Crystal structures reveal an induced-fit binding of a substrate-like Aza-peptide epoxide to SARS coronavirus main peptidase

P1 Glutamine isosteres in the design of inhibitors of 3C/3CL protease of human viruses of the Pisoniviricetes class

Viral infections are one of the leading causes of acute morbidity in humans and much endeavour has been made by the synthetic community for the development of drugs to treat associated diseases. Peptide-based enzyme inhibitors, usually short sequences of three or four residues, are one of the classes of compounds currently under development for enhancement of their activity and pharmaceutical properties. This review reports the advances made in the design of inhibitors targeting the family of highly conserved viral proteases 3C/3CLpro, which play a key role in viral replication and present minimal homology with mammalian proteases. Particular focus is put on the reported development of P1 glutamine isosteres to generate potent inhibitors mimicking the natural substrate sequence at the site of recognition.'

Detailed Insights into the Inhibitory Mechanism of New Ebselen Derivatives against Main Protease (Mpro) of Severe Acute Respiratory Syndrome Coronavirus-2 (SARS-CoV-2)

SARS-CoV-2 main protease (M^pro/3CL^pro) is a crucial target for therapeutics, which is responsible for viral polyprotein cleavage and plays a vital role in virus replication and survival. Recent studies suggest that 2-phenylbenzisoselenazol-3(2H)-one (ebselen) is a potent covalent inhibitor of M^pro, which affects its enzymatic activity and virus survival. Herein, we synthesized various ebselen derivatives to understand the mechanism of M^pro inhibition by ebselen. Using ebselen derivatives, we characterized the detailed interaction mechanism with M^pro. We discovered that modification of the parent ebselen inhibitor with an electron-withdrawing group (NO₂) increases the inhibition efficacy by 2-fold. We also solved the structure of an M^pro complex with an ebselen derivative showing the mechanism of inhibition by blocking the catalytic Cys145 of M^pro. Using a combination of crystal structures and LC-MS data, we showed that M^pro hydrolyzes the new ebselen derivative and leaves behind selenium (Se) bound with Cys145 of the catalytic dyad of M^pro. We also described the binding profile of ebselen-based inhibitors using molecular modeling predictions supported by binding and inhibition assays. Furthermore, we have also solved the crystal structure of catalytically inactive mutant H41N-M^pro, which represents the inactive state of the protein where the substrate binding pocket is blocked. The inhibited structure of H41N-M^pro shows gatekeeper residues in the substrate binding pocket responsible for blocking the substrate binding; mutation of these gatekeeper residues leads to hyperactive M^pro.

Identification of doxorubicin as a potential therapeutic against SARS-CoV-2 (COVID-19) protease: a molecular docking and dynamics simulation studies

After one year, the COVID-19 pandemic caused by SARS-CoV-2 is still the largest concern for the scientific community. Of the many recognized drug targets of SARS-CoV-2, the main protease is one of the most important target due to its function in viral replication. We conducted an in silico study with repurposing drugs of antibiotics class against virus protease and peptidase using AutoDock tool. The following significant binding energy interaction was observed with protease (PDB: 6LU7) like piperacillin -7.25; tobramycin -9.20 and doxorubicin (Doxo) -10.04 kcal/mol and with peptidase (PDB: 2GTB) piperacillin -7.08; tobramycin -8.54 and Doxo -9.89 kcal/mol. Furthermore, the interaction and stability behavior of the Doxo-protease and Doxo-peptidase complexes were analyzed for a 100-nanosecond (ns) time. Calculated RMSD values observed using molecular dynamics simulation (MDS) were found to be 0.15-0.25 nm, RMSF calculation per residues showed a value near 0.2 nm and Rg values remained approximately 2.25 nm. MM-PBSA analysis of total binding energy calculation of Doxo-protease and Doxo-peptidase complexes are found to be -148.692 and -105.367 kJ/mol, respectively. Moreover, amino acid residue ASP-197 showed the lowest contribution binding energy i.e. -18.1185 kJ/mol, and amino acid residue ASP-187 showed -17.0267 kJ/mol contribution energy. Thus, significant docking interaction and stable dynamicity of Doxo-protease complex with time was suggested that Doxo could be a choice to inhibit potentially the viral proteases that could prevent the entry inside the host cell to control the COVID-19 disease. Communicated by Ramaswamy H. Sarma.

Porcine Epidemic Diarrhea: Causative Agent, Epidemiology, Clinical Characteristics, and Treatment Strategy Targeting Main Protease

Aims:

This aimed to study the causative agent, epidemiology, clinical characteristics, and treatment strategy targeting the main protease in porcine epidemic diarrhea.

Background:

Porcine epidemic diarrhea (PED) is a contagious intestinal viral infection causing severe diarrhea, vomiting, and dehydration in pigs. High rates of mortalities and severe morbidities, approaching 100%, are reported in piglets infected with PEDV. In recent years, PED has been observed to influence the swine-farming nations in Europe, Asia, the USA, South Korea, and Canada. The PED virus (PEDV) transmission takes place through a faecal-oral route.

Objective:

The objective is to review the characteristics of PEDV and its role in the disease. In addition, we aim to outline some possible methods to combat PED infection, including targeting the main protease of coronavirus and their future perspectives.

Method:

This study is a review of literature on the PED virus.

Results:

Apart from symptomatic treatment and supportive care, there is no available specific treatment for PEDV. Appropriate disinfectants and cleaning are pivotal for the control of PEDV. To date, apart from anti-PEDV inhibitors, there are no specific drugs available commercially to treat the disease. Therefore, 3C-like protease (3CLpro) in PEDV that has highly conserved structure and catalytic mechanism serves as an alluring drug as it plays a vital role during viral polyprotein processing at the time of infection.

Conclusion:

A well synchronized and collective effort of scientists, swine veterinarians, pork industry experts, and associated authorities is essential for the accomplishment of proper execution of these required measures.

