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.     

Long-range cooperative interactions modulate dimerization in SARS 3CLpro

Severe acute respiratory syndrome (SARS) is an infectious disease caused by the human coronavirus, SARS-CoV. The main viral protease, SARS 3CLpro, is a validated target for the development of antiviral therapies. Since the enzyme is a homodimer and the individual monomers are inactive, two approaches are being used to develop inhibitors: enzyme activity inhibitors that target the active site and dimerization inhibitors. Dimerization inhibitors are usually targeted to the dimerization interface and need to compete with the attractive forces between subunits to be effective. In this paper, we show that the dimerization of SARS 3CLpro is also under allosteric control and that additional and energetically more favorable target sites away from the dimerization interface may also lead to subunit dissociation. We previously identified a cluster of conserved serine residues (Ser139, Ser144, and Ser147) located adjacent to the active site of 3CLpro that could effectively be targeted to inactivate the protease [Bacha, U et al. (2004) Biochemistry 43, 4906-4912]. Mutation of any of these serine residues to alanine had a debilitating effect on the catalytic activity of 3CLpro. In particular, the mutation of Ser147, which does not make any contact with the opposing subunit and is located approximately 9 A away from the dimer interface, totally inhibited dimerization and resulted in a complete loss of enzymatic activity. The finding that residues away from the dimer interface are able to control dimerization defines alternative targets for the design of dimerization inhibitors.     

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.     

Mutation of Gly-11 on the dimer interface results in the complete crystallographic dimer dissociation of severe acute respiratory syndrome coronavirus 3C-like protease: crystal structure with molecular dynamics simulations

SARS-CoV 3C-like protease (3CL(pro)) is an attractive target for anti-severe acute respiratory syndrome (SARS) drug discovery, and its dimerization has been extensively proved to be indispensable for enzymatic activity. However, the reason why the dissociated monomer is inactive still remains unclear due to the absence of the monomer structure. In this study, we showed that mutation of the dimer-interface residue Gly-11 to alanine entirely abolished the activity of SARS-CoV 3CL(pro). Subsequently, we determined the crystal structure of this mutant and discovered a complete crystallographic dimer dissociation of SARS-CoV 3CL(pro). The mutation might shorten the alpha-helix A' of domain I and cause a mis-oriented N-terminal finger that could not correctly squeeze into the pocket of another monomer during dimerization, thus destabilizing the dimer structure. Several structural features essential for catalysis and substrate recognition are severely impaired in the G11A monomer. Moreover, domain III rotates dramatically against the chymotrypsin fold compared with the dimer, from which we proposed a putative dimerization model for SARS-CoV 3CL(pro). As the first reported monomer structure for SARS-CoV 3CL(pro), the crystal structure of G11A mutant might provide insight into the dimerization mechanism of the protease and supply direct structural evidence for the incompetence of the dissociated monomer.     

Characterization of trans- and cis-cleavage activity of the SARS coronavirus 3CLpro protease: basis for the in vitro screening of anti-SARS drugs

Severe acute respiratory syndrome (SARS) has been globally reported. A novel coronavirus (CoV), SARS-CoV, was identified as the etiological agent of the disease. SARS-CoV 3C-like protease (3CLpro) mediates the proteolytic processing of replicase polypeptides 1a and 1ab into functional proteins, playing an important role in viral replication. In this study, we demonstrated the expression of the SARS-CoV 3CLpro in Escherichia coli and Vero cells, and then characterized the in vitro trans-cleavage and the cell-based cis-cleavage by the 3CLpro. Mutational analysis of the 3CLpro demonstrated the importance of His41, Cys145, and Glu166 in the substrate-binding subsite S1 for keeping the proteolytic activity. In addition, alanine substitution of the cleavage substrates indicated that Gln-(P1) in the substrates mainly determined the cleavage efficiency. Therefore, this study not only established the quantifiable and reliable assay for the in vitro and cell-based measurement of the 3CLpro activity, but also characterized the molecular interaction of the SARS-CoV 3CLpro with the substrates. The results will be useful for the rational development of the anti-SARS drugs.     

