Extensive targeting of chemical space at the prime side of ketoamide inhibitors of rhomboid proteases by branched substituents empowers their selectivity and potency.

Rhomboid intramembrane serine proteases have been implicated in several pathologies, and emerge as attractive pharmacological target candidates. The most potent and selective rhomboid inhibitors available to date are peptidyl α-ketoamides, but their selectivity for diverse rhomboid proteases and strategies to modulate it in relevant contexts are poorly understood. This gap, together with the lack of suitable in vitro models, hinders ketoamide development for relevant eukaryotic rhomboid enzymes. Here we explore the structure-activity relationship principles of rhomboid inhibiting ketoamides by medicinal chemistry and enzymatic in vitro and in-cell assays with recombinant rhomboid proteases GlpG, human mitochondrial rhomboid PARL and human RHBDL2. We use X-ray crystallography in lipidic cubic phase to understand the binding mode of one of the best ketoamide inhibitors synthesized here containing a branched terminal substituent bound to GlpG. In addition, to extend the interpretation of the co-crystal structure, we use quantum mechanical calculations and quantify the relative importance of interactions along the inhibitor molecule. These combined experimental analyses implicates that more extensive exploration of chemical space at the prime side is unexpectedly powerful for the selectivity of rhomboid inhibiting ketoamides. Together with variations in the peptide sequence at the non-prime side, or its non-peptidic alternatives, this strategy enables targeted tailoring of potent and selective ketoamides towards diverse rhomboid proteases including disease-relevant ones such as PARL and RHBDL2.

SQM2.20: Semiempirical quantum-mechanical scoring function yields DFT-quality protein-ligand binding affinity predictions in minutes.

Accurate estimation of protein-ligand binding affinity is the cornerstone of computer-aided drug design. We present a universal physics-based scoring function, named SQM2.20, addressing key terms of binding free energy using semiempirical quantum-mechanical computational methods. SQM2.20 incorporates the latest methodological advances while remaining computationally efficient even for systems with thousands of atoms. To validate it rigorously, we have compiled and made available the PL-REX benchmark dataset consisting of high-resolution crystal structures and reliable experimental affinities for ten diverse protein targets. Comparative assessments demonstrate that SQM2.20 outperforms other scoring methods and reaches a level of accuracy similar to much more expensive DFT calculations. In the PL-REX dataset, it achieves excellent correlation with experimental data (average R2 = 0.69) and exhibits consistent performance across all targets. In contrast to DFT, SQM2.20 provides affinity predictions in minutes, making it suitable for practical applications in hit identification or lead optimization.

On the Metal-Aided Catalytic Mechanism for Phosphodiester Bond Cleavage Performed by Nanozymes.

Recent studies have shown that gold nanoparticles (AuNPs) functionalized with Zn(II) complexes can cleave phosphate esters and nucleic acids. Remarkably, such synthetic nanonucleases appear to catalyze metal (Zn)-aided hydrolytic reactions of nucleic acids similar to metallonuclease enzymes. To clarify the reaction mechanism of these nanocatalysts, here we have comparatively analyzed two nanonucleases with a >10-fold difference in the catalytic efficiency for the hydrolysis of the 2-hydroxypropyl-4-nitrophenylphosphate (HPNP, a typical RNA model substrate). We have used microsecond-long atomistic simulations, integrated with NMR experiments, to investigate the structure and dynamics of the outer coating monolayer of these nanoparticles, either alone or in complex with HPNP, in solution. We show that the most efficient one is characterized by coating ligands that promote a well-organized monolayer structure, with the formation of solvated bimetallic catalytic sites. Importantly, we have found that these nanoparticles can mimic two-metal-ion enzymes for nucleic acid processing, with Zn ions that promote HPNP binding at the reaction center. Thus, the two-metal-ion-aided hydrolytic strategy of such nanonucleases helps in explaining their catalytic efficiency for substrate hydrolysis, in accordance with the experimental evidence. These mechanistic insights reinforce the parallelism between such functionalized AuNPs and proteins toward the rational design of more efficient catalysts.

A Gold Nanoparticle Nanonuclease Relying on a Zn(II) Mononuclear Complex.

Similarly to enzymes, functionalized gold nanoparticles efficiently catalyze chemical reactions, hence the term nanozymes. Herein, we present our results showing how surface-passivated gold nanoparticles behave as synthetic nanonucleases, able to cleave pBR322 plasmid DNA with the highest efficiency reported so far for catalysts based on a single metal ion mechanism. Experimental and computational data indicate that we have been successful in creating a catalytic site precisely mimicking that suggested for natural metallonucleases relying on a single metal ion for their activity. It comprises one Zn(II) ion to which a phosphate diester of DNA is coordinated. Importantly, as in nucleic acids-processing enzymes, a positively charged arginine plays a key role by assisting with transition state stabilization and by reducing the pKa of the nucleophilic alcohol of a serine. Our results also show how designing a catalyst for a model substrate (bis-p-nitrophenylphosphate) may provide wrong indications as for its efficiency when it is tested against the real target (plasmid DNA).

