A minimal model for stabilization of biomolecules by hydrocarbon cross-linking

Exploring the Effect of Hydrocarbon Cross-Linkers on the Structure and Binding of Stapled p53 Peptides

Short-length peptides are used as therapeutics due to their high target specificity and low toxicity; for example, peptides are designed for targeting the interaction between oncogenic protein p53 and E3 ubiquitin ligase MDM2. These peptide therapeutics form a class of successful inhibitors. To design such peptide-based inhibitors, stapling is one of the methods in which amino acid side chains are stitched together to get conformationally rigid peptides, ensuring effective binding to their partners. In the current work, we use computer simulations to investigate p53 peptides stapled with hydrocarbon chains of different lengths and positions of attachment to the peptide. We subsequently analyze their binding efficiency with MDM2. The introduction of stapling agents restricts the conformational dynamics of peptides, resulting in higher persistence of helicity. The efficiency of the stapling agents has also been verified imposing these stapled peptides to adverse conditions viz. thermal and chemical denaturation. In addition, the conformational exploration of peptides has been investigated using temperature replica exchange molecular dynamics (T-REMD) simulations. From both the unbiased and T-REMD simulations, p53 with a long hydrocarbon cross-linker shows a more conformationally rigid structure having high helicity compared to other stapled peptides. The rigidity gained due to cross-linking reduces the entropy of the peptide in the free state and thereby facilitates the complexation process. From the binding studies, we have shown that the peptide having multiple short staples has a larger enthalpy change during binding, resulting from its orientation and interactions with residues in the binding interface. On the other hand, a peptide with a single long stapling agent shows less entropic penalty than other systems. Our study suggests a plausible rationale for the relation between the length and the position of attachment of cross-linkers to peptides and their binding affinity for target partners.

Macrocyclic α helical peptide therapeutic modality: A perspective of learnings and challenges

Macrocyclic α-helical peptides have emerged as a compelling new therapeutic modality to tackle targets confined to the intracellular compartment. Within the scope of hydrocarbon-stapling there has been significant progress to date, including the first stapled α-helical peptide to enter into clinical trials. The principal design concept of stapled α-helical peptides is to mimic a cognate (protein) ligand relative to binding its target via an α-helical interface. However, it was the proclivity of such stapled α-helical peptides to exhibit cell permeability and proteolytic stability that underscored their promise as unique macrocyclic peptide drugs for intracellular targets. This perspective highlights key learnings as well as challenges in basic research with respect to structure-based design, innovative chemistry, cell permeability and proteolytic stability that are essential to fulfill the promise of stapled α-helical peptide drug development.

Small-Molecule and Peptide Inhibitors of the Pro-Survival Protein Mcl-1

The ability of protein-protein interactions to regulate cellular processes in both beneficial and detrimental ways has made them obvious drug targets. The Bcl-2 family of proteins undergo a series of protein-protein interactions which regulate the intrinsic cell-death pathway. The pro-survival members of the Bcl-2 family, including Bcl-2, Bcl-xL , and Mcl-1, are commonly overexpressed in a number of human cancers. Effective modulators of members of the Bcl-2 family have been developed and are undergoing clinical trials, but the efficient modulation of Mcl-1 is still not represented in the clinic. In addition, Mcl-1 is a major cause of resistance to radio- and chemotherapies, including inhibitors that target other Bcl-2 family members. Subsequently, the inhibition of Mcl-1 has become of significant interest to the scientific community. This review covers the progress made to date in modulating the activity of Mcl-1, by both stapled peptides and small molecules. The development of peptides as drug candidates, and the advancement of experimental and computational techniques used to discover small molecules are also highlighted.

Hydrocarbon stapled peptides as modulators of biological function

Peptide-based drug discovery has experienced a significant upturn within the past decade since the introduction of chemical modifications and unnatural amino acids has allowed for overcoming some of the drawbacks associated with peptide therapeutics. Strengthened by such features, modified peptides become capable of occupying a niche that emerges between the two major classes of today's therapeutics-small molecules (<500 Da) and biologics (>5000 Da). Stabilized α-helices have proven particularly successful at impairing disease-relevant PPIs previously considered "undruggable." Among those, hydrocarbon stapled α-helical peptides have emerged as a novel class of potential peptide therapeutics. This review provides a comprehensive overview of the development and applications of hydrocarbon stapled peptides discussing the benefits and limitations of this technique.