Recent development of 3C and 3CL protease inhibitors for anti-coronavirus and anti-picornavirus drug discovery

SARS-CoV (severe acute respiratory syndrome-associated coronavirus) caused infection of ~8000 people and death of ~800 patients around the world during the 2003 outbreak. In addition, picornaviruses such as enterovirus, coxsackievirus and rhinovirus also can cause life-threatening diseases. Replication of picornaviruses and coronaviruses requires 3Cpro (3C protease) and 3CLpro (3C-like protease) respectively, which are structurally analogous with chymotrypsin-fold, but the former is a monomer and the latter is dimeric due to an extra third domain for dimerization. Subtle structural differences in the S2 and S3 pockets of these proteases make inhibitors selective, but some dual inhibitors have been discovered. Our findings as summarized in the present review provide new potential anti-coronavirus and anti-picornavirus therapeutic agents and a clue to convert 3CLpro inhibitors into 3Cpro inhibitors and vice versa.     

Without its N-finger, the main protease of severe acute respiratory syndrome coronavirus can form a novel dimer through its C-terminal domain

The main protease (M(pro)) of severe acute respiratory syndrome coronavirus (SARS-CoV) plays an essential role in the extensive proteolytic processing of the viral polyproteins (pp1a and pp1ab), and it is an important target for anti-SARS drug development. It was found that SARS-CoV M(pro) exists in solution as an equilibrium of both monomeric and dimeric forms, and the dimeric form is the enzymatically active form. However, the mechanism of SARS-CoV M(pro) dimerization, especially the roles of its N-terminal seven residues (N-finger) and its unique C-terminal domain in the dimerization, remain unclear. Here we report that the SARS-CoV M(pro) C-terminal domain alone (residues 187 to 306; M(pro)-C) is produced in Escherichia coli in both monomeric and dimeric forms, and no exchange could be observed between them at room temperature. The M(pro)-C dimer has a novel dimerization interface. Meanwhile, the N-finger deletion mutant of SARS-CoV M(pro) also exists as both a stable monomer and a stable dimer, and the dimer is formed through the same C-terminal-domain interaction as that in the M(pro)-C dimer. However, no C-terminal domain-mediated dimerization form can be detected for wild-type SARS-CoV M(pro). Our study results help to clarify previously published controversial claims about the role of the N-finger in SARS-CoV M(pro) dimerization. Apparently, without the N-finger, SARS-CoV M(pro) can no longer retain the active dimer structure; instead, it can form a new type of dimer which is inactive. Therefore, the N-finger of SARS-CoV M(pro) is not only critical for its dimerization but also essential for the enzyme to form the enzymatically active dimer.     

Development of a red-shifted fluorescence-based assay for SARS-coronavirus 3CL protease: identification of a novel class of anti-SARS agents from the tropical marine sponge Axinella corrugata

SARS-coronavirus (SARS-CoV) encodes a main protease, 3CLpro, which plays an essential role in the viral life cycle and is currently the prime target for discovering new anti-coronavirus agents. In this article, we report our success in developing a novel red-shifted (RS) fluorescence-based assay for 3CLpro and its application for identifying small-molecule anti-SARS agents from marine organisms. We have synthesised and characterised the first generation of a red-shifted internally quenched fluorogenic substrate (RS-IQFS) for 3CLpro based on resonance energy transfer between the donor and acceptor pair CAL Fluor Red 610 and Black Hole Quencher-1 (Km and kcat values of 14 microM and 0.65 min-1). The RS-IQFS primary sequence was selected based on the results of our screening analysis of 3CLpro performed using a series of blue-shifted (BS)-IQFSs corresponding to the 3CLpro-mediated cleavage junctions of the SARS-CoV polyproteins. In contrast to BS-IQFSs, the RS-IQFS was not susceptible to fluorescence interference from coloured samples and allowed for successful screening of marine natural products and identification of a coumarin derivative, esculetin-4-carboxylic acid ethyl ester, a novel 3CLpro inhibitor (IC50=46 microM) and anti-SARS agent (EC50=112 microM; median toxic concentration>800 microM) from the tropical marine sponge Axinella corrugata.     

Structure-based design, synthesis, and biological evaluation of peptidomimetic SARS-CoV 3CLpro inhibitors

Structure-based design, synthesis, and biological evaluation of a series of peptidomimetic severe acute respiratory syndrome-coronavirus chymotrypsin-like protease inhibitors are described. These inhibitors were designed and synthesized based upon our X-ray crystal structure of inhibitor 1 bound to SARS-CoV 3CLpro. Incorporation of Boc-Ser as the P(4)-ligand resulted in enhanced SARS-CoV 3CLpro inhibitory activity. Structural analysis of the inhibitor-bound X-ray structure revealed high binding affinity toward the enzyme.     