In silico studies on structural inhibition of SARS-CoV-2 main protease Mpro by major secondary metabolites of Andrographis paniculata and Cinchona officinalis

The COVID-19 infection by Novel Corona Virus (SARS-CoV-2) has become one of the largest pandemic diseases, with cumulative confirmed infections of 275,233,892 and 5,364,996 deaths to date according to World Health Organization. Due to the absence of any approved antiviral drug to treat COVID-19, its lethality is getting severe with time. The main protease of SARS-CoV-2, M^pro is considered one of the potential drug targets because of its role in processing proteins translated from viral RNA. In the present study, four of the plant metabolites, 14-deoxy-11,12-didehydroandrographolide, andrograpanin, quinine, cinchonine from two eminent medicinal plants Andrographis paniculata and Cinchona officinalis, have been evaluated against the main protease of SARS-CoV-2 through in-silico molecular docking and molecular dynamics simulation study. From the result interpretations, it is found that andrograpanin has strong binding affinities with the target protein in its active site with potential negative energies. Molecular Dynamic simulation and MMGBSA studies suggest that earlier reported N3 inhibitor and andrograpanin exhibit effective binding interactions involving identical amino acid residues with the same binding pockets of the main protease of SARS-CoV-2. Therefore, the theoretical experiment suggests that andrograpanin, could be considered the promising inhibitor against SARS-CoV-2 M^pro.

Structure-Based Optimization of ML300-Derived, Noncovalent Inhibitors Targeting the Severe Acute Respiratory Syndrome Coronavirus 3CL Protease (SARS-CoV-2 3CLpro)

Starting from the MLPCN probe compound ML300, a structure-based optimization campaign was initiated against the recent severe acute respiratory syndrome coronavirus (SARS-CoV-2) main protease (3CLpro). X-ray structures of SARS-CoV-1 and SARS-CoV-2 3CLpro enzymes in complex with multiple ML300-based inhibitors, including the original probe ML300, were obtained and proved instrumental in guiding chemistry toward probe compound 41 (CCF0058981). The disclosed inhibitors utilize a noncovalent mode of action and complex in a noncanonical binding mode not observed by peptidic 3CLpro inhibitors. In vitro DMPK profiling highlights key areas where further optimization in the series is required to obtain useful in vivo probes. Antiviral activity was established using a SARS-CoV-2-infected Vero E6 cell viability assay and a plaque formation assay. Compound 41 demonstrates nanomolar activity in these respective assays, comparable in potency to remdesivir. These findings have implications for antiviral development to combat current and future SARS-like zoonotic coronavirus outbreaks.

Allosteric Regulation of 3CL Protease of SARS-CoV-2 and SARS-CoV Observed in the Crystal Structure Ensemble

The 3C-like protease (3CL^pro) of SARS-CoV-2 is a potential therapeutic target for COVID-19. Importantly, it has an abundance of structural information solved as a complex with various drug candidate compounds. Collecting these crystal structures (83 Protein Data Bank (PDB) entries) together with those of the highly homologous 3CL^pro of SARS-CoV (101 PDB entries), we constructed the crystal structure ensemble of 3CL^pro to analyze the dynamic regulation of its catalytic function. The structural dynamics of the 3CL^pro dimer observed in the ensemble were characterized by the motions of four separate loops (the C-loop, E-loop, H-loop, and Linker) and the C-terminal domain III on the rigid core of the chymotrypsin fold. Among the four moving loops, the C-loop (also known as the oxyanion binding loop) causes the order (active)-disorder (collapsed) transition, which is regulated cooperatively by five hydrogen bonds made with the surrounding residues. The C-loop, E-loop, and Linker constitute the major ligand binding sites, which consist of a limited variety of binding residues including the substrate binding subsites. Ligand binding causes a ligand size dependent conformational change to the E-loop and Linker, which further stabilize the C-loop via the hydrogen bond between the C-loop and E-loop. The T285A mutation from SARS-CoV 3CL^pro to SARS-CoV-2 3CL^pro significantly closes the interface of the domain III dimer and allosterically stabilizes the active conformation of the C-loop via hydrogen bonds with Ser1 and Gly2; thus, SARS-CoV-2 3CL^pro seems to have increased activity relative to that of SARS-CoV 3CL^pro.

Molybdenum Carbonyl Complexes with Benzimidazole Derivatives Against SARS-CoV-2 by Molecular Docking and DFT/TDDFT Methods

Benzimidazole derivative molecules attract attention of scientists due to their bioactivities. The dramatic changes in recorded activities according to the type and position of the substituents motivate synthesis and analysis of new molecules. Commercial benzimidazole-based molecules have been used in therapeutic procedures. It is known that the activities of metal complexes with benzimidazole derivative ligands have different activities when compared to the benzimidazole main structure. Nowadays, one of the most important health problems is COVID-19, which caused the pandemic that we are still experiencing. Although vaccine studies are important to overcome acute problems, regarding the possible post-vaccination adverse effects, the need for new drugs against the virus is obvious. Considering the urgency and the limited facilities during the pandemic, preliminary in silico studies of candidate molecules are essential. In this study, {[bis-(N-benzylbenzimidazole)] tetracarbonylmolybdenum}, {[bis-(N-4-chlorobenzylbenzimidazole)] tetracarbonylmolybdenum} and {[bis-(N-4-methoxybenzylbenzimidazole)] tetracarbonylmolybdenum} were synthesized and characterized. The optimization and the structural analysis of these molecules were performed by DFT/TDDFT methods. The molecules were docked into SARS coronavirus main peptidase (PDB ID: 2gtb), COVID-19 main protease in complex with Z219104216 (PDB ID: 5r82), COVID-19 main protease in complex with an inhibitor N3 (PDB ID: 6lu7) and Papain-like protease of SARS-CoV-2 (PDB ID: 6w9c) crystal structures for evaluation of their anti-viral activity.

