AI-guided pipeline for protein-protein interaction drug discovery identifies a SARS-CoV-2 inhibitor.

Protein-protein interactions (PPIs) offer great opportunities to expand the druggable proteome and therapeutically tackle various diseases, but remain challenging targets for drug discovery. Here, we provide a comprehensive pipeline that combines experimental and computational tools to identify and validate PPI targets and perform early-stage drug discovery. We have developed a machine learning approach that prioritizes interactions by analyzing quantitative data from binary PPI assays or AlphaFold-Multimer predictions. Using the quantitative assay LuTHy together with our machine learning algorithm, we identified high-confidence interactions among SARS-CoV-2 proteins for which we predicted three-dimensional structures using AlphaFold-Multimer. We employed VirtualFlow to target the contact interface of the NSP10-NSP16 SARS-CoV-2 methyltransferase complex by ultra-large virtual drug screening. Thereby, we identified a compound that binds to NSP10 and inhibits its interaction with NSP16, while also disrupting the methyltransferase activity of the complex, and SARS-CoV-2 replication. Overall, this pipeline will help to prioritize PPI targets to accelerate the discovery of early-stage drug candidates targeting protein complexes and pathways.

Sulfonated red and far-red rhodamines to visualize SNAP- and Halo-tagged cell surface proteins.

The (in)ability to permeate membranes is a key feature of chemical biology probes that defines their suitability for specific applications. Here we report sulfonated rhodamines that endow xanthene dyes with cellular impermeability for analysis of surface proteins. We fuse charged sulfonates to red and far-red dyes to obtain Sulfo549 and Sulfo646, respectively, and further link these to benzylguanine and choloralkane substrates for SNAP-tag and Halo-tag labelling. Sulfonated rhodamine-conjugated fluorophores maintain desirable photophysical properties, such as brightness and photostability. While transfected cells with a nuclear localized SNAP-tag remain unlabelled, extracellular exposed tags can be cleanly visualized. By multiplexing with a permeable rhodamine, we are able to differentiate extra- and intracellular SNAP- and Halo-tags, including those installed on the glucagon-like peptide-1 receptor, a prototypical class B G protein-coupled receptor. Sulfo549 and Sulfo646 also labelled transfected neurons derived from induced pluripotent stem cells (iPSCs), allowing STED nanoscopy of the axonal membrane. Together, this work provides a new avenue for rendering dyes impermeable for exclusive extracellular visualization via self-labelling protein tags. We anticipate that Sulfo549, Sulfo646 and their congeners will be useful for a number of cell biology applications where labelling of intracellular sites interferes with accurate surface protein analysis.

Probing the Interactions of Sulfur-Containing Histidine Compounds with Human Gamma-Glutamyl Transpeptidase.

Gamma-glutamyl transpeptidase (GGT) is a cell surface enzyme involved in glutathione metabolism and maintenance of redox homeostasis. High expression of GGT on tumor cells is associated with an increase of cell proliferation and resistance against chemotherapy. GGT inhibitors that have been evaluated in clinical trials are too toxic for human use. We have previously identified ovothiols, 5(Nπ)-methyl-thiohistidines of marine origin, as non-competitive-like inhibitors of GGT that are more potent than the known GGT inhibitor, 6-diazo-5-oxo-l-norleucine (DON), and are not toxic for human embryonic cells. We extended these studies to the desmethylated form of ovothiol, 5-thiohistidine, and confirmed that this ovothiol derivative also acts as a non-competitive-like GGT inhibitor, with a potency comparable to ovothiol. We also found that both 5-thiohistidine derivatives act as reversible GGT inhibitors compared to the irreversible DON. Finally, we probed the interactions of 5-thiohistidines with GGT by docking analysis and compared them with the 2-thiohistidine ergothioneine, the physiological substrate glutathione, and the DON inhibitor. Overall, our results provide new insight for further development of 5-thiohistidine derivatives as therapeutics for GGT-positive tumors.

Small Molecules Targeting Human N-Acetylmannosamine Kinase.

N-Acetylmannosamine kinase (MNK) plays a key role in the biosynthesis of sialic acids and glycosylation of proteins. Sialylated glycoconjugates affect a large number of biological processes, including immune modulation and cancer transformation. In search of effective inhibitors of MNK we applied high-throughput screening of drug-like small molecules. By applying different orthogonal assays for their validation we identified four potential MNK-specific inhibitors with IC50 values in the low-micromolar range. Molecular modelling of the inhibitors into the active site of MNK supports their binding to the sugar or the ATP-binding pocket of the enzyme or both. These compounds are promising for downregulation of the sialic acid content of glycoconjugates and for studying the functional contribution of sialic acids to disease development.

