Current Topics in Medicinal Chemistry - Volume 15, Issue 20, 2015

Many key cellular events determining the thin line between healthy and oncogenic behavior rely on the proper functioning of protein-protein interactions (PPIs). Alterations that affect the affinity of a protein-protein binding site may destabilize a desired healthy interaction, or stabilize an oncogenic interaction. The understanding that there are a few key hot-spot residues that are mainly responsible for the binding energy of an interaction greatly widened the prospects of targeting oncogenic protein-protein interfaces enabling the use of small ligands in addition to biological molecules such as peptides and antibodies. Taming oncogenic signaling requires a deep understanding of protein interactions and their networks. Traditional representation of PPIs in signaling pathways as nodes and edges falls short of expressing interaction specific modulation of signals. Structural networks, deciphering which sites on a protein structure are responsible for each of the many interactions it may carry out, help understanding specific oncogenic mutations on signaling. We describe the key features of PPIs and their targeting, together with the advantages of structural networks, and provide four case studies demonstrating different opportunities for the aim of modulating oncogenic interactions.

The majority of functionally important biological processes are regulated by allosteric communication within individual proteins and across protein complexes. The proteins controlling these communication networks respond to changes in the cellular environment by switching between different conformational states. Targeting the interface residues mediating these processes through the rational identification of molecules modulating or mimicking their effects holds great therapeutic potential. Protein-protein interactions (PPIs) have shown to have a high degree of plasticity since they occur through small regions, called hot spots, which are included in binding surfaces or in binding clefts of the proteins and are characterized by a high degree of complementarity. This prompted several researchers to compare the protein structure to human grammar proposing terms like “protein language”. The decoding of this language represent a new paradigm not only to clarify the dynamics of many biological processes but also to improve the opportunities in drug discovery. In this review, we try to give an overview on intra-molecular and inter-molecular protein communication mechanisms describing the protein interaction domains (PIDs) and short linear motifs (SLiMs), which delineate the authentic syntactic and semantic units in a protein. Moreover, we illustrate some novel approaches performed on natural compounds and on synthetic derivatives aimed at developing new classes of potential drugs able to interfere with intra-molecular and inter-molecular protein communication.

In recent years the ever so complex field of drug discovery has embraced novel design strategies based on biophysical fragment screening (fragment-based drug design; FBDD) using nuclear magnetic resonance spectroscopy (NMR) and/or structure-guided approaches, most often using X-ray crystallography and computer modeling. Experience from recent years unveiled that these methods are more effective and less prone to artifacts compared to biochemical high-throughput screening (HTS) of large collection of compounds in designing protein inhibitors. Hence these strategies are increasingly becoming the most utilized in the modern pharmaceutical industry. Nonetheless, there is still an impending need to develop innovative and effective strategies to tackle other more challenging targets such as those involving protein-protein interactions (PPIs). While HTS strategies notoriously fail to identify viable hits against such targets, few successful examples of PPIs antagonists derived by FBDD strategies exist. Recently, we reported on a new strategy that combines some of the basic principles of fragment-based screening with combinatorial chemistry and NMR-based screening. The approach, termed HTS by NMR, combines the advantages of combinatorial chemistry and NMR-based screening to rapidly and unambiguously identify bona fide inhibitors of PPIs. This review will reiterate the critical aspects of the approach with examples of possible applications.

Proteins are not static objects. To carry out their functions in the cells and participate in biochemical interaction networks, proteins have to explore different conformational substates, which favor the adaptation to different partners and ultimately allow them to respond to changes in the environment. In this paper we discuss the implications of including the atomistic description of protein dynamics and flexibility in the context of drug discovery and design. The underlying idea is that a better understanding of the atomistic details of molecular recognition phenomena and conformational cross-talk between a ligand and a receptor can in fact translate in unexplored opportunities for the discovery of new drug like molecules. We will illustrate and discuss dynamics-based pharmacophores, the discovery of cryptic binding sites, the characterization and exploitation of allosteric regulation mechanisms and the definition of potential protein-protein interaction sites as potential sources of new bases for the rational design of small molecules endowed with specific biological functions. Overall, the inclusion of protein flexibility in the drug discovery process is starting to attract attention not only in the academic but also in the industrial community. This is supported by experimental tests that prove the actual feasibility of considering the explicit dynamics of drug-protein interactions at all relevant levels of resolution and the use of multiple receptor conformations in drug discovery, as affordable complements (if not an alternative) to classical High Throughput Screening (HTS) efforts based on static structures.

Protein-protein interactions (PPI) are at the center of molecular mechanisms of life. The protein ligands convene for regulation of biological function: adding, enhancing or inhibiting activity, for assistance in structural integrity or to enable subsequent PPI. All these general roles of PPI are represented in the proteasome, the giant proteolytic factory universally present in human cells. The proteasome is a renowned target for anti-cancer drugs and a considered target for drugs curbing inflammation. The essential function of the proteasome, the degradation of a majority of intracellular proteins via the ubiquitin-proteasome pathway, relies on proper interactions between multiple subunits of the enzyme and between multiple modules forming distinct super-assemblies covered by the “proteasome” name. The interface regions between constitutive, alternative or transient protein components of the proteasome provide a rich platform for design of drugs with potentially very diverse actions. Still, the resource remains largely untapped since all proteasometargeting drugs used so far in humans are classical competitive inhibitors blocking catalytic centers. In this review, we will discuss the opportunities and challenges of targeting PPI in the hub enzyme for intracellular protein catabolism, the proteasome.

Biological systems rely on the establishment of interactions between biomolecules, which take place in the aqueous environment of the cell. It was already demonstrated that a small set of residues at the interface, Hot-Spots(HS), contributes significantly to the binding free energy. However, these energetic determinants of affinity and specificity are still not fully understood. Moreover, the contribution of water to their HS character is also poorly characterized. In this review, we have focused on the structural data available that support the occlusion of HS from solvent, and therefore the “Oring theory”not only on protein-protein but also on protein-DNA complexes. We also emphasized the use of Solvent Accessible Surface Area (SASA) features in a variety of machine-learning approaches that aim to detect binding HS.

PAR1, member of the family of protease-activated receptors, is a GPCR whose activation requires a proteolytic cleavage at its extracellular N-terminus to unveil a tethered activating ligand. Although thrombin is the main activator of this receptor, diverse other proteases can also activate and disarm PAR1. Besides, tethered activating ligand-based peptides (PAR-APs) can also activate the receptor. PAR1 mainly signals via G proteins but, it can also signal using β-arrestin pathways and by transactivation of other receptors. This complex PAR1 interactome is completed with the receptor desensitization, trafficking, and degradation. PAR1 has shown species-, cellular-, and physiological or pathological state-dependent specificity. This review try to give an overview on the complex PAR1 interactome, its therapeutic impact upon the cardiovascular, immune and nervous systems, inflammation and cancer, as well as, on its modulation with agonists and antagonists.