Macrocyclic α-Helical Peptide Drug Discovery

Macrocyclic α-helical peptides have emerged as a promising new drug class and within the scope of hydrocarbon-stapled peptides such molecules have advanced into the clinic. The overarching concept of designing proteomimetics of an α-helical ‘ligand’ which binds its cognate ‘target’ relative to α-helical interfacing protein-protein interactions has been well-validated and expanded through numerous investigations for a plethora of therapeutic targets oftentimes referred to as “undruggable” with respect to other modalities (e.g., small-molecule or proteins). This chapter highlights the evolution of macrocyclic α-helical peptides in terms of target space, biophysical and computational chemistry, structural diversity and synthesis, drug design and chemical biology. It is noteworthy that hydrocarbon-stapled peptides have successfully risen to the summit of such drug discovery campaigns.

The use of a neutral peptide aptamer scaffold to anchor BH3 peptides constitutes a viable approach to studying their function

The B-cell CLL/lymphoma-2 (Bcl-2) family of proteins are important regulators of the intrinsic pathway of apoptosis, and their interactions, driven by Bcl-2 homology (BH) domains, are of great interest in cancer research. Particularly, the BH3 domain is of clinical relevance, as it promotes apoptosis through activation of Bcl-2-associated x protein (Bax) and Bcl-2 antagonist killer (Bak), as well as by antagonising the anti-apoptotic Bcl-2 family members. Although investigated extensively in vitro, the study of the BH3 domain alone inside cells is more problematic because of diminished secondary structure of the unconstrained peptide and a lack of stability. In this study, we report the successful use of a novel peptide aptamer scaffold – Stefin A quadruple mutant – to anchor and present the BH3 domains from Bcl-2-interacting mediator of cell death (Bim), p53 upregulated modulator of apoptosis (Puma), Bcl-2-associated death promoter (Bad) and Noxa, and demonstrate its usefulness in the study of the BH3 domains in vivo. When expressed intracellularly, anchored BH3 peptides exhibit much the same binding specificities previously established in vitro, however, we find that, at endogenous expression levels, Bcl-2 does not bind to any of the anchored BH3 domains tested. Nonetheless, when expressed inside cells the anchored PUMA and Bim BH3 α-helices powerfully induce cell death in the absence of efficient targeting to the mitochondrial membrane, whereas the Noxa helix requires a membrane insertion domain in order to kill Mcl-1-dependent myeloma cells. Finally, the binding of the Bim BH3 peptide to Bax was the only interaction with a pro-apoptotic effector protein observed in this study.

Quantitative analysis of the effects of photoswitchable distance constraints on the structure of a globular protein

Photoswitchable distance constraints in the form of photoisomerizable chemical cross-links offer a general approach to the design of reversibly photocontrolled proteins. To apply these effectively, however, one must have guidelines for the choice of cross-linker structure and cross-linker attachment sites. Here we investigate the effects of varying cross-linker structure on the photocontrol of folding of the Fyn SH3 domain, a well-studied model protein. We develop a theoretical framework based on an explicit-chain model of protein folding, modified to include detailed model linkers, that allows prediction of the effect of a given linker on the free energy of folding of a protein. Using this framework, we were able to quantitatively explain the experimental result that a longer, but somewhat flexible, cross-linker is less destabilizing to the folded state than a shorter more rigid cross-linker. The models also suggest how misfolded states may be generated by cross-linking, providing a rationale for altered dynamics seen in nuclear magnetic resonance analyses of these proteins. The theoretical framework is readily portable to any protein of known folded state structure and thus can be used to guide the design of photoswitchable proteins generally.