Substrate Hunting for the Myxobacterial CYP260A1 Revealed New 1α-Hydroxylated Products from C-19 Steroids.

Cytochromes P450 catalyze a variety of synthetically useful reactions. However, it is difficult to determine their physiological or artificial functions when a plethora of orphan P450 systems are present in a genome. CYP260A1 from Sorangium cellulosum So ce56 is a new member among the 21 available P450s in the strain. To identify putative substrates for CYP260A1 we used high-throughput screening of a compound library (ca. 17,000 ligands). Structural analogues of the type I hits were searched for biotechnologically relevant compounds, and this led us to select C-19 steroids as potential substrates. We identified efficient surrogate redox partners for CYP260A1, and an Escherichia coli-based whole-cell biocatalyst system was developed to convert testosterone, androstenedione, and their derivatives methyltestosterone and 11-oxoandrostenedione. A detailed (1) H and (13) C NMR characterization of the product(s) from C-19 steroids revealed that CYP260A1 is the very first 1α-steroid hydroxylase.

Fluorescent mimetics of CMP-Neu5Ac are highly potent, cell-permeable polarization probes of eukaryotic and bacterial sialyltransferases and inhibit cellular sialylation.

Oligosaccharides of the glycolipids and glycoproteins at the outer membranes of human cells carry terminal neuraminic acids, which are responsible for recognition events and adhesion of cells, bacteria, and virus particles. The synthesis of neuraminic acid containing glycosides is accomplished by intracellular sialyl transferases. Therefore, the chemical manipulation of cellular sialylation could be very important to interfere with cancer development, inflammations, and infections. The development and applications of the first nanomolar fluorescent inhibitors of sialyl transferases are described herein. The obtained carbohydrate-nucleotide mimetics were found to bind all four commercially available and tested eukaryotic and bacterial sialyl transferases in a fluorescence polarization assay. Moreover, it was observed that the anionic mimetics intruded rapidly and efficiently into cells in vesicles and translocated to cellular organelles surrounding the nucleus of CHO cells. The new compounds inhibit cellular sialylation in two cell lines and open new perspectives for investigations of cellular sialylation.

The opening dynamics of the lateral gate regulates the activity of rhomboid proteases.

Rhomboid proteases hydrolyze substrate helices within the lipid bilayer to release soluble domains from the membrane. Here, we investigate the mechanism of activity regulation for this unique but wide-spread protein family. In the model rhomboid GlpG, a lateral gate formed by transmembrane helices TM2 and TM5 was previously proposed to allow access of the hydrophobic substrate to the shielded hydrophilic active site. In our study, we modified the gate region and either immobilized the gate by introducing a maleimide-maleimide (M2M) crosslink or weakened the TM2/TM5 interaction network through mutations. We used solid-state nuclear magnetic resonance (NMR), molecular dynamics (MD) simulations, and molecular docking to investigate the resulting effects on structure and dynamics on the atomic level. We find that variants with increased dynamics at TM5 also exhibit enhanced activity, whereas introduction of a crosslink close to the active site strongly reduces activity. Our study therefore establishes a strong link between the opening dynamics of the lateral gate in rhomboid proteases and their enzymatic activity.

AI-guided pipeline for protein-protein interaction drug discovery identifies a SARS-CoV-2 inhibitor.

Protein-protein interactions (PPIs) offer great opportunities to expand the druggable proteome and therapeutically tackle various diseases, but remain challenging targets for drug discovery. Here, we provide a comprehensive pipeline that combines experimental and computational tools to identify and validate PPI targets and perform early-stage drug discovery. We have developed a machine learning approach that prioritizes interactions by analyzing quantitative data from binary PPI assays and AlphaFold-Multimer predictions. Using the quantitative assay LuTHy together with our machine learning algorithm, we identified high-confidence interactions among SARS-CoV-2 proteins for which we predicted three-dimensional structures using AlphaFold Multimer. We employed VirtualFlow to target the contact interface of the NSP10-NSP16 SARS-CoV-2 methyltransferase complex by ultra-large virtual drug screening. Thereby, we identified a compound that binds to NSP10 and inhibits its interaction with NSP16, while also disrupting the methyltransferase activity of the complex, and SARS-CoV-2 replication. Overall, this pipeline will help to prioritize PPI targets to accelerate the discovery of early-stage drug candidates targeting protein complexes and pathways.