Challenges of short substrate analogues as SARS-CoV-2 main protease inhibitors

Specific anti-coronaviral drugs complementing available vaccines are urgently needed to fight the COVID-19 pandemic. Given its high conservation across the betacoronavirus genus and dissimilarity to human proteases, the SARS-CoV-2 main protease (M^pro) is an attractive drug target. SARS-CoV-2 M^pro inhibitors have been developed at unprecedented speed, most of them being substrate-derived peptidomimetics with cysteine-modifying warheads. In this study, M^pro has proven resistant towards the identification of high-affinity short substrate-derived peptides and peptidomimetics without warheads. 20 cyclic and linear substrate analogues bearing natural and unnatural residues, which were predicted by computational modelling to bind with high affinity and designed to establish structure-activity relationships, displayed no inhibitory activity at concentrations as high as 100 μM. Only a long linear peptide covering residues P₆ to P₅' displayed moderate inhibition (K_i = 57 µM). Our detailed findings will inform current and future drug discovery campaigns targeting M^pro.

Constituents of buriti oil (Mauritia flexuosa L.) like inhibitors of the SARS-Coronavirus main peptidase: an investigation by docking and molecular dynamics

Statistics show alarming numbers of infected and killed in the world, caused by the Covid-19 pandemic, which still doesn't have a specific treatment and effective in combating all efforts to seek treatments and medications against this disease. Natural products are of relevant interest in the search for new drugs. Thus, Buriti oil (Mauritia flexuosa L.) is a natural product extracted from the fruit of the palm and is quite common in the legal Amazon region, Brazil. In the present work, the anti-Covid-19 biological activity of some constituents of Buriti oil was investigated using in silico methods of Molecular Docking and Molecular Dynamics Simulations. The main results of Molecular Docking revealed favorable interaction energies in the formation of the 2GTB peptidase complex (main peptidase of SARS-CoV) with the 13-cis-β-carotene ligands (ΔGbind = -10.23Kcal mol-1), 9-cis -β-carotene (ΔGbind = -9.82Kcal mol-1), and α-carotene (ΔGbind = -8.34Kcal mol-1). Molecular Dynamics simulations demonstrated considerable interaction for these ligands with emphasis on α-carotene. Such theoretical results encourage and enable a direction for experimental studies in vitro and in vivo, essential in the development of new drugs with enzymatic inhibitory action for Covid-19.Communicated by Ramaswamy H. Sarma.

A Blueprint for High Affinity SARS-CoV-2 Mpro Inhibitors from Activity-Based Compound Library Screening Guided by Analysis of Protein Dynamics

The SARS-CoV-2 coronavirus outbreak continues to spread at a rapid rate worldwide. The main protease (Mpro) is an attractive target for anti-COVID-19 agents. Unexpected difficulties have been encountered in the design of specific inhibitors. Here, by analyzing an ensemble of ∼30 000 SARS-CoV-2 Mpro conformations from crystallographic studies and molecular simulations, we show that small structural variations in the binding site dramatically impact ligand binding properties. Hence, traditional druggability indices fail to adequately discriminate between highly and poorly druggable conformations of the binding site. By performing ∼200 virtual screenings of compound libraries on selected protein structures, we redefine the protein's druggability as the consensus chemical space arising from the multiple conformations of the binding site formed upon ligand binding. This procedure revealed a unique SARS-CoV-2 Mpro blueprint that led to a definition of a specific structure-based pharmacophore. The latter explains the poor transferability of potent SARS-CoV Mpro inhibitors to SARS-CoV-2 Mpro, despite the identical sequences of the active sites. Importantly, application of the pharmacophore predicted novel high affinity inhibitors of SARS-CoV-2 Mpro, that were validated by in vitro assays performed here and by a newly solved X-ray crystal structure. These results provide a strong basis for effective rational drug design campaigns against SARS-CoV-2 Mpro and a new computational approach to screen protein targets with malleable binding sites.

Perspectives on plant flavonoid quercetin-based drugs for novel SARS-CoV-2

Background

The world pandemic COVID-19 caused by SARS-CoV-2 is currently claiming thousands of lives. Flavonoids abundantly present in the fruits and vegetables, especially quercetin, are shown to have antiviral activities.

Main text

This paper reviews the capability of the plant flavonoid quercetin to fight the novel coronavirus and the possibility for drug development based on this. The mode of action explaining the known pathways through which this molecule succeeds in the antiviral activity, action of quercetin on SARS-CoV-2 main protease 3CL^pro, antiviral activities of its derivatives on human viruses, effect of combination of zinc co-factor along with quercetin in the COVID-19 treatment, and the regulation of miRNA genes involved in the viral pathogenesis are discussed. Proof for this concept is provided following the virtual screening using ten key enzymes of SARS-CoV-2 and assessing their interactions. Active residues in the 3D structures have been predicted using CASTp and were docked against quercetin. Key proteins 3CL^pro, spike glycoprotein/ human ACE2-BOAT1 complex, RNA-dependent RNA polymerase, main peptidase, spike glycoprotein, RNA replicase, RNA binding protein, papain-like protease, SARS papain-like protease/ deubiquitinase, and complex of main peptidase with an additional Ala at the N-terminus of each protomer, have shown the binding energies ranging between − 6.71 and − 3.37 kcal/ Mol, showing that quercetin is a potential drug candidate inhibiting multiple SARS-CoV-2 enzymes.

Conclusion

The antiviral properties of flavonoid and the molecular mechanisms involved are reviewed. Further, proof for this concept is given by docking of key proteins from SARS-CoV-2 with quercetin.

