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Papain-like protease is an essential enzyme in the proteolytic processing required for the replication of SARS-CoV-2. Accordingly, such an enzyme is an important target for the development of anti-SARS-CoV-2 agents which may reduce the mortality associated with outbreaks of SARS-CoV-2. A set of 69 semi-synthesized molecules that exhibited the structural features of SARS-CoV-2 papain-like protease inhibitors (PLPI) were docked against the coronavirus papain-like protease (PLpro) enzyme (PDB ID: (4OW0). Docking studies showed that derivatives 34 and 58 were better than the co-crystallized ligand while derivatives 17, 28, 31, 40, 41, 43, 47, 54, and 65 exhibited good binding modes and binding free energies. The pharmacokinetic profiling study was conducted according to the four principles of the Lipinski rules and excluded derivative 31. Furthermore, ADMET and toxicity studies showed that derivatives 28, 34, and 47 have the potential to be drugs and have been demonstrated as safe when assessed via seven toxicity models. Finally, comparing the molecular orbital energies and the molecular electrostatic potential maps of 28, 34, and 47 against the co-crystallized ligand in a DFT study indicated that 28 is the most promising candidate to interact with the target receptor (PLpro).

The pandemic caused by Severe Acute Respiratory Syndrome Coronavirus 2 (SARS-CoV-2) continues to expand. Papain-like protease (PLpro) is one of two SARS-CoV-2 proteases potentially targetable with antivirals. PLpro is an attractive target because it plays an essential role in cleavage and maturation of viral polyproteins, assembly of the replicase-transcriptase complex, and disruption of host responses. We report a substantive body of structural, biochemical, and virus replication studies that identify several inhibitors of the SARS-CoV-2 enzyme. We determined the high resolution structure of wild-type PLpro, the active site C111S mutant, and their complexes with inhibitors. This collection of structures details inhibitors recognition and interactions providing fundamental molecular and mechanistic insight into PLpro. All compounds inhibit the peptidase activity of PLpro in vitro, some block SARS-CoV-2 replication in cell culture assays. These findings will accelerate structure-based drug design efforts targeting PLpro to identify high-affinity inhibitors of clinical value. The SARS-CoV-2 papain-like protease (PLpro) is of interest as an antiviral drug target. Here, the authors synthesize and characterise naphthalene-based inhibitors for PLpro and present the crystal structures of PLpro in its apo state and with the bound inhibitors, which is of interest for further structure-based drug design efforts.

Cyclotides are plant defense peptides that have been extensively investigated for pharmaceutical and agricultural applications, but key details of their posttranslational biosynthesis have remained elusive. Asparaginyl endopeptidases are crucial in the final stage of the head-to-tail cyclization reaction, but the enzyme(s) involved in the prerequisite steps of N-terminal proteolytic release were unknown until now. Here we use activity-guided fractionation to identify specific members of papain-like cysteine proteases involved in the N-terminal cleavage of cyclotide precursors. Through both characterization of recombinantly produced enzymes and in planta peptide cyclization assays, we define the molecular basis of the substrate requirements of these enzymes, including the prototypic member, here termed kalatase A. The findings reported here will pave the way for improving the efficiency of plant biofactory approaches for heterologous production of cyclotide analogs of therapeutic or agricultural value.

Plants deploy a sophisticated immune system to cope with different microbial pathogens and other invaders. Recent research provides an increasing body of evidence for papain-like cysteine proteases (PLCPs) being central hubs in plant immunity. PLCPs are required for full resistance of plants to various pathogens. At the same time, PLCPs are targeted by secreted pathogen effectors to suppress immune responses. Consequently, they are subject to a co-evolutionary host–pathogen arms race. When activated, PLCPs induce a broad spectrum of defense responses including plant cell death. While the important role of PLCPs in plant immunity has become more evident, it remains largely elusive how these enzymes are activated and which signaling pathways are triggered to orchestrate different downstream responses.

The Papain-like protease (PLpro) is a domain of a multi-functional, non-structural protein 3 of coronaviruses. PLpro cleaves viral polyproteins and posttranslational conjugates with poly-ubiquitin and protective ISG15, composed of two ubiquitin-like (UBL) domains. Across coronaviruses, PLpro showed divergent selectivity for recognition and cleavage of posttranslational conjugates despite sequence conservation. We show that SARS-CoV-2 PLpro binds human ISG15 and K48-linked di-ubiquitin (K48-Ub 2 ) with nanomolar affinity and detect alternate weaker-binding modes. Crystal structures of untethered PLpro complexes with ISG15 and K48-Ub 2 combined with solution NMR and cross-linking mass spectrometry revealed how the two domains of ISG15 or K48-Ub 2 are differently utilized in interactions with PLpro. Analysis of protein interface energetics predicted differential binding stabilities of the two UBL/Ub domains that were validated experimentally. We emphasize how substrate recognition can be tuned to cleave specifically ISG15 or K48-Ub 2 modifications while retaining capacity to cleave mono-Ub conjugates. These results highlight alternative druggable surfaces that would inhibit PLpro function. Understanding mechanisms of PLpro substrate selectivity offers new ways to decouple substrate activities and will inform new therapeutic strategies. Here, the authors use multi-disciplinary approaches to uncover how PLpro from SARS-CoV-2 can discriminate between different substrates.