Identification of CYP106A2 as a regioselective allylic bacterial diterpene hydroxylase.

The cytochrome P450 monooxygenase CYP106A2 from Bacillus megaterium ATCC 13368 catalyzes hydroxylations of a variety of 3-oxo-Δ(4) -steroids such as progesterone and deoxycorticosterone (DOC), mainly in the 15ß-position. We combined a high-throughput screening and a rational approach for identifying new substrates of CYP106A2. The diterpene resin acid abietic acid was found to be a substrate and was docked into the active site of a CYP106A2 homology model to provide further inside into the structural basis of the regioselectivity of hydroxylation. The products of the hydroxylation reaction were analyzed by HPLC and the V(max) and K(m) values were calculated. The corresponding reaction products were analyzed by NMR spectroscopy and identified as 12α- and 12ß-hydroxyabietic acid. CYP106A2 was therefore identified as the first reported bacterial cytochrome P450 diterpene hydroxylase. Furthermore, an effective whole-cell catalyst for the selective allylic 12α- and 12ß-hydroxylation was applied to produce the hydroxylated products.

Cyanobacterial electron carrier proteins as electron donors to CYP106A2 from Bacillus megaterium ATCC 13368.

The CYP450 from Bacillus megaterium (BmCYP106A2) catalyzes the 15beta-hydroxylation of several steroids and also synthesizes mono-hydroxylated 9alpha- and 11alpha-OH-progesterone. This study reports on the ability of BmCYP106A2 to be efficiently reduced by the photosynthetic flavodoxin and, particularly, ferredoxin electron carriers from the cyanobacterium Anabaena. These results open the possibility for the design of a hybrid system to provide reducing equivalents for the hydroxylation process. Additionally, they suggest that despite the interaction of BmCYP106A2 with these proteins, particularly with flavodoxin, they do not rely on a precise complementarity of the reacting molecules, rearrangements might be required and alternative binding modes might contribute to the observed electron transfer reactions.

Design of chemical libraries with potentially bioactive molecules applying a maximum common substructure concept.

Success in small molecule screening relies heavily on the preselection of compounds. Here, we present a strategy for the enrichment of chemical libraries with potentially bioactive compounds integrating the collected knowledge of medicinal chemistry. Employing a genetic algorithm, substructures typically occurring in bioactive compounds were identified using the World Drug Index. Availability of compounds containing the selected substructures was analysed in vendor libraries, and the substructure-specific sublibraries were assembled. Compounds containing reactive, undesired functional groups were omitted. Using a diversity filter for both physico-chemical properties and the substructure composition, the compounds of all the sublibraries were ranked. Accordingly, a screening collection of 16,671 compounds was selected. Diversity and chemical space coverage of the collection indicate that it is highly diverse and well-placed in the chemical space spanned by bioactive compounds. Furthermore, secondary assay-validated hits presented in this study show the practical relevance of our library design strategy.

The transcription factor STAT5 catalyzes Mannich ligation reactions yielding inhibitors of leukemic cell proliferation.

Protein-templated fragment ligations have been established as a powerful method for the assembly and detection of optimized protein ligands. Initially developed for reversible ligations, the method has been expanded to irreversible reactions enabling the formation of super-additive fragment combinations. Here, protein-induced Mannich ligations are discovered as a biocatalytic reaction furnishing inhibitors of the transcription factor STAT5. STAT5 protein catalyzes multicomponent reactions of a phosphate mimetic, formaldehyde, and 1H-tetrazoles yielding protein ligands with greatly increased binding affinity and ligand efficiency. Reactions are induced under physiological conditions selectively by native STAT5 but not by other proteins. Formation of ligation products and (auto-)inhibition of the reaction are quantified and the mechanism is investigated. Inhibitors assembled by STAT5 block specifically the phosphorylation of this protein in a cellular model of acute myeloid leukemia (AML), DNA-binding of STAT5 dimers, expression of downstream targets of the transcription factor, and the proliferation of cancer cells in mice.

Theoretical and experimental evaluation of a CYP106A2 low homology model and production of mutants with changed activity and selectivity of hydroxylation.

