Evaluating Stability and Activity of SARS-CoV-2 PLpro for High-throughput Screening of Inhibitors

Improving soluble recombinant SARS-CoV-2 papain-like protease production in Escherichia coli through chaperonin and maltose-binding protein tag: purification and kinetic characterization

Although COVID-19 is now becoming endemic, SARS-CoV-2 persists potential jeopardy to clinically vulnerable populations. Hence, further study is still necessary to discover novel antiviral agents against SARS-CoV-2 for proactive preparedness. SARS-CoV-2 papain-like protease (PL Pro) is a target enzyme for searching anti-Covid candidates. Our prior study revealed the major formation of inclusion bodies during PL Pro expression in E. coli RIPL. In this study, we tried using chaperonin in the E. coli Arctic Express system and both codon optimization and maltose-binding protein (MBP) fusion protein to make PL Pro more soluble. Recombinant PL Pro encoded on the pET21d(+) plasmid was expressed in E. coli Arctic express. However, the soluble protein yield remained low and unstable due to suboptimal codon usage in the insert gene. Whereas, fusion of the MBP protein with optimized codon of PL Pro enhanced the enzyme expression and solubility. Recombinant PL Pro cleaved the linker between MBP and PL Pro, which served as a cleavage site recognized by PL Pro (LKGG↓A). The purified enzyme from a 200-mL culture generated 1 mL of pure PL Pro enzyme at a 1.913 mg/mL concentration. It exhibited favorable activity against the Z-RLRGG-AMC substrate, with a Km value of 33.40 μM and a Vmax of 5.10 RFU/min.

Thiol-Reactive or Redox-Active: Revising a Repurposing Screen Led to a New Invalidation Pipeline and Identified a True Noncovalent Inhibitor Against Papain-like Protease from SARS-CoV-2

The SARS-CoV-2 papain-like protease PLpro has multiple roles in the viral replication cycle, related to both its polypeptide cleavage function and its ability to antagonize the host immune response. Targeting the PLpro function is recognized as a promising mechanism to modulate viral replication, while supporting host immune responses. However, the development of PLpro-specific inhibitors remains challenging. Comprehensive investigations utilizing enzymatic, binding studies, and cellular assays revealed the previously reported inhibitors to act in an unspecific manner. At present, GRL-0617 and its derivatives remain the best-validated compounds with demonstrated antiviral activity in cells and in mouse models. In this study, we refer to the pitfalls of the redox sensitivity of PLpro. Using a screening-based approach to identify inhibitors of PLpro's proteolytic activity, we made extensive efforts to validate active compounds over a range of conditions and readouts, emphasizing the need for comprehensive orthogonal data when profiling putative PLpro inhibitors. The remaining active compound, CPI-169, was shown to be a noncovalent inhibitor capable of competing with GRL-0617 in NMR-based experiments, suggesting that it occupied a similar binding site and inhibited viral replication in Vero-E6 cells, opening new design opportunities for further development as antiviral agents.

Novel compounds with dual inhibition activity against SARS-CoV-2 critical enzymes RdRp and human TMPRSS2

COVID-19 caused major worldwide problems. The spread of variants and limited treatment encouraged the design of novel anti-SARS-CoV-2 compounds. A series of compounds RH1-23 were designed to dually target RNA-dependent RNA polymerase (RdRp) and transmembrane serine protease 2 (TMPRSS2). Compared to remdesivir, in vitro screening indicated the highest selectivity and potent activity of RH11-13 with half maximum inhibitory concentration (IC50) 3.9, 5.7, and 19.72 nM, respectively. RH11-12 showed superior inhibition activity against TMPRSS2 and RdRP with IC50 (1.7 and 4.2), and (6.1 and 4.42) nM, respectively. WaterMap analysis and molecular dynamics studies demonstrated the superior enzyme binding activity of RH11 and RH12. On Vero-E6 cells, RH11 and RH12 significantly inhibited the viral replication with 66 % and 63.2 %, and viral adsorption with 44 % and 65 %, alongside virucidal effect with 51.40 % and 90.5 %, respectively. Furthermore, the potent activity of RH12 was tested on TMPRSS2-expressing cells (Calu-3) compared to camostat. RH12 exhibited selectivity index (26.05) similar to camostat (28.01) and comparable to its SI on Vero-E6 cells (22.6). RH12 demonstrated also a significant inhibition of the viral adsorption on Calu-3 cells with 60 % inhibition at 30 nM. The designed compounds exhibited good physiochemical properties. These findings indicate a broad-spectrum antiviral efficacy of the designed compounds, particularly RH12, with a promise for further development.