Direct-acting antivirals are needed to combat coronavirus disease 2019 (COVID-19), which is caused by severe acute respiratory syndrome-coronavirus-2 (SARS-CoV-2). The papain-like protease (PLpro) domain of Nsp3 from SARS-CoV-2 is essential for viral replication. In addition, PLpro dysregulates the host immune response by cleaving ubiquitin and interferon-stimulated gene 15 protein from host proteins. As a result, PLpro is a promising target for inhibition by small-molecule therapeutics. Here we design a series of covalent inhibitors by introducing a peptidomimetic linker and reactive electrophile onto analogs of the noncovalent PLpro inhibitor GRL0617. The most potent compound inhibits PLpro with k inact /K I  = 9,600 M −1 s −1 , achieves sub-μM EC 50 values against three SARS-CoV-2 variants in mammalian cell lines, and does not inhibit a panel of human deubiquitinases (DUBs) at >30 μM concentrations of inhibitor. An X-ray co-crystal structure of the compound bound to PLpro validates our design strategy and establishes the molecular basis for covalent inhibition and selectivity against structurally similar human DUBs. These findings present an opportunity for further development of covalent PLpro inhibitors. The development of direct-acting antivirals to combat COVID-19 remains an important goal. Here the authors design covalent inhibitors that target the papain-like protease from SARS-CoV-2. The most promising inhibitor blocks viral replication in mammalian cells.

Papain-like cysteine proteases (PLCPs) are a large class of proteolytic enzymes associated with development, immunity, and senescence. Although many properties have been described for individual proteases, the distribution of these characteristics has not been studied collectively. Here, we analyzed 723 plant PLCPs and classify them into nine subfamilies that are present throughout the plant kingdom. Analysis of these subfamilies revealed previously unreported distinct subfamily-specific functional and structural characteristics. For example, the NPIR and KDEL localization signals are distinctive for subfamilies, and the carboxyl-terminal granulin domain occurs in two PLCP subfamilies, in which some individual members probably evolved by deletion of the granulin domains. We also discovered a conserved double cysteine in the catalytic site of SAG12-like proteases and two subfamily-specific disulfides in RD19A-like proteases. Protease activity profiling of representatives of the PLCP subfamilies using novel fluorescent probes revealed striking polymorphic labeling profiles and remarkably distinct pH dependency. Competition assays with peptide-epoxide scanning libraries revealed common and unique inhibitory fingerprints. Finally, we expand the detection of PLCPs by identifying common and organ-specific protease activities and identify previously undetected proteases upon labeling with cell-penetrating probes in vivo. This study provides the plant protease research community with tools for further functional annotation of plant PLCPs.

The severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) pandemic has made it clear that combating coronavirus outbreaks benefits from a combination of vaccines and therapeutics. A promising drug target common to all coronaviruses—including SARS-CoV, MERS-CoV, and SARS-CoV-2—is the papain-like protease (PLpro). PLpro cleaves part of the viral replicase polyproteins into non-structural protein subunits, which are essential to the viral replication cycle. Additionally, PLpro can cleave both ubiquitin and the ubiquitin-like protein ISG15 from host cell substrates as a mechanism to evade innate immune responses during infection. These roles make PLpro an attractive antiviral drug target. Here we demonstrate that ubiquitin variants (UbVs) can be selected from a phage-displayed library and used to specifically and potently block SARS-CoV-2 PLpro activity. A crystal structure of SARS-CoV-2 PLpro in complex with a representative UbV reveals a dimeric UbV bound to PLpro at a site distal to the catalytic site. Yet, the UbV inhibits the essential cleavage activities of the protease in vitro and in cells, and it reduces viral replication in cell culture by almost five orders of magnitude.

Pelodiscus sinensis meat is a nutritional food and tonic with angiotensin-converting enzyme (ACE) inhibitory activities. To identify the bioactive substances responsible, several bioinformatics methods were integrated to enable a virtual screening for bioactive peptides in proteins identified within a water-soluble protein fraction of Pelodiscus sinensis meat by Shotgun proteomics. The peptides were generated from the identified proteins by in silico proteolysis using six proteases. A comparison of the numbers of proteins suitable for digestion with each enzyme and the iBAQ (intensity-based absolute quantification) values for these proteins revealed that bromelain and papain were the most suitable proteases for this sample. Next, the water solubility, toxicity, and ADMET (absorption/distribution/metabolism/excretion/toxicity) properties of these peptides were evaluated in silico. Finally, a novel ACE inhibitory peptide IEWEF with an IC50 value of 41.33 µM was identified. The activity of the synthesized peptide was verified in vitro, and it was shown to be a non-competitive ACE inhibitor. Molecular docking revealed that IEWEF could tightly bind to C-ACE, and N-ACE with energies less than 0 kJ mol−1, and the peptide IEWEF can form hydrogen bonds with C-ACE and N-ACE respectively. These results provide evidence that bioactive peptides in the water-soluble protein fraction account for (at least) some of the ACE inhibitory activities observed in Pelodiscus sinensis meat. Furthermore, our research provides a workflow for the efficient identification of novel ACE inhibitory peptides from complex protein mixtures.

This study explores various methods for the covalent immobilization of cysteine proteases (ficin, papain, and bromelain). Covalent immobilization involves the formation of covalent bonds between the enzyme and a carrier or between enzyme molecules themselves without a carrier using a crosslinking agent. This process enhances the stability of the enzyme and allows for the creation of preparations with specific and controlled properties. The objective of this study is to evaluate the impact of covalent immobilization under different conditions on the proteolytic activity of the enzymes. The most favorable results were achieved by immobilizing ficin and bromelain through covalent bonding to medium and high molecular weight chitosans, using 5 and 3.33% glutaraldehyde solutions, respectively. For papain, 5 and 6.67% glutaraldehyde solutions proved to be more effective as crosslinking agents. These findings indicate that covalent immobilization can enhance the performance of these enzymes as biocatalysts, with potential applications in various biotechnological fields.