A Novel Determinant of PSMD9 PDZ Binding Guides the Evolution of the First Generation of Super Binding Peptides.

The PDZ domain is one of the most widespread protein interaction domains found in nature. Due to their integral role in numerous biological functions, their ability to act as scaffolds for signal amplification, and the occurrence of mutations linked to human diseases, PDZ domains are attractive therapeutic targets. On the basis of the differential binding affinities of selected C-terminal peptides of the human proteome for one such PDZ domain (PSMD9) and by exploring structure-activity relationships, we design and convert a low-affinity tetrapeptide (∼439 µM) to a tight binding sequence (∼5 µM). The peptide inhibits PSMD9-hnRNPA1 interactions that are critical in basal and stimulus-induced NF-κB signaling and a potential therapeutic target in cancers, including chemotherapy or radiation-induced therapy resistance. Extensive application of computer modeling, including ligand mapping and all-atom molecular dynamics simulations, helps us to rationalize the structural basis for the huge differences in binding affinity and inform us about the residue-wise contributions to the binding energy. Our findings are in accord with the classical preference of the (PSMD9) PDZ domain for C-terminal sequences that contain hydrophobic residues at the P0 (C-terminal) position. In addition, for the first time, we identify a hitherto unknown occupancy for cysteine at the P-2 position that drives high-affinity interaction in a PDZ domain.

Incorporation of Putative Helix-Breaking Amino Acids in the Design of Novel Stapled Peptides: Exploring Biophysical and Cellular Permeability Properties.

Stapled α-helical peptides represent an emerging superclass of macrocyclic molecules with drug-like properties, including high-affinity target binding, protease resistance, and membrane permeability. As a model system for probing the chemical space available for optimizing these properties, we focused on dual Mdm2/MdmX antagonist stapled peptides related to the p53 N-terminus. Specifically, we first generated a library of ATSP-7041 (Chang et al., 2013) analogs iteratively modified by L-Ala and D-amino acids. Single L-Ala substitutions beyond the Mdm2/(X) binding interfacial residues (i.e., Phe3, Trp7, and Cba10) had minimal effects on target binding, α-helical content, and cellular activity. Similar binding affinities and cellular activities were noted at non-interfacial positions when the template residues were substituted with their d-amino acid counterparts, despite the fact that d-amino acid residues typically 'break' right-handed α-helices. d-amino acid substitutions at the interfacial residues Phe3 and Cba10 resulted in the expected decreases in binding affinity and cellular activity. Surprisingly, substitution at the remaining interfacial position with its d-amino acid equivalent (i.e., Trp7 to d-Trp7) was fully tolerated, both in terms of its binding affinity and cellular activity. An X-ray structure of the d-Trp7-modified peptide was determined and revealed that the indole side chain was able to interact optimally with its Mdm2 binding site by a slight global re-orientation of the stapled peptide. To further investigate the comparative effects of d-amino acid substitutions we used linear analogs of ATSP-7041, where we replaced the stapling amino acids by Aib (i.e., R84 to Aib4 and S511 to Aib11) to retain the helix-inducing properties of α-methylation. The resultant analog sequence Ac-Leu-Thr-Phe-Aib-Glu-Tyr-Trp-Gln-Leu-Cba-Aib-Ser-Ala-Ala-NH2 exhibited high-affinity target binding (Mdm2 Kd = 43 nM) and significant α-helicity in circular dichroism studies. Relative to this linear ATSP-7041 analog, several d-amino acid substitutions at Mdm2(X) non-binding residues (e.g., d-Glu5, d-Gln8, and d-Leu9) demonstrated decreased binding and α-helicity. Importantly, circular dichroism (CD) spectroscopy showed that although helicity was indeed disrupted by d-amino acids in linear versions of our template sequence, stapled molecules tolerated these residues well. Further studies on stapled peptides incorporating N-methylated amino acids, l-Pro, or Gly substitutions showed that despite some positional dependence, these helix-breaking residues were also generally tolerated in terms of secondary structure, binding affinity, and cellular activity. Overall, macrocyclization by hydrocarbon stapling appears to overcome the destabilization of α-helicity by helix breaking residues and, in the specific case of d-Trp7-modification, a highly potent ATSP-7041 analog (Mdm2 Kd = 30 nM; cellular EC50 = 600 nM) was identified. Our findings provide incentive for future studies to expand the chemical diversity of macrocyclic α-helical peptides (e.g., d-amino acid modifications) to explore their biophysical properties and cellular permeability. Indeed, using the library of 50 peptides generated in this study, a good correlation between cellular permeability and lipophilicity was observed.