Residues in the fructose-binding pocket are required for ketohexokinase-A activity

Excessive fructose consumption is a primary contributor to the global surges in obesity, cancer, and metabolic syndrome. Fructolysis is not robustly regulated and is initiated by ketohexokinase (KHK). In this study, we determined the crystal structure of KHK-A, one of two human isozymes of KHK, in the apo-state at 1.85 Å resolution, and we investigated the roles of residues in the fructose-binding pocket by mutational analysis. Introducing alanine at D15, N42, or N45 inactivated KHK-A, whereas mutating R141 or K174 reduced activity and thermodynamic stability. Kinetic studies revealed that the R141A and K174A mutations reduced fructose affinity by 2- to 4-fold compared to WT KHK-A, without affecting ATP affinity. Molecular dynamics simulations provided mechanistic insights into the potential roles of the mutated residues in ligand coordination and the maintenance of an open state in one monomer and a closed state in the other. Protein-protein interactome analysis indicated distinct expression patterns and downregulation of partner proteins in different tumor tissues, warranting a reevaluation of KHK's role in cancer development and progression. The connections between different cancer genes and the KHK signaling pathway suggest that KHK is a potential target for preventing cancer metastasis. This study enhances our understanding of KHK-A's structure and function and offers valuable insights into potential targets for developing treatments for obesity, cancer, and metabolic syndrome.

Inhibitory effects of senkyuchachosan on SARS-CoV-2 papain-like protease activity in vitro

Papain-like protease (PLpro) enzyme plays a vital role in viral replication as it breaks down polyproteins and disrupts the host's immune response. There are few reports on Kampo formulas that focus on PLpro activity. In this study, we evaluated the inhibitory effects of senkyuchachosan, a traditional Japanese medicine, on PLpro of SARS-CoV-2, the virus responsible for causing COVID-19. We purified the PLpro enzyme and conducted in vitro enzymatic assays using specific substrates. Among the nine crude drugs present in senkyuchachosan, four (Cyperi Rhizoma, Schizonepetae Spica, Menthae Herba, and Camelliae sinensis Folium [CsF]) strongly inhibited PLpro activity. CsF, derived from Camellia sinensis (green tea), contains polyphenols, including catechins and tannins. To confirm that the PLpro inhibitory effects of senkyuchachosan predominantly stem from tannins, the tannins were removed from the decoction using polyvinylpolypyrrolidone (PVPP). The inhibitory effect of senkyuchachosan on PLpro activity was reduced by the removal of PVPP. In addition, the tannin fraction obtained from the CsF extracts showed significant PLpro inhibitory effects. These findings lay the groundwork for the potential development of therapeutic agents that target SARS-CoV-2 infection by intervening in proteolytic cleavage of the virus.

Utilizing sinapic acid as an inhibitory antiviral agent against MERS-CoV PLpro

Concerns about the social and economic collapse, high mortality rates, and stress on the healthcare system are developing due to the coronavirus onslaught in the form of various species and their variants. In the recent past, infections brought on by coronaviruses severe acute respiratory syndrome coronaviruses (SARS-CoV and SARS-CoV-2) as well as middle east respiratory syndrome coronavirus (MERS-CoV) have been reported. There is a severe lack of medications to treat various coronavirus types including MERS-CoV which is hazard to public health due to its ability for pandemic spread by human-to-human transmission. Here, we utilized sinapic acid (SA) against papain-like protease (PLpro), a crucial enzyme involved in MERS-CoV replication, because phytomedicine derived from nature has less well-known negative effects. The thermal shift assay (TSA) was used in the current study to determine whether the drug interact with the recombinant MERS-CoV PLpro. Also, inhibition assay was conducted as the hydrolysis of fluorogenic peptide from the Z-RLRGG-AMC-peptide bond in the presence of SA to determine the level of inhibition of the MERS-CoV PLpro. To study the structural binding efficiency Autodock Vina was used to dock SA to the MERS-CoV PLpro and results were analyzed using PyMOL and Maestro Schrödinger programs. Our results show a convincing interaction between SA and the MERS protease, as SA reduced MERS-CoV PLpro in a dose-dependent way IC50 values of 68.58 μM (of SA). The TSA showed SA raised temperature of melting to 54.61 °C near IC50 and at approximately 2X IC50 concentration (111.5 μM) the Tm for SA + MERS-CoV PLpro was 59.72 °C. SA was docked to MERS-CoV PLpro to identify the binding site. SA bound to the blocking loop (BL2) region of MERS-CoV PLpro interacts with F268, E272, V275, and P249 residues of MERS-CoV PLpro. The effectiveness of protease inhibitors against MERS-CoV has been established and SA is already known for broad range biological activity including antiviral properties; it can be a suitable candidate for anti-MERS-CoV treatment.