Graphical abstract

Antiviral Effects of Polyphenols from Marine Algae

The disease-preventive and medicinal properties of plant polyphenolic compounds have long been known. As active ingredients, they are used to prevent and treat many noncommunicable diseases. In recent decades, marine macroalgae have attracted the attention of biotechnologists and pharmacologists as a promising and almost inexhaustible source of polyphenols. This heterogeneous group of compounds contains many biopolymers with unique structure and biological properties that exhibit high anti-infective activity. In the present review, the authors focus on the antiviral potential of polyphenolic compounds (phlorotannins) from marine algae and consider the mechanisms of their action as well as other biological properties of these compounds that have effects on the progress and outcome of viral infections. Effective nutraceuticals, to be potentially developed on the basis of algal polyphenols, can also be used in the complex therapy of viral diseases. It is necessary to extend in vivo studies on laboratory animals, which subsequently will allow proceeding to clinical tests. Polyphenolic compounds have a great potential as active ingredients to be used for the creation of new antiviral pharmaceutical substances.

Protein structural heterogeneity: A hypothesis for the basis of proteolytic recognition by the main protease of SARS-CoV and SARS-CoV-2

The main protease (M^pro) of SARS-CoV and SARS-CoV-2 is a key enzyme in viral replication and a promising target for the development of antiviral therapeutics. The understanding of this protein is based on a number of observations derived from earlier x-ray structures, which mostly consider substrates or ligands as the main reason behind modulation of the active site. This lead to the concept of substrate-induced subsite cooperativity as an initial attempt to explain the dual binding specificity of this enzyme in recognizing the cleavage sequences at its N- and C-termini, which are important processing steps in obtaining the mature protease. The presented hypothesis proposes that structural heterogeneity is a property of the enzyme, independent of the presence of a substrate or ligand. Indeed, the analysis of M^pro structures of SARS-CoV and SARS-CoV-2 reveals a conformational diversity for the catalytically competent state in ligand-free structures. Variation in the binding site appears to result from flexibility at residues lining the S₁ subpocket and segments incorporating methionine 49 and glutamine 189. The structural evidence introduces “structure-based recognition” as a new paradigm in substrate proteolysis by M^pro. In this concept, the binding space in subpockets of the enzyme varies in a non-cooperative manner, causing distinct conformations, which recognize and process different cleavage sites, as the N- and C-termini. Insights into the recognition basis of the protease provide explanation to the ordered processing of cleavage sites. The hypothesis expands the conformational space of the enzyme and consequently opportunities for drug development and repurposing efforts.

Innate immune evasion mediated by picornaviral 3C protease: Possible lessons for coronaviral 3C-like protease?

Severe acute respiratory syndrome coronavirus-2 is the etiological agent of the ongoing pandemic of coronavirus disease-2019, a multi-organ disease that has triggered an unprecedented global health and economic crisis. The virally encoded 3C-like protease (3CL^pro ), which is named after picornaviral 3C protease (3C^pro ) due to their similarities in substrate recognition and enzymatic activity, is essential for viral replication and has been considered as the primary drug target. However, information regarding the cellular substrates of 3CL^pro and its interaction with the host remains scarce, though recent work has begun to shape our understanding more clearly. Here we summarized and compared the mechanisms by which picornaviruses and coronaviruses have evolved to evade innate immune surveillance, with a focus on the established role of 3C^pro in this process. Through this comparison, we hope to highlight the potential action and mechanisms that are conserved and shared between 3C^pro and 3CL^pro . In this review, we also briefly discussed current advances in the development of broad-spectrum antivirals targeting both 3C^pro and 3CL^pro .

In silico analysis of phytochemicals as potential inhibitors of proteases involved in SARS-CoV-2 infection

In silico analysis of six phytochemicals, flabelliferin, marmelosin, piperine, ocimin, curcumin and leucoanthocyanin, along with three drug compounds, nelfinavir, remdesivir and hydroxychloroquine, as positive control against drug targets of one SARS-CoV-2 viral protease, COVID-19 main protease (SARS CoV-2 3CL^pro/M^pro), two coronavirus proteases, SARS-CoV main peptidase (SARS CoV M^pro), SARS-CoV main proteinase (SARS CoV 3CL^pro), and one human cellular transmembrane serine proteinase (TMPRSS2), was carried out. Except leucoanthocyanin all other phytochemicals proved better than all three positive control drugs against SARS-CoV main peptidase, whereas, flabelliferin was found to be the potential inhibitor for SARS-CoV main proteinase out performing all the positive control drugs and phytochemicals. Amongst the compounds studied, the best inhibitor for COVID-19 main protease was nelfinavir followed by flabelliferin and ocimin. Flabelliferin was found to the best promising inhibitor of human cellular transmembrane serine proteinase, followed by nelfinavir, curcumin, piperine and marmelosin. The result on the inhibition of human cellular transmembrane serine proteinase against COVID-19 has a stable therapeutic advantage as mutation may quickly occur on viral drug targets. Hence, all the phytochemicals tested in the present study are the potential inhibitors of the all the four drug targets and can form a part of therapeutics against COVID-19 with further clinical studies.

Communicated by Ramaswamy H. Sarma

Conserved interactions required for inhibition of the main protease of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2)

The COVID-19 pandemic caused by the SARS-CoV-2 requires a fast development of antiviral drugs. SARS-CoV-2 viral main protease (Mpro, also called 3C-like protease, 3CLpro) is a potential target for drug design. Crystal and co-crystal structures of the SARS-CoV-2 Mpro have been solved, enabling the rational design of inhibitory compounds. In this study we analyzed the available SARS-CoV-2 and the highly similar SARS-CoV-1 crystal structures. We identified within the active site of the Mpro, in addition to the inhibitory ligands' interaction with the catalytic C145, two key H-bond interactions with the conserved H163 and E166 residues. Both H-bond interactions are present in almost all co-crystals and are likely to occur also during the viral polypeptide cleavage process as suggested from docking of the Mpro cleavage recognition sequence. We screened in silico a library of 6900 FDA-approved drugs (ChEMBL) and filtered using these key interactions and selected 29 non-covalent compounds predicted to bind to the protease. Additional screen, using DOCKovalent was carried out on DrugBank library (11,414 experimental and approved drugs) and resulted in 6 covalent compounds. The selected compounds from both screens were tested in vitro by a protease activity inhibition assay. Two compounds showed activity at the 50 µM concentration range. Our analysis and findings can facilitate and focus the development of highly potent inhibitors against SARS-CoV-2 infection.