Steroids are important pharmaceutically active compounds. In contrast to the liver drug-metabolising cytochrome P450s, which metabolise a variety of substrates, steroid hydroxylases generally display a rather narrow substrate specificity. It is therefore a challenging goal to change their regio- and stereoselectivity. CYP106A2 is one of only a few bacterial steroid hydroxylases and hydroxylates 3-oxo-Delta4-steroids mainly in 15beta-position. In order to gain insights into the structure and function of this enzyme, whose crystal structure is unknown, a homology model has been created. The substrate progesterone was then docked into the active site to predict which residues might affect substrate binding. The model was substantiated by using a combination of theoretical and experimental investigations. First, numerous computational structure evaluation tools assessed the plausibility of its protein geometry and its quality. Second, the model explains many key properties of common cytochrome P450s. Third, two sets of mutants have been heterologously expressed, and the influence of the mutations on the catalytic activity towards deoxycorticosterone and progesterone has been studied experimentally: the first set comprises six mutations located in the structurally variable regions of this enzyme that are very difficult to predict by cytochrome P450 modelling (K27R, I86T, E90V, I71T, D185G and I215T). For these positions, no participation in the active-site formation was predicted, or could be experimentally demonstrated. The second set comprises five mutants in substrate recognition site 6 (S394I, A395L, T396R, G397P and Q398S). For these residues, participation in active-site formation and an influence on substrate binding was predicted by docking. These mutants are based on an alignment with human CYP11B1, and in fact most of these mutants altered the active-site structure and the hydroxylation activity of CYP106A2 dramatically.

Modulation of aldosterone and cortisol synthesis on the molecular level.

CYP11B1 and the closely related CYP11B2 are involved in the production of adrenal steroid hormones. Although in human their primary structure is 93% identical they are involved in the biosynthesis of functionally diverse products, such as glucocorticoids and mineralocorticoids, respectively. In contrast, bovine CYP11B1 combines both activities in one single enzyme. The CYP11B family belongs to class I cytochromes P450 that have been described in bacteria and mitochondria and receive their electrons from a low molecular weight iron sulphur protein which is reduced by a NADPH-dependent FAD-containing reductase. In this review, we summarise the current knowledge on the modulation of aldosterone and cortisol synthesis by transcriptional regulation, on the molecular level as consequence of mutations found in patients suffering from steroid hormone-related diseases as well as introduced by site-directed mutagenesis and as consequence of protein-protein interaction with both CYP11A1 and the natural redox partner adrenodoxin.

Deletions in the loop surrounding the iron-sulfur cluster of adrenodoxin severely affect the interactions with its native redox partners adrenodoxin reductase and cytochrome P450(scc) (CYP11A1).

The redox active iron-sulfur center of bovine adrenodoxin is coordinated by four cysteine residues in positions 46, 52, 55 and 92 and is covered by a loop containing the residues Glu-47, Gly-48, Thr-49, Leu-50 and Ala-51. In plant-type [2Fe-2S] ferredoxins, the corresponding loop consists of only four amino acids. The loop is positioned at the surface of the proteins and forms a boundary separating the [2Fe-2S] cluster from solvent. In order to analyze the biological function of the five amino acids of the loop in adrenodoxin (Adx) for this electron transfer protein each residue was deleted by site-directed mutagenesis. The resulting five recombinant Adx variants show dramatic differences among each other regarding their spectroscopic characteristics and functional properties. The redox potential is affected differently depending on the position of the conducted deletion. In contrast, all mutations in the protein loop influence the binding to the redox partners adrenodoxin reductase (AdR) and cytochrome P450(scc) (CYP11A1) indicating the importance of this loop for the physiological function of this iron--sulfur protein.

Design of a general-purpose European compound screening library for EU-OPENSCREEN.

