected countries. There are more than 219 countries badly affected by the COVID-19. There are no particular small molecule inhibitors to combat the dreadful virus. Many antivirals, antimalarials, antiparasitic, antibacterials, immunosuppressive anti-inflammatory, and immune stimulatory agents have been repurposed for the treatment of COVID19. But the exact mechanism of action of these drugs towards COVID-19 targets has not been experimented with yet. Under the effect of chemotherapeutics, the virus may change its genetic material and produces various strains, which are the main reasons behind the dreadful attack of COVID-19. The nuclear genetic components are composed of main protease and RNA-dependent RNA polymerase (RdRp) which are responsible for producing nascent virion and viral replication in the host cells. To explore the biochemical mechanisms of various small molecule inhibitors, structure-based drug design can be attempted utilizing NMR crystallography. The process identifies and validates the target protein involved in the disease pathogenesis by the binding of a chemical ligand at a well-defined pocket on the protein surface. In this way, the mode of binding of the ligands inside the target cavity can be predicted for the design of potent SARS-CoV-2 inhibitors.