A large-scale screen to target SARS-CoV-2: The severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) genome is initially expressed as two large polyproteins. Its main protease, M ^pro , is essential to yield functional viral proteins, making it a key drug target. Günther et al. used x-ray crystallography to screen more than 5000 compounds that are either approved drugs or drugs in clinical trials. The screen identified 37 compounds that bind to M ^pro . High-resolution structures showed that most compounds bind at the active site but also revealed two allosteric sites where binding of a drug causes conformational changes that affect the active site. In cell-based assays, seven compounds had antiviral activity without toxicity. The most potent, calpeptin, binds covalently in the active site, whereas the second most potent, pelitinib, binds at an allosteric site. Science , this issue p. 642

The most frequently occurring resistance of Gram-negative bacteria against tetracyclines is triggered by drug recognition of the Tet repressor. This causes dissociation of the repressor-operator DNA complex and enables expression of the resistance protein TetA, which is responsible for active efflux of tetracycline. The 2.5 angstrom resolution crystal structure of the homodimeric Tet repressor complexed with tetracycline-magnesium reveals detailed drug recognition. The orientation of the operator-binding helix-turn-helix motifs of the repressor is inverted in comparison with other DNA binding proteins. The repressor-drug complex is unable to interact with DNA because the separation of the DNA binding motifs is 5 angstroms wider than usually observed.

The fungal proteinase K (EC 3.4.21.14) is a very potent unusually stable member of the subtilisin family. Its X‐ray structure determined at 0.15‐nm resolution shows two bound Ca²⁺ ions. Cal isin near‐ideal pentagonal bipyramidal configuration with Asp200 carboxylate and Prol75 peptide C=O in an apical, and Vai177 peptide C=O and four water molecules in an equatorial position, whereas Ca2 displays incomplete octahedral coordination with the carboxylate of Asp260, the peptide C=O of Vail6 and two water molecules. Scatchard analysis of the titration of Ca²⁺‐free proteinase K with Ca²⁺ yields a single dissociation constant (7.6±2.5) × 10 ⁻⁸ M associated with the tightly bound Cal whereas Ca² is so weakly bound that it cannot be titrated. If proteinase K is depleted of Ca²⁺ by treatment with EDTA, followed by gel filtration, its enzymatic activity drops within 6 h to 20% of its original value, without autolysis. Addition of excess Ca²⁺ immediately raises the residual activity to 28%, but full activity is not achieved. Removal of Ca²⁺ triggers a conformational change of the substrate recognition site because there is a direct connection, via secondary structure hydrogen bonds, between the Cal binding site and the substrate‐recognition site. This is indicated further by circular dichroism and fluorescence‐spectroscopic data, and by reversed‐phase FPLC, carried out in the presence and absence of Ca²⁺, but the overall structure of the enzyme is not affected. Depletion of Ca²⁺ also influences binding of longer peptide inhibitors of the chloromethane type, it increases the rate of autolysis after about 48 h, it reduces the thermal stability (measured by activity tests from 65 °C to 46 °C), and it enhances the deactivation by 8 M urea which inactivates to only 65%, whereas sodium dodecyl sulfate totally inactivates at a concentration of 12.5%.