Covalent narlaprevir- and boceprevir-derived hybrid inhibitors of SARS-CoV-2 main protease: room-temperature X-ray and neutron crystallography, binding thermodynamics, and antiviral activity

The COVID-19 pandemic continues to disrupt everyday life, with constantly emerging SARS-CoV-2 variants threatening to render current vaccines ineffective. Small-molecule antivirals can provide an important therapeutic treatment option that is subject to challenges caused by the virus variants. The viral main protease (Mpro) is critical for the virus replication and thus is considered an attractive drug target for specific protease inhibitors. We performed the design and characterization of three reversible covalent hybrid inhibitors BBH-1, BBH-2 and NBH-2, whose structures were derived from those of hepatitis C protease inhibitors boceprevir and narlaprevir. A joint X-ray/neutron structure of the Mpro/BBH-1 complex demonstrated that a Cys145 thiolate reaction with the inhibitor’s keto-warhead creates a negatively charged oxyanion, similar to that proposed for the Mpro-catalyzed peptide bond hydrolysis. Protonation states of the ionizable residues in the Mpro active site adapt to the inhibitor, which appears to be an intrinsic property of Mpro. Structural comparisons of the hybrid inhibitors with PF-07321332 revealed unconventional interactions of PF-07321332 with Mpro which may explain its more favorable enthalpy of binding and consequently higher potency. BBH-1, BBH-2 and NBH-2 demonstrated comparable antiviral properties in vitro relative to PF-07321332, making them good candidates for further design of improved antivirals.

1 Overall quality at a glance i ○ The following experimental techniques were used to determine the structure:

X-RAY DIFFRACTION
The reported resolution of this entry is 1.80 Å.
Percentile scores (ranging between 0-100) for global validation metrics of the entry are shown in the following graphic. The table shows the number of entries on which the scores are based. The table below summarises the geometric issues observed across the polymeric chains and their fit to the electron density. The red, orange, yellow and green segments of the lower bar indicate the fraction of residues that contain outliers for >=3, 2, 1 and 0 types of geometric quality criteria respectively. A grey segment represents the fraction of residues that are not modelled. The numeric value for each fraction is indicated below the corresponding segment, with a dot representing fractions <=5% The upper red bar (where present) indicates the fraction of residues that have poor fit to the electron density. The numeric value is given above the bar.

Mol Chain Length
Quality of chain 1 A 306 2 Entry composition i ○ There are 3 unique types of molecules in this entry. The entry contains 2594 atoms, of which 0 are hydrogens and 0 are deuteriums.
In the tables below, the ZeroOcc column contains the number of atoms modelled with zero occupancy, the AltConf column contains the number of residues with at least one atom in alternate conformation and the Trace column contains the number of residues modelled with at most 2 atoms.
• Molecule 1 is a protein called 3C-like proteinase.  3 Residue-property plots i ○ These plots are drawn for all protein, RNA, DNA and oligosaccharide chains in the entry. The first graphic for a chain summarises the proportions of the various outlier classes displayed in the second graphic. The second graphic shows the sequence view annotated by issues in geometry and electron density. Residues are color-coded according to the number of geometric quality criteria for which they contain at least one outlier: green = 0, yellow = 1, orange = 2 and red = 3 or more. A red dot above a residue indicates a poor fit to the electron density (RSRZ > 2). Stretches of 2 or more consecutive residues without any outlier are shown as a green connector. Residues present in the sample, but not in the model, are shown in grey.

Mol Chain Residues
• Molecule 1: 3C-like proteinase Chain A: Xtriage's analysis on translational NCS is as follows: The largest off-origin peak in the Patterson function is 7.85% of the height of the origin peak. No significant pseudotranslation is detected.
5 Model quality i ○

Standard geometry i ○
Bond lengths and bond angles in the following residue types are not validated in this section: NB2 The Z score for a bond length (or angle) is the number of standard deviations the observed value is removed from the expected value. A bond length (or angle) with |Z| > 5 is considered an outlier worth inspection. RMSZ is the root-mean-square of all Z scores of the bond lengths (or angles).

Mol Chain Bond lengths Bond angles
There are no bond length outliers.
All (1) bond angle outliers are listed below: There are no chirality outliers.
There are no planarity outliers.