Characterization of Hydration Properties in Structural Ensembles of Biomolecules.

Solute-solvent interactions are critical for biomolecular stability and recognition. Explicit solvent molecular dynamics (MD) simulations are routinely used to probe such interactions. However, detailed analyses and interpretation of the hydration patterns seen in MD simulations can be both complex and time-consuming. A variety of approaches/tools to compute and interrogate hydration properties in structural ensembles of proteins, nucleic acids, or in general any molecule are available and are complemented here with a new and free software package ("JAL"). Central to "JAL" is an intuitive atom centric approach of computing hydration properties. In addition to the standard metrics commonly used to understand hydration, "JAL" introduces two nonstandard utilities: a program to rapidly compute buried waters in an MD trajectory and a new method to compute multiwater bridges around a solute. We demonstrate the utility of the package by probing the hydration characteristics of the tumor suppressor protein p53 and the translation initiation factor eif4E. "JAL" is hosted online and can be accessed for free at http://mspc.bii.a-star.edu.sg/minhn/jal.html .

Simulations of mutant p53 DNA binding domains reveal a novel druggable pocket.

The DNA binding domain (DBD) of the tumor suppressor p53 is the site of several oncogenic mutations. A subset of these mutations lowers the unfolding temperature of the DBD. Unfolding leads to the exposure of a hydrophobic ß-strand and nucleates aggregation which results in pathologies through loss of function and dominant negative/gain of function effects. Inspired by the hypothesis that structural changes that are associated with events initiating unfolding in DBD are likely to present opportunities for inhibition, we investigate the dynamics of the wild type (WT) and some aggregating mutants through extensive all atom explicit solvent MD simulations. Simulations reveal differential conformational sampling between the WT and the mutants of a turn region (S6-S7) that is contiguous to a known aggregation-prone region (APR). The conformational properties of the S6-S7 turn appear to be modulated by a network of interacting residues. We speculate that changes that take place in this network as a result of the mutational stress result in the events that destabilize the DBD and initiate unfolding. These perturbations also result in the emergence of a novel pocket that appears to have druggable characteristics. FDA approved drugs are computationally screened against this pocket.

The interfacial character of antibody paratopes: analysis of antibody-antigen structures.

SUMMARY: In this study, computational methods are applied to investigate the general properties of antigen engaging residues of a paratope from a non-redundant dataset of 403 antibody-antigen complexes to dissect the contribution of hydrogen bonds, hydrophobic, van der Waals contacts and ionic interactions, as well as role of water molecules in the antigen-antibody interface. Consistent with previous reports using smaller datasets, we found that Tyr, Trp, Ser, Asn, Asp, Thr, Arg, Gly, His contribute substantially to the interactions between antibody and antigen. Furthermore, antibody-antigen interactions can be mediated by interfacial waters. However, there is no reported comprehensive analysis for a large number of structured waters that engage in higher ordered structures at the antibody-antigen interface. From our dataset, we have found the presence of interfacial waters in 242 complexes. We present evidence that suggests a compelling role of these interfacial waters in interactions of antibodies with a range of antigens differing in shape complementarity. Finally, we carry out 296 835 pairwise 3D structure comparisons of 771 structures of contact residues of antibodies with their interfacial water molecules from our dataset using CLICK method. A heuristic clustering algorithm is used to obtain unique structural similarities, and found to separate into 368 different clusters. These clusters are used to identify structural motifs of contact residues of antibodies for epitope binding. AVAILABILITY AND IMPLEMENTATION: This clustering database of contact residues is freely accessible at http://mspc.bii.a-star.edu.sg/minhn/pclick.html. CONTACT: minhn@bii.a-star.edu.sg, chandra@bii.a-star.edu.sg or zhong_pingyu@immunol.a-star.edu.sg. SUPPLEMENTARY INFORMATION: Supplementary data are available at Bioinformatics online.