Mutation of Glu-166 Blocks the Substrate-Induced Dimerization of SARS Coronavirus Main Protease

The maturation of SARS coronavirus involves the autocleavage of polyproteins 1a and 1ab by the main protease (Mpro) and a papain-like protease; these represent attractive targets for the development of anti-SARS drugs. The functional unit of Mpro is a homodimer, and each subunit has a His-41?Cys-145 catalytic dyad. Current thinking in this area is that Mpro dimerization is essential for catalysis, although the influence of the substrate binding on the dimer formation has never been explored. Here, we delineate the contributions of the peptide substrate to Mpro dimerization. Enzyme kinetic assays indicate that the monomeric mutant R298A/L exhibits lower activity but in a cooperative manner. Analytical ultracentrifugation analyses indicate that in the presence of substrates, the major species of R298A/L shows a significant size shift toward the dimeric form and the monomer-dimer dissociation constant of R298A/L decreases by 12- to 17-fold, approaching that for wild-type. Furthermore, this substrate-induced dimerization was found to be reversible after substrates were removed. Based on the crystal structures, a key residue, Glu-166, which is responsible for recognizing the Gln-P1 of the substrate and binding to Ser-1 of another protomer, will interact with Asn-142 and block the S1 subsite entrance in the monomer. Our studies indicate that mutation of Glu-166 in the R298A mutant indeed blocks the substrate-induced dimerization. This demonstrates that Glu-166 plays a pivotal role in connecting the substrate binding site with the dimer interface. We conclude that protein-ligand and protein-protein interactions are closely correlated in Mpro.     

Liberation of SARS-CoV main protease from the viral polyprotein: N-terminal autocleavage does not depend on the mature dimerization mode

The main protease (M^pro) plays a vital role in proteolytic processing of the polyproteins in the replicative cycle of SARS coronavirus (SARS-CoV). Dimerization of this enzyme has been shown to be indispensable for trans-cleavage activity. However, the auto-processing mechanism of M^pro, i.e. its own release from the polyproteins through autocleavage, remains unclear. This study elucidates the relationship between the N-terminal autocleavage activity and the dimerization of “immature” M^pro. Three residues (Arg4, Glu290, and Arg298), which contribute to the active dimer conformation of mature M^pro, are selected for mutational analyses. Surprisingly, all three mutants still perform N-terminal autocleavage, while the dimerization of mature protease and trans-cleavage activity following auto-processing are completely inhibited by the E290R and R298E mutations and partially so by the R4E mutation. Furthermore, the mature E290R mutant can resume N-terminal autocleavage activity when mixed with the “immature” C145A/E290R double mutant whereas its trans-cleavage activity remains absent. Therefore, the N-terminal auto-processing of M^pro appears to require only two “immature” monomers approaching one another to form an “intermediate” dimer structure and does not strictly depend on the active dimer conformation existing in mature protease. In conclusion, an auto-release model of M^pro from the polyproteins is proposed, which will help understand the auto-processing mechanism and the difference between the autocleavage and trans-cleavage proteolytic activities of SARS-CoV M^pro.     

Increase of SARS-CoV 3CL peptidase activity due to macromolecular crowding effects in the milieu composition

The 3C-like peptidase of the severe acute respiratory syndrome virus (SARS-CoV) is strictly required for viral replication, thus being a potential target for the development of antiviral agents. In contrast to monomeric picornavirus 3C peptidases, SARS-CoV 3CLpro exists in equilibrium between the monomer and dimer forms in solution, and only the dimer is proteolytically active in dilute buffer solutions. In this study, the increase of SARS-CoV 3CLpro peptidase activity in presence of kosmotropic salts and crowding agents is described. The activation followed the Hofmeister series of anions, with two orders of magnitude enhancement in the presence of Na?SO?, whereas the crowding agents polyethylene glycol and bovine serum albumin increased the hydrolytic rate up to 3 times. Kinetic determinations of the monomer dimer dissociation constant (K(d)) indicated that activation was a result of a more active dimer, without significant changes in K(d) values. The activation was found to be independent of substrate length and was derived from both k(cat) increase and K(m) decrease. The viral peptidase activation described here could be related to the crowded intracellular environment and indicates a further fine-tuning mechanism for biological control, particularly in the microenvironment of the vesicles that are induced in host cells during positive strand RNA virus infection.     