Benchmark Data Sets of Boron Cluster Dihydrogen Bonding for the Validation of Approximate Computational Methods.

The success of approximate computational methods, such as molecular mechanics, or dispersion-corrected density functional theory, in the description of non-covalent interactions relies on accurate parameterizations. Benchmark data sets are thus required. This area is well developed for organic molecules and biomolecules but practically non-existent for boron clusters, which have been gaining in importance in modern drug as well as material design. To fill this gap, we have introduced two data sets featuring the most common non-covalent interaction of boron clusters, the dihydrogen bond, and calculated reference interaction energies at the "golden standard" CCSD(T)/CBS level. The boron clusters studied interact with formamide, methanol, water and methane at various distances and in two geometrical arrangements. The performance of the tested approximate methods is variable and recommendations for further use are given.

SQM/COSMO Scoring Function: Reliable Quantum-Mechanical Tool for Sampling and Ranking in Structure-Based Drug Design.

Invited for this month's cover is the group of Prof. Pavel Hobza, Czech Academy of Sciences, Prague. The cover picture shows a powerful automated quantum mechanics based SQM/COSMO approach to protein-ligand scoring. It comprises thorough preparation of ligand structures, extensive generation of binding complexes, fast geometry relaxation and reliable affinity prediction. Read the full text of the Minireview at 10.1002/cplu.202000120.

SQM/COSMO Scoring Function: Reliable Quantum-Mechanical Tool for Sampling and Ranking in Structure-Based Drug Design.

Quantum mechanical (QM) methods have been gaining importance in structure-based drug design where a reliable description of protein-ligand interactions is of utmost significance. However, strategies i. e. QM/MM, fragmentation or semiempirical (SQM) methods had to be pursued to overcome the unfavorable scaling of QM methods. Various SQM-based approaches have significantly contributed to the accuracy of docking and improvement of lead compounds. Parametrizations of SQM and implicit solvent methods in our laboratory have been instrumental to obtain a reliable SQM-based scoring function. The experience gained in its application for activity ranking of ligands binding to tens of protein targets resulted in setting up a faster SQM/COSMO scoring approach, which outperforms standard scoring methods in native pose identification for two dozen protein targets with ten thousand poses. Recently, SQM/COSMO was effectively applied in a proof-of-concept study of enrichment in virtual screening. Due to its superior performance, feasibility and chemical generality, we propose the SQM/COSMO approach as an efficient tool in structure-based drug design.

Ranking Power of the SQM/COSMO Scoring Function on Carbonic Anhydrase†II-Inhibitor Complexes.

Accurate prediction of protein-ligand binding affinities is essential for hit-to-lead optimization and virtual screening. The reliability of scoring functions can be improved by including quantum effects. Here, we demonstrate the ranking power of the semiempirical quantum mechanics (SQM)/implicit solvent (COSMO) scoring function by using a challenging set of 10 inhibitors binding to carbonic anhydrase II through Zn2+ in the active site. This new dataset consists of the high-resolution (1.1-1.4 Å) crystal structures and experimentally determined inhibitory constant (Ki ) values. It allows for evaluation of the common approximations, such as representing the solvent implicitly or by using a single target conformation combined with a set of ligand docking poses. SQM/COSMO attained a good correlation of R2 of 0.56-0.77 with the experimental inhibitory activities, benefiting from careful handling of both noncovalent interactions (e.g. charge transfer) and solvation. This proof-of-concept study of SQM/COSMO ranking for metalloprotein-ligand systems demonstrates its potential for hit-to-lead applications.

B-Hπ: a nonclassical hydrogen bond or dispersion contact?

Close B-Hπ contacts have recently been observed in crystallographic structures of Ir-dithiolene-phosphine complexes containing boron hydride cluster. This finding was interpreted using quantum chemical calculations as a new type of electrostatically driven nonclassical hydrogen bonding. However, such an explanation contradicts the wealth of evidence for unique noncovalent interactions of boron hydrides. Moreover, care must be exercised when computational methods are used to interpret new phenomena. Therefore, here, we cautiously examine the B-Hπ interaction by means of advanced quantum chemistry and disprove the claimed attractive electrostatic nature and rather define it as a nonspecific dispersion-driven contact. In summary, we present evidence that the crystallographically observed B-Hπ contacts do not constitute a novel type of hydrogen bonding of boron hydride clusters.

SQM/COSMO Scoring Function at the DFTB3-D3H4 Level: Unique Identification of Native Protein-Ligand Poses.