The recent outbreaks of human coronaviruses: A medicinal chemistry perspective

Coronaviruses (CoVs) infect both humans and animals. In humans, CoVs can cause respiratory, kidney, heart, brain, and intestinal infections that can range from mild to lethal. Since the start of the 21st century, three β‐coronaviruses have crossed the species barrier to infect humans: severe‐acute respiratory syndrome (SARS)‐CoV‐1, Middle East respiratory syndrome (MERS)‐CoV, and SARS‐CoV‐2 (2019‐nCoV). These viruses are dangerous and can easily be transmitted from human to human. Therefore, the development of anticoronaviral therapies is urgently needed. However, to date, no approved vaccines or drugs against CoV infections are available. In this review, we focus on the medicinal chemistry efforts toward the development of antiviral agents against SARS‐CoV‐1, MERS‐CoV, SARS‐CoV‐2, targeting biochemical events important for viral replication and its life cycle. These targets include the spike glycoprotein and its host‐receptors for viral entry, proteases that are essential for cleaving polyproteins to produce functional proteins, and RNA‐dependent RNA polymerase for viral RNA replication.

Knowledge-based structural models of SARS-CoV-2 proteins and their complexes with potential drugs

The World Health Organization (WHO) has declared the coronavirus disease 2019 (COVID‐19) caused by the novel coronavirus SARS‐CoV‐2 a pandemic. There is, however, no confirmed anti‐COVID‐19 therapeutic currently. In order to assist structure‐based discovery efforts for repurposing drugs against this disease, we constructed knowledge‐based models of SARS‐CoV‐2 proteins and compared the ligand molecules in the template structures with approved/experimental drugs and components of natural medicines. Our theoretical models suggest several drugs, such as carfilzomib, sinefungin, tecadenoson, and trabodenoson, that could be further investigated for their potential for treating COVID‐19.

Pharmacological Therapeutics Targeting RNA-Dependent RNA Polymerase, Proteinase and Spike Protein: From Mechanistic Studies to Clinical Trials for COVID-19

An outbreak of novel coronavirus-related pneumonia COVID-19, that was identified in December 2019, has expanded rapidly, with cases now confirmed in more than 211 countries or areas. This constant transmission of a novel coronavirus and its ability to spread from human to human have prompted scientists to develop new approaches for treatment of COVID-19. A recent study has shown that remdesivir and chloroquine effectively inhibit the replication and infection of severe acute respiratory syndrome coronavirus-2 (SARS-CoV-2, 2019-nCov) in vitro. In the United States, one case of COVID-19 was successfully treated with compassionate use of remdesivir in January of 2020. In addition, a clinically proven protease inhibitor, camostat mesylate, has been demonstrated to inhibit Calu-3 infection with SARS-CoV-2 and prevent SARS-2-spike protein (S protein)-mediated entry into primary human lung cells. Here, we systemically discuss the pharmacological therapeutics targeting RNA-dependent RNA polymerase (RdRp), proteinase and S protein for treatment of SARS-CoV-2 infection. This review should shed light on the fundamental rationale behind inhibition of SARS-CoV-2 enzymes RdRp as new therapeutic approaches for management of patients with COVID-19. In addition, we will discuss the viability and challenges in targeting RdRp and proteinase, and application of natural product quinoline and its analog chloroquine for treatment of coronavirus infection. Finally, determining the structural-functional relationships of the S protein of SARS-CoV-2 will provide new insights into inhibition of interactions between S protein and angiotensin-converting enzyme 2 (ACE2) and enable us to develop novel therapeutic approaches for novel coronavirus SARS-CoV-2.

An Overview of Severe Acute Respiratory Syndrome-Coronavirus (SARS-CoV) 3CL Protease Inhibitors: Peptidomimetics and Small Molecule Chemotherapy

Severe acute respiratory syndrome (SARS) is caused by a newly emerged coronavirus that infected more than 8000 individuals and resulted in more than 800 (10-15%) fatalities in 2003. The causative agent of SARS has been identified as a novel human coronavirus (SARS-CoV), and its viral protease, SARS-CoV 3CL(pro), has been shown to be essential for replication and has hence been recognized as a potent drug target for SARS infection. Currently, there is no effective treatment for this epidemic despite the intensive research that has been undertaken since 2003 (over 3500 publications). This perspective focuses on the status of various efficacious anti-SARS-CoV 3CL(pro) chemotherapies discovered during the last 12 years (2003-2015) from all sources, including laboratory synthetic methods, natural products, and virtual screening. We describe here mainly peptidomimetic and small molecule inhibitors of SARS-CoV 3CL(pro). Attempts have been made to provide a complete description of the structural features and binding modes of these inhibitors under many conditions.

Potential Broad Spectrum Inhibitors of the Coronavirus 3CLpro: A Virtual Screening and Structure-Based Drug Design Study

Human coronaviruses represent a significant disease burden; however, there is currently no antiviral strategy to combat infection. The outbreak of severe acute respiratory syndrome (SARS) in 2003 and Middle East respiratory syndrome (MERS) less than 10 years later demonstrates the potential of coronaviruses to cross species boundaries and further highlights the importance of identifying novel lead compounds with broad spectrum activity. The coronavirus 3CL(pro) provides a highly validated drug target and as there is a high degree of sequence homology and conservation in main chain architecture the design of broad spectrum inhibitors is viable. The ZINC drugs-now library was screened in a consensus high-throughput pharmacophore modeling and molecular docking approach by Vina, Glide, GOLD and MM-GBSA. Molecular dynamics further confirmed results obtained from structure-based techniques. A highly defined hit-list of 19 compounds was identified by the structure-based drug design methodologies. As these compounds were extensively validated by a consensus approach and by molecular dynamics, the likelihood that at least one of these compounds is bioactive is excellent. Additionally, the compounds segregate into 15 significantly dissimilar (p < 0.05) clusters based on shape and features, which represent valuable scaffolds that can be used as a basis for future anti-coronaviral inhibitor discovery experiments. Importantly though, the enriched subset of 19 compounds identified from the larger library has to be validated experimentally.