Repurposing existing drugs: identification of SARS-CoV-2 3C-like protease inhibitors

Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) is responsible for coronavirus disease 2019 (COVID-19). Since its emergence, the COVID-19 pandemic has not only distressed medical services but also caused economic upheavals, marking urgent the need for effective therapeutics. The experience of combating SARS-CoV and MERS-CoV has shown that inhibiting the 3-chymotrypsin-like protease (3CLpro) blocks the replication of the virus. Given the well-studied properties of FDA-approved drugs, identification of SARS-CoV-2 3CLpro inhibitors in an FDA-approved drug library would be of great therapeutic value. Here, we screened a library consisting of 774 FDA-approved drugs for potent SARS-CoV-2 3CLpro inhibitors, using an intramolecularly quenched fluorescence (IQF) peptide substrate. Ethacrynic acid, naproxen, allopurinol, butenafine hydrochloride, raloxifene hydrochloride, tranylcypromine hydrochloride, and saquinavir mesylate have been found to block the proteolytic activity of SARS-CoV-2 3CLpro. The inhibitory activity of these repurposing drugs against SARS-CoV-2 3CLpro highlights their therapeutic potential for treating COVID-19 and other Betacoronavirus infections.     

Key dimer interface residues impact the catalytic activity of 3CLpro,the main protease of SARS-CoV-2

3C-like protease (3CLpro) processes and liberates functional viral proteins essential for the maturation and infectivity of severe acute respiratory syndrome coronavirus 2, the virus responsible for COVID-19. It has been suggested that 3CLpro is catalytically active as a dimer, making the dimerization interface a target for antiviral development. Guided by structural analysis, here we introduced single amino acid substitutions at nine residues at three key sites of the dimer interface to assess their impact on dimerization and activity. We show that at site 1, alanine substitution of S1 or E166 increased by twofold or reduced relative activity, respectively. At site 2, alanine substitution of S10 or E14 eliminated activity, whereas K12A exhibited ∼60% relative activity. At site 3, alanine substitution of R4, E290, or Q299 eliminated activity, whereas S139A exhibited 46% relative activity. We further found that the oligomerization states of the dimer interface mutants varied; the inactive mutants R4A, R4Q, S10A/C, E14A/D/Q/S, E290A, and Q299A/E were present as dimers, demonstrating that dimerization is not an indication of catalytically active 3CLpro. In addition, present mostly as monomers, K12A displayed residual activity, which could be attributed to the conspicuous amount of dimer present. Finally, differential scanning calorimetry did not reveal a direct relationship between the thermodynamic stability of mutants with oligomerization or catalytic activity. These results provide insights on two allosteric sites, R4/E290 and S10/E14, that may promote the design of antiviral compounds that target the dimer interface rather than the active site of severe acute respiratory syndrome coronavirus 2 3CLpro.     

Inhibition of SARS-CoV 3CL protease by flavonoids

Abstract

There were severe panics caused by Severe Acute Respiratory Syndrome (SARS) and Middle-East Respiratory Syndrome-Coronavirus. Therefore, researches targeting these viruses have been required. Coronaviruses (CoVs) have been rising targets of some flavonoids. The antiviral activity of some flavonoids against CoVs is presumed directly caused by inhibiting 3C-like protease (3CLpro). Here, we applied a flavonoid library to systematically probe inhibitory compounds against SARS-CoV 3CLpro. Herbacetin, rhoifolin and pectolinarin were found to efficiently block the enzymatic activity of SARS-CoV 3CLpro. The interaction of the three flavonoids was confirmed using a tryptophan-based fluorescence method, too. An induced-fit docking analysis indicated that S1, S2 and S3′ sites are involved in binding with flavonoids. The comparison with previous studies showed that Triton X-100 played a critical role in objecting false positive or overestimated inhibitory activity of flavonoids. With the systematic analysis, the three flavonoids are suggested to be templates to design functionally improved inhibitors.     