We have recently introduced the "SQM/COSMO" scoring function which combines a semiempirical quantum mechanical description of noncovalent interactions at the PM6-D3H4X level and the COSMO implicit model of solvation. This approach outperformed standard scoring functions but faced challenges with a metalloprotein featuring a Zn2+···S- interaction. Here, we invoke SCC-DFTB3-D3H4, a higher-level SQM method, and observe improved behavior for the metalloprotein and high-quality results for the other systems. This method holds promise for diverse protein-ligand complexes including metalloproteins.

Superior Performance of the SQM/COSMO Scoring Functions in Native Pose Recognition of Diverse Protein-Ligand Complexes in Cognate Docking.

General and reliable description of structures and energetics in protein-ligand (PL) binding using the docking/scoring methodology has until now been elusive. We address this urgent deficiency of scoring functions (SFs) by the systematic development of corrected semiempirical quantum mechanical (SQM) methods, which correctly describe all types of noncovalent interactions and are fast enough to treat systems of thousands of atoms. Two most accurate SQM methods, PM6-D3H4X and SCC-DFTB3-D3H4X, are coupled with the conductor-like screening model (COSMO) implicit solvation model in so-called "SQM/COSMO" SFs and have shown unique recognition of native ligand poses in cognate docking in four challenging PL systems, including metalloprotein. Here, we apply the two SQM/COSMO SFs to 17 diverse PL complexes and compare their performance with four widely used classical SFs (Glide XP, AutoDock4, AutoDock Vina, and UCSF Dock). We observe superior performance of the SQM/COSMO SFs and identify challenging systems. This method, due to its generality, comparability across the chemical space, and lack of need for any system-specific parameters, gives promise of becoming, after comprehensive large-scale testing in the near future, a useful computational tool in structure-based drug design and serving as a reference method for the development of other SFs.

The SQM/COSMO filter: reliable native pose identification based on the quantum-mechanical description of protein-ligand interactions and implicit COSMO solvation.

Current virtual screening tools are fast, but reliable scoring is elusive. Here, we present the 'SQM/COSMO filter', a novel scoring function featuring a quantitative semiempirical quantum mechanical (SQM) description of all types of noncovalent interactions coupled with implicit COSMO solvation. We show unequivocally that it outperforms eight widely used scoring functions. The accuracy and chemical generality of the SQM/COSMO filter make it a perfect tool for late stages of virtual screening.

The non-planarity of the benzene molecule in the X-ray structure of the chelated bismuth(III) heteroboroxine complex is not supported by quantum mechanical calculations.

The non-planarity of the benzene moiety in the crystal of a chelated bismuth(iii) heteroboroxine complex was not supported by DFT-D quantum chemical calculations. The observed bent structure of benzene is in fact a superimposition (thermal average) of the ensemble of thermally populated benzene structures in the complex studied.

Atomic resolution crystal structure of Sapp2p, a secreted aspartic protease from Candida parapsilosis.

The virulence of the Candida pathogens is enhanced by the production of secreted aspartic proteases, which therefore represent possible targets for drug design. Here, the crystal structure of the secreted aspartic protease Sapp2p from Candida parapsilosis was determined. Sapp2p was isolated from its natural source and crystallized in complex with pepstatin A, a classical aspartic protease inhibitor. The atomic resolution of 0.83 Šallowed the protonation states of the active-site residues to be inferred. A detailed comparison of the structure of Sapp2p with the structure of Sapp1p, the most abundant C. parapsilosis secreted aspartic protease, was performed. The analysis, which included advanced quantum-chemical interaction-energy calculations, uncovered molecular details that allowed the experimentally observed equipotent inhibition of both isoenzymes by pepstatin A to be rationalized.

Characteristics of a σ-Hole and the Nature of a Halogen Bond.

The nature of halogen bonding in 128 complexes was investigated using advanced quantum mechanical calculations. First, isolated halogen donors were studied and their σ-holes were described in terms of size and magnitude. Later, both partners in the complex were considered and their interaction was described in terms of DFT-SAPT decomposition. The whole set of complexes under study was split into two categories on the basis of their stabilisation energy. The first subset with 38 complexes possesses stabilisation energies in the range 7-32 kcal/mol, while the second subset with 90 complexes has stabilisation energies smaller than 7 kcal/mol. The first subset is characterised by small intermolecular distances (less than 2.5 Å) and a significant contraction of van der Waals (vdW) distance (sum of vdW radii). Here the polarisation/electrostatic energy is dominant, mostly followed by induction and dispersion energies. The importance of induction energy reflects the charge-transfer character of the respective halogen bonds. Intermolecular distances in the second subset are large and the respective contraction of vdW distance upon the formation of a halogen bond is much smaller. Here the dispersion energy is mostly dominant, followed by polarisation and induction energies. Considering the whole set of complexes, we conclude that the characteristic features of their halogen bonds arise from the concerted action of polarisation and dispersion energies and neither of these energies can be considered as dominant. Finally, the magnitude of the σ-hole and DFT-SAPT stabilisation energy correlates only weakly within the whole set of complexes.