This work describes a collaborative effort to define and apply a protocol for the rational selection of a general-purpose screening library, to be used by the screening platforms affiliated with the EU-OPENSCREEN initiative. It is designed as a standard source of compounds for primary screening against novel biological targets, at the request of research partners. Given the general nature of the potential applications of this compound collection, the focus of the selection strategy lies on ensuring chemical stability, absence of reactive compounds, screening-compliant physicochemical properties, loose compliance to drug-likeness criteria (as drug design is a major, but not exclusive application), and maximal diversity/coverage of chemical space, aimed at providing hits for a wide spectrum of drugable targets. Finally, practical availability/cost issues cannot be avoided. The main goal of this publication is to inform potential future users of this library about its conception, sources, and characteristics. The outline of the selection procedure, notably of the filtering rules designed by a large committee of European medicinal chemists and chemoinformaticians, may be of general methodological interest for the screening/medicinal chemistry community. The selection task of 200K molecules out of a pre-filtered set of 1.4M candidates was shared by five independent European research groups, each picking a subset of 40K compounds according to their own in-house methodology and expertise. An in-depth analysis of chemical space coverage of the library serves not only to characterize the collection, but also to compare the various chemoinformatics-driven selection procedures of maximal diversity sets. Compound selections contributed by various participating groups were mapped onto general-purpose self-organizing maps (SOMs) built on the basis of marketed drugs and bioactive reference molecules. In this way, the occupancy of chemical space by the EU-OPENSCREEN library could be directly compared with distributions of known bioactives of various classes. This mapping highlights the relevance of the selection and shows how the consensus reached by merging the five different 40K selections contributes to achieve this relevance. The approach also allows one to readily identify subsets of target- or target-class-oriented compounds from the EU-OPENSCREEN library to suit the needs of the diverse range of potential users. The final EU-OPENSCREEN library, assembled by merging five independent selections of 40K compounds from various expert groups, represents an excellent example of a Europe-wide collaborative effort toward the common objective of building best-in-class European open screening platforms.

Alzheimer's disease: identification and development of ß-secretase (BACE-1) binding fragments and inhibitors by dynamic ligation screening (DLS).

The application of dynamic ligation screening (DLS), a methodology for fragment-based drug discovery (FBDD), to the aspartic protease ß-secretase (BACE-1) is reported. For this purpose, three new fluorescence resonance energy transfer (FRET) substrates were designed and synthesized. Their kinetic parameters (Vmax , KM , and kcat ) were determined and compared with a commercial substrate. Secondly, a peptide aldehyde was designed as a chemically reactive inhibitor (CRI) based on the Swedish mutation substrate sequence. Incubation of this CRI with the protease, a FRET substrate, and one amine per well taken from an amine library, which was assembled by a maximum common substructure (MCS) approach, revealed the fragment 3-(3-aminophenyl)-2H-chromen-2-one (1) to be a competitive BACE-1 inhibitor that enhanced the activity of the CRI. Irreversibly formed fragment combination products of 1 with the initial peptide sequence were active and confirmed the targeting of the active site through the ethane-1,2-diamine isostere. Finally, structure-assisted combination of fragment 1 with secondary fragments that target the S1 site in hit optimization yielded novel, entirely fragment-based BACE-1 inhibitors with up to 30-fold improved binding affinity. Interactions with the protein were explained by molecular modeling studies, which indicate that the new fragment combinations interact with the catalytic aspartic acid dyad, as well as with the adjacent binding sites required for potency.

The structure of human ADP-ribosylhydrolase 3 (ARH3) provides insights into the reversibility of protein ADP-ribosylation.

Posttranslational modifications are used by cells from all kingdoms of life to control enzymatic activity and to regulate protein function. For many cellular processes, including DNA repair, spindle function, and apoptosis, reversible mono- and polyADP-ribosylation constitutes a very important regulatory mechanism. Moreover, many pathogenic bacteria secrete toxins which ADP-ribosylate human proteins, causing diseases such as whooping cough, cholera, and diphtheria. Whereas the 3D structures of numerous ADP-ribosylating toxins and related mammalian enzymes have been elucidated, virtually nothing is known about the structure of protein de-ADP-ribosylating enzymes. Here, we report the 3Dstructure of human ADP-ribosylhydrolase 3 (hARH3). The molecular architecture of hARH3 constitutes the archetype of an all-alpha-helical protein fold and provides insights into the reversibility of protein ADP-ribosylation. Two magnesium ions flanked by highly conserved amino acids pinpoint the active-site crevice. Recombinant hARH3 binds free ADP-ribose with micromolar affinity and efficiently de-ADP-ribosylates poly- but not monoADP-ribosylated proteins. Docking experiments indicate a possible binding mode for ADP-ribose polymers and suggest a reaction mechanism. Our results underscore the importance of endogenous ADP-ribosylation cycles and provide a basis for structure-based design of ADP-ribosylhydrolase inhibitors.