Hydration effects on the efficacy of the Epidermal growth factor receptor kinase inhibitor afatinib.

Small molecules targeting the EGFR tyrosine kinase domain have been used with some success at treating patients with non-small cell lung cancer driven by activating mutations in the kinase domain. The initial class of inhibitors displaced ATP noncovalently but were rendered ineffective due to the development of resistance mutations in the kinase domain. These were overcome by the development of covalent inhibitors such as afatinib which also bind in the ATP pocket. However pooled analysis of two recent clinical trials LUX-3 and LUX-6 demonstrated an unprecedented overall survival benefit of afatinib over chemotherapy for the EGFR 19del , but not the EGFR L858R . In the current study we use modelling and simulations to show that structural constraints in EGFR 19del deletion result in significantly attenuated flexibilities in the binding pocket resulting in strong hydrogen and halogen bonds with afatinib in the EGFR 19del ; these constraints are modulated by buried water and result in the differential affinities of afatinib for the different mutants. SNP analysis of residues surrounding the buried water points to the likelihood of further differential effects of afatinib and provides a compelling case for investigating the effects of the SNPs towards further stratification of patients for ensuring the most effective use of afatinib.

Water-Bridge Mediates Recognition of mRNA Cap in eIF4E.

Ligand binding pockets in proteins contain water molecules, which play important roles in modulating protein-ligand interactions. Available crystallographic data for the 5' mRNA cap-binding pocket of the translation initiation factor protein eIF4E shows several structurally conserved waters, which also persist in molecular dynamics simulations. These waters engage an intricate hydrogen-bond network between the cap and protein. Two crystallographic waters in the cleft of the pocket show a high degree of conservation and bridge two residues, which are part of an evolutionarily conserved scaffold. This appears to be a preformed recognition module for the cap with the two structural waters facilitating an efficient interaction. This is also recapitulated in a new crystal structure of the apo protein. These findings open new windows for the design and screening of compounds targeting eIF4E.

Small Molecules Targeting the Inactive Form of the Mnk1/2 Kinases.

Overexpression of the eukaryotic initiation factor 4E (eIF4E) is linked to a variety of cancers. Both mitogen-activated protein kinases-interacting kinases 1 and 2 (Mnk1/2) activate the oncogene eIF4E through posttranslational modification (phosphorylating it at the conserved Ser209). Inhibition of Mnk prevents eIF4E phosphorylation, making the Mnk-eIF4E axis a potential therapeutic target for oncology. Recently, the design and synthesis of a series of novel potent compounds inhibiting the Mnk1/2 kinases were carried out in-house. Here, we describe computational models of the interactions between Mnk1/2 kinases and these inhibitors. Molecular modeling combined with free energy calculations show that these compounds bind to the inactive forms of the kinases. All compounds adopt similar conformations in the catalytic sites of both kinases, stabilized by hydrogen bonds with the hinge regions and with the catalytic Lys78 (Mnk1) and Lys113 (Mnk2). These hydrogen bond interactions clearly play a critical role in determining the conformational stability and potency of the compounds. We also find that van der Waals interactions with an allosteric pocket are key to their binding and potency. Two distinct hydration sites that appear to further stabilize the ligand binding/interactions were observed. Critically, the inclusion of explicit water molecules in the calculations results in improving the agreement between calculated and experimental binding free energies.

Wetting of nonconserved residue-backbones: A feature indicative of aggregation associated regions of proteins.

Aggregation is an irreversible form of protein complexation and often toxic to cells. The process entails partial or major unfolding that is largely driven by hydration. We model the role of hydration in aggregation using "Dehydrons." "Dehydrons" are unsatisfied backbone hydrogen bonds in proteins that seek shielding from water molecules by associating with ligands or proteins. We find that the residues at aggregation interfaces have hydrated backbones, and in contrast to other forms of protein-protein interactions, are under less evolutionary pressure to be conserved. Combining evolutionary conservation of residues and extent of backbone hydration allows us to distinguish regions on proteins associated with aggregation (non-conserved dehydron-residues) from other interaction interfaces (conserved dehydron-residues). This novel feature can complement the existing strategies used to investigate protein aggregation/complexation.