Chalcogen and pnicogen bonds in complexes of neutral icosahedral and bicapped square-antiprismatic heteroboranes.

A systematic quantum mechanical study of σ-hole (chalcogen, pnicogen, and halogen) bonding in neutral experimentally known closo-heteroboranes is performed. Chalcogens and pnicogens are incorporated in the borane cage, whereas halogens are considered as exo-substituents of dicarbaboranes. The chalcogen and pnicogen atoms in the heteroborane cages have partial positive charge and thus more positive σ-holes. Consequently, these heteroboranes form very strong chalcogen and pnicogen bonds. Halogen atoms in dicarbaboranes also have a highly positive σ-hole, but only in the case of C-bonded halogen atoms. In such cases, the halogen bond of heteroboranes is also strong and comparable to halogen bonds in organic compounds with several electron-withdrawing groups being close to the halogen atom involved in the halogen bond.

Carborane-based carbonic anhydrase inhibitors: insight into CAII/CAIX specificity from a high-resolution crystal structure, modeling, and quantum chemical calculations.

Carborane-based compounds are promising lead structures for development of inhibitors of carbonic anhydrases (CAs). Here, we report structural and computational analysis applicable to structure-based design of carborane compounds with selectivity toward the cancer-specific CAIX isoenzyme. We determined the crystal structure of CAII in complex with 1-methylenesulfamide-1,2-dicarba-closo-dodecaborane at 1.0 Å resolution and used this structure to model the 1-methylenesulfamide-1,2-dicarba-closo-dodecaborane interactions with CAIX. A virtual glycine scan revealed the contributions of individual residues to the energy of binding of 1-methylenesulfamide-1,2-dicarba-closo-dodecaborane to CAII and CAIX, respectively.

The dominant role of chalcogen bonding in the crystal packing of 2D/3D aromatics.

The chalcogen bond is a nonclassical σ-hole-based noncovalent interaction with emerging applications in medicinal chemistry and material science. It is found in organic compounds, including 2D aromatics, but has so far never been observed in 3D aromatic inorganic boron hydrides. Thiaboranes, harboring a sulfur heteroatom in the icosahedral cage, are candidates for the formation of chalcogen bonds. The phenyl-substituted thiaborane, synthesized and crystalized in this study, forms sulfur⋅⋅⋅π type chalcogen bonds. Quantum chemical analysis revealed that these interactions are considerably stronger than both in their organic counterparts and in the known halogen bond. The reason is the existence of a highly positive σ-hole on the positively charged sulfur atom. This discovery expands the possibilities of applying substituted boron clusters in crystal engineering and drug design.

QM/MM calculations reveal the different nature of the interaction of two carborane-based sulfamide inhibitors of human carbonic anhydrase II.

The crystal structures of two novel carborane-sulfamide inhibitors in the complex with human carbonic anhydrase II (hCAII) have been studied using QM/MM calculations. Even though both complexes possess the strongly interacting sulfamide···zinc ion motif, the calculations have revealed the different nature of binding of the carborane parts of the inhibitors. The neutral closo-carborane cage was bound to hCAII mainly via dispersion interactions and formed only very weak dihydrogen bonds. On the contrary, the monoanionic nido cage interacted with the protein mainly via electrostatic interactions. It formed short and strong dihydrogen bonds (stabilization of up to 4.2 kcal/mol; H···H distances of 1.7 Å) with the polar hydrogen of protein NH2 groups. This type of binding is unique among all of the classical organic and inorganic inhibitors of hCAII. Virtual glycine scanning allowed us to identify the amino-acid side chains, which made important contributions to ligand-binding energies. In summary, using QM/MM calculations, we have provided a detailed understanding of the differences between the interactions of two carborane sulfamides, identified the amino acids of hCAII with which they interact, and thus paved the way for the computer-aided rational design of selective boron-cluster-containing hCAII inhibitors.

The Semiempirical Quantum Mechanical Scoring Function for In Silico Drug Design.

This Minireview discusses the latest developments in modern quantum mechanics (QM)-based computer-aided drug design, especially using semiempirical QM (SQM) methods. It first tackles biochemical and biophysical quantities and the approaches to their measurements. Protein-ligand affinities are determined mostly by noncovalent interactions. The text thus illustrates how these can be accurately treated with SQM methods. Next, a construction of a modern SQM-based scoring function is presented and its applications listed. In summary, SQM-based scoring is a promising modern efficient strategy to be exploited in computer-aided drug design.