Predicting anti-COVID-19 potential: in silico analysis of Mauritine compound from Ziziphus-spina christi as a promising papain-like protease (PLpro) inhibitor

The COVID-19 pandemic caused by the SARS-CoV-2 virus, recognized by the World Health Organization (WHO), has led to 164,523,894 confirmed cases and 3,412,032 deaths globally as of May 20, 2021. SARS-CoV-2 encodes crucial proteases for its replication cycle, including the papain-like protease (PLpro), presenting a potential target for developing COVID-19 treatments. Mauritine, a cyclopeptide alkaloid found in the Ziziphus-spina christi plant, exhibits antiviral properties and was investigated for its affinity and toxicity towards PLpro using molecular docking through MGLTools 1.5.6 with Autodock Tools 4.2. Preceding this, toxicity and ADME prediction were performed via Toxtree 3.1.0 software and SwissADME servers. Results from molecular docking revealed free binding energy values of -8.58; -7.73; -8.36; -6.07; -6.67; -7.83; -7.67; -7.40; and -6.87 Kcal/mol for Mauritine-A, Mauritine-B, Mauritine-C, Mauritine-D, Mauritine-F, Mauritine-H, Mauritine-J, Mauritine-L, and Mauritine-M, respectively. Correspondingly, inhibition constants were 0.51724; 2.14; 0.7398; 35.43; 12.95; 1.83; 2.38; 3.80; and 9.17 µM, respectively. Interactions observed included hydrogen bonds, hydrophobic interactions, and electrostatic interactions between the Mauritine compounds and the receptor. Mauritine-A and Mauritine-C emerged as a promising anti-COVID-19 candidate due to its superior affinity compared to other derivatives, as indicated by research findings. Interestingly, Mauritine-A and Mauritine-C exhibits notable stability as depicted by the RMSD and RMSF graphs, along with a considerable MM-PBSA binding free energy value of -162.431 and -137.500 kJ/mol, respectively.Communicated by Ramaswamy H. Sarma.

Development of 2-chloroquinoline based heterocyclic frameworks as dual inhibitors of SARS-CoV-2 MPro and PLPro

Evolution of new variants of SARS-CoV-2 warrant the need for the continued efforts in identifying target-oriented new drugs. Dual targeting agents against M^Pro and PL^Pro not only overcome the incomplete efficacy but also the drug resistance, which is common problem. Since both these are cysteine proteases, we designed 2-chloroquinoline based molecules with additional imine moiety in the middle as possible nucleophilic warheads. In the first round of design and synthesis, three molecules (C3, C4 and C5) inhibited (Ki < 2 μM) only M^Pro by binding covalently to C145 and one molecule (C10) inhibited both the proteases non-covalently (Ki < 2 μM) with negligible cytotoxicity. Further conversion of the imine in C10 to azetidinone (C11) improved the potency against both the enzymes in the nanomolar range (820 nM against M^Pro and 350 nM against PL^Pro) with no cytotoxicity. Conversion of imine to thiazolidinone (C12), reduced the inhibition by 3-5 folds against both the enzymes. Biochemical and computational studies suggest that C10-C12 bind in the substrate binding pocket of M^Pro and in the BL2 loop of the PL^Pro. Since these dual inhibitors have least cytotoxicity, they could be further explored as therapeutics against the SARS-CoV-2 and other analogous viruses.

Identification and characterization of aurintricarboxylic acid as a potential inhibitor of SARS-CoV-2 PLpro

As the global health crisis due to evolution of mutations in SARS-CoV-2 continues, it is important to develop several effective antivirals to control the disease. Targeting papain-like protease (PLpro) of SARS-CoV-2 for drug development is a promising strategy due to its dual role in promoting viral replication and dysregulating host immune responses. Here, we screened a library of compounds to find potential inhibitors of PLpro. We find aurintricarboxylic acid (ATA) inhibits PLpro with K_i and IC₅₀ values of 16 μM and 30 μM, respectively. The binding of ATA to PLpro was further characterized using isothermal titration calorimetry, differential scanning fluorimetry, dynamic light scattering and circular dichroism spectrometry. In vitro assays showed the antiviral potential of ATA with IC₅₀ of 50 μM. In vivo efficacy was studied in Syrian hamsters and the results are being discussed.