Structures of the Middle East respiratory syndrome coronavirus 3C-like protease reveal insights into substrate specificity

Middle East respiratory syndrome coronavirus (MERS‐CoV) is a highly pathogenic virus that causes severe respiratory illness accompanied by multi‐organ dysfunction, resulting in a case fatality rate of approximately 40%. As found in other coronaviruses, the majority of the positive‐stranded RNA MERS‐CoV genome is translated into two polyproteins, one created by a ribosomal frameshift, that are cleaved at three sites by a papain‐like protease and at 11 sites by a 3C‐like protease (3CL^pro). Since 3CL^pro is essential for viral replication, it is a leading candidate for therapeutic intervention. To accelerate the development of 3CL^pro inhibitors, three crystal structures of a catalytically inactive variant (C148A) of the MERS‐CoV 3CL^pro enzyme were determined. The aim was to co‐crystallize the inactive enzyme with a peptide substrate. Fortuitously, however, in two of the structures the C‐terminus of one protomer is bound in the active site of a neighboring molecule, providing a snapshot of an enzyme–product complex. In the third structure, two of the three protomers in the asymmetric unit form a homodimer similar to that of SARS‐CoV 3CL^pro; however, the third protomer adopts a radically different conformation that is likely to correspond to a crystallographic monomer, indicative of substantial structural plasticity in the enzyme. The results presented here provide a foundation for the structure‐based design of small‐molecule inhibitors of the MERS‐CoV 3CL^pro enzyme.

Evidence for substrate binding-induced zwitterion formation in the catalytic Cys-His dyad of the SARS-CoV main protease

The coronavirus main protease (M(pro)) represents an attractive drug target for antiviral therapy of coronavirus (CoV) infections, including severe acute respiratory syndrome (SARS). The SARS-CoV M(pro) and related CoV proteases have several distinct features, such as an uncharged Cys-His catalytic dyad embedded in a chymotrypsin-like protease fold, that clearly separate these enzymes from archetypical cysteine proteases. To further characterize the catalytic system of CoV main proteases and to obtain information about improved inhibitors, we performed comprehensive simulations of the proton-transfer reactions in the SARS-CoV M(pro) active site that lead to the Cys(-)/His(+) zwitterionic state required for efficient proteolytic activity. Our simulations, comprising the free enzyme as well as substrate-enzyme and inhibitor-enzyme complexes, lead us to predict that zwitterion formation is fostered by substrate binding but not inhibitor binding. This indicates that M(pro) employs a substrate-induced catalytic mechanism that further enhances its substrate specificity. Our computational data are in line with available experimental results, such as X-ray geometries, measured pKa values, mutagenesis experiments, and the measured differences between the kinetic parameters of substrates and inhibitors. The data also provide an atomistic picture of the formerly postulated electrostatic trigger involved in SARS-CoV M(pro) activity. Finally, they provide information on how a specific microenvironment may finely tune the activity of M(pro) toward specific viral protein substrates, which is known to be required for efficient viral replication. Our simulations also indicate that the low inhibition potencies of known covalently interacting inhibitors may, at least in part, be attributed to insufficient fostering of the proton-transfer reaction. These findings suggest ways to achieve improved inhibitors.

From SARS to MERS: crystallographic studies on coronaviral proteases enable antiviral drug design

This review focuses on the important contributions that macromolecular crystallography has made over the past 12 years to elucidating structures and mechanisms of the essential proteases of coronaviruses, the main protease (M(pro) ) and the papain-like protease (PL(pro) ). The role of X-ray crystallography in structure-assisted drug discovery against these targets is discussed. Aspects dealt with in this review include the emergence of the SARS coronavirus in 2002-2003 and of the MERS coronavirus 10 years later and the origins of these viruses. The crystal structure of the free SARS coronavirus M(pro) and its dependence on pH is discussed, as are efforts to design inhibitors on the basis of these structures. The mechanism of maturation of the enzyme from the viral polyprotein is still a matter of debate. The crystal structure of the SARS coronavirus PL(pro) and its complex with ubiquitin is also discussed, as is its orthologue from MERS coronavirus. Efforts at predictive structure-based inhibitor development for bat coronavirus M(pro) s to increase the preparedness against zoonotic transmission to man are described as well. The paper closes with a brief discussion of structure-based discovery of antivirals in an academic setting.

Dynamically-driven enhancement of the catalytic machinery of the SARS 3C-like protease by the S284-T285-I286/A mutations on the extra domain

Previously we revealed that the extra domain of SARS 3CLpro mediated the catalysis via different mechanisms. While the R298A mutation completely abolished the dimerization, thus resulting in the inactive catalytic machinery, N214A inactivated the enzyme by altering its dynamics without significantly perturbing its structure. Here we studied another mutant with S284-T285-I286 replaced by Ala (STI/A) with a 3.6-fold activity increase and slightly enhanced dimerization. We determined its crystal structure, which still adopts the dimeric structure almost identical to that of the wild-type (WT), except for slightly tighter packing between two extra-domains. We then conducted 100-ns molecular dynamics (MD) simulations for both STI/A and WT, the longest reported so far for 3CLpro. In the simulations, two STI/A extra domains become further tightly packed, leading to a significant volume reduction of the nano-channel formed by residues from both catalytic and extra domains. The enhanced packing appears to slightly increase the dynamic stability of the N-finger and the first helix residues, which subsequently triggers the redistribution of dynamics over residues directly contacting them. This ultimately enhances the dynamical stability of the residues constituting the catalytic dyad and substrate-binding pockets. Further correlation analysis reveals that a global network of the correlated motions exists in the protease, whose components include all residues identified so far to be critical for the dimerization and catalysis. Most strikingly, the N214A mutation globally decouples this network while the STI/A mutation alters the correlation pattern. Together with previous results, the present study establishes that besides the classic structural allostery, the dynamic allostery also operates in the SARS 3CLpro, which is surprisingly able to relay the perturbations on the extra domain onto the catalytic machinery to manifest opposite catalytic effects. Our results thus imply a promising avenue to design specific inhibitors for 3CL proteases by disrupting their dynamic correlation network.