Efficient quantification of the importance of contacts for the dynamical stability of proteins

Understanding the stability of the native state and the dynamics of a protein is of great importance for all areas of biomolecular design. The efficient estimation of the influence of individual contacts between amino acids in a protein structure is a first step in the reengineering of a particular protein for technological or pharmacological purposes. At the same time, the functional annotation of molecular evolution can be facilitated by such insight. Here, we use a recently suggested, information theoretical measure in biomolecular design - the Kullback-Leibler-divergence - to quantify and therefore rank residue-residue contacts within proteins according to their overall contribution to the molecular mechanics. We implement this protocol on the basis of a reduced molecular model, which allows us to use a well-known lemma of linear algebra to speed up the computation. The increase in computational performance is around 10(1)- to 10(4)-fold. We applied the method to two proteins to illustrate the protocol and its results. We found that our method can reliably identify key residues in the molecular mechanics and the protein fold in comparison to well-known properties in the serine protease inhibitor. We found significant correlations to experimental results, e.g., dissociation constants and Φ values.

All-atom model for stabilization of alpha-helical structure in peptides by hydrocarbon staples

Recent work has shown that the incorporation of an all-hydrocarbon "staple" into peptides can greatly increase their alpha-helix propensity, leading to an improvement in pharmaceutical properties such as proteolytic stability, receptor affinity, and cell permeability. Stapled peptides thus show promise as a new class of drugs capable of accessing intractable targets such as those that engage in intracellular protein-protein interactions. The extent of alpha-helix stabilization provided by stapling has proven to be substantially context dependent, requiring cumbersome screening to identify the optimal site for staple incorporation. In certain cases, a staple encompassing one turn of the helix (attached at residues i and i+4) furnishes greater helix stabilization than one encompassing two turns (i,i+7 staple), which runs counter to expectation based on polymer theory. These findings highlight the need for a more thorough understanding of the forces that underlie helix stabilization by hydrocarbon staples. Here we report all-atom Monte Carlo folding simulations comparing unmodified peptides derived from RNase A and BID BH3 with various i,i+4 and i,i+7 stapled versions thereof. The results of these simulations were found to be in quantitative agreement with experimentally determined helix propensities. We also discovered that staples can stabilize quasi-stable decoy conformations, and that the removal of these states plays a major role in determining the helix stability of stapled peptides. Finally, we critically investigate why our method works, exposing the underlying physical forces that stabilize stapled peptides.

Relating sequence evolution of HIV1-protease to its underlying molecular mechanics

We investigate the connection between sequence evolution of the human immunodeficiency virus (HIV) type 1 protease under neutral selection or selective pressure induced by protease inhibitors and the functional and molecular-stability characteristics of the molecule in the physical domain. To this end we analyze sequence data on more than 45,000 patients with bioinformatical tools, namely mutual information between residue pairings. In addition we perform extensive computations on the molecular mechanics of the molecule subject to artificial mutations. The changes in the mechanics and dynamics of the molecule in three-dimensional space upon perturbation are then related to the sequence stability as described by the mutual information. We distinguish physical interactions by their evolutionary background and give hints for potential new drug targets. In addition we discuss how such targets can be efficiently chosen to give the HI virus less opportunity to develop resistance towards such drugs while maintaining the protease function at the same time. The interactions between residue no. 28 and 23' in different chains as well as the interaction between residue no. 92 and 94 within one chain were identified as particular crucial. In addition we find interactions in the beta-sheet-dimerization interface to be important for conserving the protein function and stability while these are at the same time evolutionary conserved - implications of and comparisons to experimental results are finally discussed.

Information theoretical measures to analyze trajectories in rational molecular design

We develop a new methodology to analyze molecular dynamics trajectories and other time series data from simulation runs. This methodology is based on an information measure of the difference between distributions of various data extract from such simulations. The method is fast as it only involves the numerical integration/summation of the distributions in one dimension while avoiding sampling issues at the same time. The method is most suitable for applications in which different scenarios are to be compared, e.g. to guide rational molecular design. We show the power of the proposed method in an application of rational drug design by reduced model computations on the BH3 motif in the apoptosis inducing BCL(2) protein family.