Cinanserin is an inhibitor of the 3C-like proteinase of severe acute respiratory syndrome coronavirus and strongly reduces virus replication in vitro

The 3C-like proteinase (3CLpro) of severe acute respiratory syndrome-associated coronavirus (SARS-CoV) is one of the most promising targets for anti-SARS-CoV drugs due to its crucial role in the viral life cycle. In this study, a database containing structural information of more than 8,000 existing drugs was virtually screened by a docking approach to identify potential binding molecules of SARS-CoV 3CLpro. As a target for screening, both a homology model and the crystallographic structure of the binding pocket of the enzyme were used. Cinanserin (SQ 10,643), a well-characterized serotonin antagonist that has undergone preliminary clinical testing in humans in the 1960s, showed a high score in the screening and was chosen for further experimental evaluation. Binding of both cinanserin and its hydrochloride to bacterially expressed 3CLpro of SARS-CoV and the related human coronavirus 229E (HCoV-229E) was demonstrated by surface plasmon resonance technology. The catalytic activity of both enzymes was inhibited with 50% inhibitory concentration (IC50) values of 5 microM, as tested with a fluorogenic substrate. The antiviral activity of cinanserin was further evaluated in tissue culture assays, namely, a replicon system based on HCoV-229E and quantitative test assays with infectious SARS-CoV and HCoV-229E. All assays revealed a strong inhibition of coronavirus replication at nontoxic drug concentrations. The level of virus RNA and infectious particles was reduced by up to 4 log units, with IC50 values ranging from 19 to 34 microM. These findings demonstrate that the old drug cinanserin is an inhibitor of SARS-CoV replication, acting most likely via inhibition of the 3CL proteinase.     

The papain-like protease of severe acute respiratory syndrome coronavirus has deubiquitinating activity

Replication of the genomic RNA of severe acute respiratory syndrome coronavirus (SARS-CoV) is mediated by replicase polyproteins that are processed by two viral proteases, papain-like protease (PLpro) and 3C-like protease (3CLpro). Previously, we showed that SARS-CoV PLpro processes the replicase polyprotein at three conserved cleavage sites. Here, we report the identification and characterization of a 316-amino-acid catalytic core domain of PLpro that can efficiently cleave replicase substrates in trans-cleavage assays and peptide substrates in fluorescent resonance energy transfer-based protease assays. We performed bioinformatics analysis on 16 papain-like protease domains from nine different coronaviruses and identified a putative catalytic triad (Cys1651-His1812-Asp1826) and zinc-binding site. Mutagenesis studies revealed that Asp1826 and the four cysteine residues involved in zinc binding are essential for SARS-CoV PLpro activity. Molecular modeling of SARS-CoV PLpro suggested that this catalytic core may also have deubiquitinating activity. We tested this hypothesis by measuring the deubiquitinating activity of PLpro by two independent assays. SARS CoV-PLpro hydrolyzed both diubiquitin and ubiquitin-7-amino-4-methylcoumarin (AMC) substrates, and hydrolysis of ubiquitin-AMC is approximately 180-fold more efficient than hydrolysis of a peptide substrate that mimics the PLpro replicase recognition sequence. To investigate the critical determinants recognized by PLpro, we performed site-directed mutagenesis on the P6 to P2' residues at each of the three PLpro cleavage sites. We found that PLpro recognizes the consensus cleavage sequence LXGG, which is also the consensus sequence recognized by cellular deubiquitinating enzymes. This similarity in the substrate recognition sites should be considered during the development of SARS-CoV PLpro inhibitors.     

A novel auto-cleavage assay for studying mutational effects on the active site of severe acute respiratory syndrome coronavirus 3C-like protease

The 3C-like protease (3CLpro) of severe acute respiratory syndrome (SARS) has been proposed as an attractive target for drug design. His41 and Cys145 were essential for the active site as the principal catalytic residues. In this study, we mutated the two sites, expressed four resulting mutants in Escherichia coli and characterized. All mutants showed undetectable activity in trans-cleavage assay. In addition, we introduced a 31-mer peptide containing an auto-cleavage site to the N-terminal of the proteases and found the peptide could be cleaved efficiently by 3CLsc itself, but, among the four mutants, only the mutant Cys145-->Ser showed residual activity as detected by the auto-cleavage assay. The data supported the proposition unequivocally that SARS-CoV 3CLpro was a member of serine proteases involving His41 and Cys145 residues at the active site. The auto-cleavage assay also provided a sensitive and reliable compensation to the traditional trans-cleavage assay.     

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.