Too-close contacts i ○
In the following The all-atom clashscore is defined as the number of clashes found per 1000 atoms (including hydrogen atoms). The all-atom clashscore for this structure is 2.
All (8)  There are no symmetry-related clashes.

Protein backbone i ○
In the following table, the Percentiles column shows the percent Ramachandran outliers of the chain as a percentile score with respect to all X-ray entries followed by that with respect to entries of similar resolution.
The Analysed column shows the number of residues for which the backbone conformation was analysed, and the total number of residues. There are no Ramachandran outliers to report.

Protein sidechains i ○
In the following table, the Percentiles column shows the percent sidechain outliers of the chain as a percentile score with respect to all X-ray entries followed by that with respect to entries of similar resolution.
The Analysed column shows the number of residues for which the sidechain conformation was analysed, and the total number of residues. All (7) residues with a non-rotameric sidechain are listed below:

Mol Chain
Full wwPDB X-ray Structure Validation Report 7TFR

Non-standard residues in protein, DNA, RNA chains i ○
There are no non-standard protein/DNA/RNA residues in this entry.

Carbohydrates i ○
There are no monosaccharides in this entry.

Ligand geometry i ○
1 ligand is modelled in this entry.
In the following table, the Counts columns list the number of bonds (or angles) for which Mogul statistics could be retrieved, the number of bonds (or angles) that are observed in the model and the number of bonds (or angles) that are defined in the Chemical Component Dictionary. The Link column lists molecule types, if any, to which the group is linked. The Z score for a bond length (or angle) is the number of standard deviations the observed value is removed from the expected value. A bond length (or angle) with |Z| > 2 is considered an outlier worth inspection. RMSZ is the root-mean-square of all Z scores of the bond lengths (or angles). There are no chirality outliers.

Mol Type Chain Res Link
All (1) torsion outliers are listed below: Mol Chain Res Type Atoms 2 A 401 NB2 C30-C24-N20-C18 There are no ring outliers.
No monomer is involved in short contacts.
The following is a two-dimensional graphical depiction of Mogul quality analysis of bond lengths, bond angles, torsion angles, and ring geometry for all instances of the Ligand of Interest. In addition, ligands with molecular weight > 250 and outliers as shown on the validation Tables will also be included. For torsion angles, if less then 5% of the Mogul distribution of torsion angles is within 10 degrees of the torsion angle in question, then that torsion angle is considered an outlier. Any bond that is central to one or more torsion angles identified as an outlier by Mogul will be highlighted in the graph. For rings, the root-mean-square deviation (RMSD) between the ring in question and similar rings identified by Mogul is calculated over all ring torsion angles. If the average RMSD is greater than 60 degrees and the minimal RMSD between the ring in question and any Mogul-identified rings is also greater than 60 degrees, then that ring is considered an outlier. The outliers are highlighted in purple. The color gray indicates Mogul did not find sufficient equivalents in the CSD to analyse the geometry.

Ligand NB2 A 401
Bond lengths Bond angles

Other polymers i ○
There are no such residues in this entry.

Polymer linkage issues i ○
There are no chain breaks in this entry. 6 Fit of model and data i ○ 6.1 Protein, DNA and RNA chains i ○ In the following table, the column labelled '#RSRZ> 2' contains the number (and percentage) of RSRZ outliers, followed by percent RSRZ outliers for the chain as percentile scores relative to all X-ray entries and entries of similar resolution. The OWAB column contains the minimum, median, 95 th percentile and maximum values of the occupancy-weighted average B-factor per residue. The column labelled 'Q< 0.9' lists the number of (and percentage) of residues with an average occupancy less than 0.9. 6.2 Non-standard residues in protein, DNA, RNA chains i ○ There are no non-standard protein/DNA/RNA residues in this entry.

Carbohydrates i ○
There are no monosaccharides in this entry.

Ligands i ○
In the following The following is a graphical depiction of the model fit to experimental electron density of all instances of the Ligand of Interest. In addition, ligands with molecular weight > 250 and outliers as shown on the geometry validation Tables will also be included. Each fit is shown from different orientation to approximate a three-dimensional view.