Discovery of N-(benzo[1,2,3]triazol-1-yl)-N-(benzyl)acetamido)phenyl) carboxamides as severe acute respiratory syndrome coronavirus (SARS-CoV) 3CLpro inhibitors: identification of ML300 and noncovalent nanomolar inhibitors with an induced-fit binding

Herein we report the discovery and SAR of a novel series of SARS-CoV 3CLpro inhibitors identified through the NIH Molecular Libraries Probe Production Centers Network (MLPCN). In addition to ML188, ML300 represents the second probe declared for 3CLpro from this collaborative effort. The X-ray structure of SARS-CoV 3CLpro bound with a ML300 analog highlights a unique induced-fit reorganization of the S2-S4 binding pockets leading to the first sub-micromolar noncovalent 3CLpro inhibitors retaining a single amide bond.

Benchmark Study for the Cysteine-Histidine Proton Transfer Reaction in a Protein Environment: Gas Phase, COSMO, QM/MM Approaches

Proton transfer reactions are of crucial interest for the investigation of proteins. We have investigated the accuracy of commonly used quantum chemical methods for the description of proton transfer reactions in different environments (gas phase, COSMO, QM/MM) using the proton transfer between the catalytic dyad residues cysteine 145 and histidine 41 of SARS coronavirus main protease as a case study. The test includes thermodynamic, kinetic, and structural properties. The study comprises computationally demanding ab initio approaches (HF, CC2, MP2, SCS-CC2, SCS-MP2, CCSD(T)), popular density functional theories (BLYP, B3LYP, M06-2X), and semiempirical methods (MNDO/d, AM1, RM1, PM3, PM6). The approximated coupled cluster approach LCCSD(T) is taken as a reference method. We find that the robustness of the tested methods with respect to the environment correlates well with the level of theory. As an example HF, CC2, MP2, and their SCS variants show similar errors for gas phase, COSMO, or QM/MM computations. In contrast for semiempirical methods, the errors strongly diversify if one goes from gas phase to COSMO or QM/MM. Particular problems are observed for the recent semiempirical methods PM6 and RM1, which show the best performance for gas phase calculations but possess larger errors in conjunction with COSMO. Finally, a combination of SCS-MP2 and B3LYP or M06-2X allows reliable estimates about remaining errors.

SARS coronavirus main proteinase 3.4.22.69

EC number

 3.4.22.69

Recommended name

 SARS coronavirus main proteinase

Synonyms

 3C-like protease <2,3> [9,16,38,49,51]

 3CL protease <2> [14,48]

 3cLpro <1,2,3> [7,11,13,16,19,28,38,49,51]

 C30.004 (Merops-ID)

 Mpro

 SARS 3C-like protease <2> [17]

 SARS 3C-like proteinase <2> [15,18,27]

 SARS 3CL protease <2> [31]

 SARS 3CLpro <2> [49]

 SARS CoV main proteinase <2> [1,2,4,5]

 SARS CoVMpro <2> [33]

 SARS Mpro <2> [25]

 SARS coronavirus 3C-like protease <2> [48]

 SARS coronavirus 3C-like proteinase <2> [50]

 SARS coronavirus 3CL protease <2> [20]

 SARS coronavirus main peptidase <2> [23]

 SARS coronavirus main protease <2> [25]

 SARS coronavirus main proteinase <2> [5,33]

 SARS main protease <2> [12,25]

 SARS-3CL protease <2> [48]

 SARS-3CLpro <2> [29,50]

 SARS-CoV 3C-like peptidaseSARS-CoV 3C-like peptidase<2> [24]

 SARS-CoV 3C-like protease<1> [19]

 SARS-CoV 3CL protease <2> [22,30,44,46]

 SARS-CoV 3CLpro <2> [32,36,38,44,45]

 SARS-CoV 3CLpro enzyme <2> [11]

 SARS-CoV Mpro <2> [21,40]

 SARS-CoV main protease <2> [21,26,43]

 SARS-coronavirus 3CL protease <2> [8]

 SARS-coronavirus main protease <2> [47]

 TGEV Mpro

 coronavirus 3C-like protease <1> [19]

 porcine transmissible gastroenteritis virus Mpro

 severe acute respiratory syndrome coronavirus 3C-like protease <2> [41,42]

 severe acute respiratory syndrome coronavirus main protease <2> [21]

 severe acute respiratory syndrome coronavirus main proteinase <2> [33]

CAS registry number

 218925-73-6

 37353-41-6

From head to toe of the norovirus 3C-like protease

Noroviruses are major causative agents of viral gastroenteritis in humans. Currently, there are no therapeutic medications to treat noroviral infections, nor are there effective vaccines against these pathogens. The viral 3C-like protease is solely responsible for the maturation of viral protein components. The crystal structures of the proteases were resolved at high atomic resolution. The protease was also explored by means of mutagenesis. These studies revealed the active-site amino acid residues and factors determining and affecting substrate specificity as well as the principle of architecting the protease molecule. The possible mechanism of proteolysis was also suggested. Consideration of the data accumulated thus far will be useful for development of therapeutic drugs targeting the viral protease.

Inhibitors of SARS-3CLpro: virtual screening, biological evaluation, and molecular dynamics simulation studies

SARS-CoV from the coronaviridae family has been identified as the etiological agent of Severe Acute Respiratory Syndrome (SARS), a highly contagious upper respiratory disease that reached epidemic status in 2002. SARS-3CL(pro), a cysteine protease indispensible to the viral life cycle, has been identified as one of the key therapeutic targets against SARS. A combined ligand and structure-based virtual screening was carried out against the Asinex Platinum collection. Multiple low micromolar inhibitors of the enzyme were identified through this search, one of which also showed activity against SARS-CoV in a whole cell CPE assay. Furthermore, multinanosecond explicit solvent simulations were carried out using the docking poses of the identified hits to study the overall stability of the binding site interactions as well as identify important changes in the interaction profile that were not apparent from the docking study. Cumulative analysis of the evaluated compounds and the simulation studies led to the identification of certain protein-ligand interaction patterns which would be useful in further structure based design efforts.

Dynamically-driven inactivation of the catalytic machinery of the SARS 3C-like protease by the N214A mutation on the extra domain

Despite utilizing the same chymotrypsin fold to host the catalytic machinery, coronavirus 3C-like proteases (3CLpro) noticeably differ from picornavirus 3C proteases in acquiring an extra helical domain in evolution. Previously, the extra domain was demonstrated to regulate the catalysis of the SARS-CoV 3CLpro by controlling its dimerization. Here, we studied N214A, another mutant with only a doubled dissociation constant but significantly abolished activity. Unexpectedly, N214A still adopts the dimeric structure almost identical to that of the wild-type (WT) enzyme. Thus, we conducted 30-ns molecular dynamics (MD) simulations for N214A, WT, and R298A which we previously characterized to be a monomer with the collapsed catalytic machinery. Remarkably, three proteases display distinctive dynamical behaviors. While in WT, the catalytic machinery stably retains in the activated state; in R298A it remains largely collapsed in the inactivated state, thus implying that two states are not only structurally very distinguishable but also dynamically well separated. Surprisingly, in N214A the catalytic dyad becomes dynamically unstable and many residues constituting the catalytic machinery jump to sample the conformations highly resembling those of R298A. Therefore, the N214A mutation appears to trigger the dramatic change of the enzyme dynamics in the context of the dimeric form which ultimately inactivates the catalytic machinery. The present MD simulations represent the longest reported so far for the SARS-CoV 3CLpro, unveiling that its catalysis is critically dependent on the dynamics, which can be amazingly modulated by the extra domain. Consequently, mediating the dynamics may offer a potential avenue to inhibit the SARS-CoV 3CLpro.

Severe acute respiratory syndrome (SARS) is the first emerging infectious disease of the 21st century which has not only caused rapid infection and death, but also triggered a dramatic social crisis. Its 3C-like protease is crucial for reproducing virus and thus represents a top target for drug design. Interestingly, unlike 3C protease such as from picorovirus, the SARS protease evolutionarily acquired a C-terminal extra domain with previously-unknown function. Immediately after SARS outbreak, we revealed that the extra domain was able to regulate the catalysis by controlling the dimerization essential for activity. Here, we studied one mutant with only slightly-weakened dimerization but almost completely abolished activity. We determined its three-dimensional structure but very unexpectedly it is almost identical to that of the wild-type enzyme. Therefore, we initiated 30-ns molecular dynamic simulations for five forms of the enzyme and the results demonstrate that the dynamical changes in this mutant are responsible for its inactivation. Therefore, the extra domain can also control the catalysis by modulating the enzyme dynamics. This is not only of fundamental significance to understanding how enzymes evolve, but also implies a novel avenue for design of anti-SARS molecules.

A structural view of the inactivation of the SARS coronavirus main proteinase by benzotriazole esters

The main proteinase (M^pro) of the severe acute respiratory syndrome (SARS) coronavirus is a principal target for the design of anticoronaviral compounds. Benzotriazole esters have been reported as potent nonpeptidic inhibitors of the enzyme, but their exact mechanism of action remains unclear. Here we present crystal structures of SARS-CoV M^pro, the active-site cysteine of which has been acylated by benzotriazole esters that act as suicide inhibitors. In one of the structures, the thioester product has been hydrolyzed and benzoic acid is observed to bind to the hydrophobic S2 pocket. This structure also features the enzyme with a shortened N-terminal segment (“amputated N finger”). The results further the understanding of the important role of the N finger for catalysis as well as the design of benzotriazole inhibitors with improved specificity.

Mechanism for controlling the dimer-monomer switch and coupling dimerization to catalysis of the severe acute respiratory syndrome coronavirus 3C-like protease

Unlike 3C protease, the severe acute respiratory syndrome coronavirus (SARS-CoV) 3C-like protease (3CLpro) is only enzymatically active as a homodimer and its catalysis is under extensive regulation by the unique extra domain. Despite intense studies, two puzzles still remain: (i) how the dimer-monomer switch is controlled and (ii) why dimerization is absolutely required for catalysis. Here we report the monomeric crystal structure of the SARS-CoV 3CLpro mutant R298A at a resolution of 1.75 A. Detailed analysis reveals that Arg298 serves as a key component for maintaining dimerization, and consequently, its mutation will trigger a cooperative switch from a dimer to a monomer. The monomeric enzyme is irreversibly inactivated because its catalytic machinery is frozen in the collapsed state, characteristic of the formation of a short 3(10)-helix from an active-site loop. Remarkably, dimerization appears to be coupled to catalysis in 3CLpro through the use of overlapped residues for two networks, one for dimerization and another for the catalysis.

Structure-based virtual screening against SARS-3CL(pro) to identify novel non-peptidic hits

Severe acute respiratory syndrome is a highly infectious upper respiratory tract disease caused by SARS-CoV, a previously unidentified human coronavirus. SARS-3CL(pro) is a viral cysteine protease critical to the pathogen's life cycle and hence a therapeutic target of importance. The recently elucidated crystal structures of this enzyme provide an opportunity for the discovery of inhibitors through rational drug design. In the current study, Gold docking program was utilized to conduct extensive docking studies against the target crystal structure to develop a robust and predictive docking protocol. The validated docking protocol was used to conduct a structure-based virtual screening of the Asinex Platinum collection. Biological evaluation of a screened selection of compounds was carried out to identify novel inhibitors of the viral protease.