Trp-Trp Cross-Linking: A Structure–Reactivity Relationship in the Formation and Design of Hyperstable Peptide β-Hairpin and α-Helix Scaffolds

The Potential of Peptide-Based Inhibitors in Disrupting Protein-Protein Interactions for Targeted Cancer Therapy

Protein-protein interactions (PPIs) form an intricate cellular network known as the interactome, which is essential for various cellular processes, such as gene regulation, signal transduction, and metabolic pathways. The dysregulation of this network has been closely linked to various disease states. In cancer, these aberrant PPIs, termed oncogenic PPIs (OncoPPIs), are involved in tumour formation and proliferation. Therefore, the inhibition of OncoPPIs becomes a strategy for targeted cancer therapy. Small molecule inhibitors have been the dominant strategy for PPI inhibition owing to their small size and ability to cross cell membranes. However, peptide-based inhibitors have emerged as compelling alternatives, offering distinct advantages over small molecule inhibitors. Peptides, with their larger size and flexible backbones, can effectively engage with the broad interfaces of PPIs. Their high specificity, lower toxicity, and ease of modification make them promising candidates for targeted cancer therapy. Over the past decade, significant advancements have been made in developing peptide-based inhibitors. This review discusses the critical aspects of targeting PPIs, emphasizes the significance of OncoPPIs in cancer therapy, and explores the advantages of using peptide-based inhibitors as therapeutic agents. It also highlights recent progress in peptide design aimed at overcoming the limitations of peptide therapeutics, offering a comprehensive overview of the current landscape and potential of peptide-based inhibitors in cancer treatment.

Empowering canonical biochemicals with cross-linked novelty: Recursions in applications of protein cross-links

Diversity in the biochemical workhorses of the cell-that is, proteins-is achieved by the innumerable permutations offered primarily by the 20 canonical L-amino acids prevalent in all biological systems. Yet, proteins are known to additionally undergo unusual modifications for specialized functions. Of the various post-translational modifications known to occur in proteins, the recently identified non-disulfide cross-links are unique, residue-specific covalent modifications that confer additional structural stability and unique functional characteristics to these biomolecules. We review an exclusive class of amino acid cross-links encompassing aromatic and sulfur-containing side chains, which not only confer superior biochemical characteristics to the protein but also possess additional spectroscopic features that can be exploited as novel chromophores. Studies of their in vivo reaction mechanism have facilitated their specialized in vitro applications in hydrogels and protein anchoring in monolayer chips. Furthering the discovery of unique canonical cross-links through new chemical, structural, and bioinformatics tools will catalyze the development of protein-specific hyperstable nanostructures, superfoods, and biotherapeutics.

Recent Advances in Metal-Free Peptide Stapling Strategies

Protein-protein interactions (PPIs) pose challenges for intervention through small molecule drugs, protein drugs, and linear peptides due to inherent limitations such as inappropriate size, poor stability, and limited membrane penetrance. The emergence of stapled α-helical peptides presents a promising avenue as potential competitors for inhibiting PPIs, demonstrating enhanced structural stability and increased tolerance to proteolytic enzymes. This review aims to provide an overview of metal-free stapling strategies involving two identical natural amino acids, two different natural amino acids, non-natural amino acids, and multicomponent reactions. The primary objective is to delineate comprehensive peptide stapling approaches and foster innovative ideation among readers by accentuating methodologies published within the past five years and elucidating evolving trends in stapled peptides.

Design of Functional Globular β-Sheet Miniproteins

The design of discrete β-sheet peptides is far less advanced than e. g. the design of α-helical peptides. The reputation of β-sheet peptides as being poorly soluble and aggregation-prone often hinders active design efforts. Here, we show that this reputation is unfounded. We demonstrate this by looking at the β-hairpin and WW domain. Their structure and folding have been extensively studied and they have long served as model systems to investigate protein folding and folding kinetics. The resulting fundamental understanding has led to the development of hyperstable β-sheet scaffolds that fold at temperatures of 100 °C or high concentrations of denaturants. These have been used to design functional miniproteins with protein or nucleic acid binding properties, in some cases with such success that medical applications are conceivable. The β-sheet scaffolds are not always completely rigid, but can be specifically designed to respond to changes in pH, redox potential or presence of metal ions. Some engineered β-sheet peptides also exhibit catalytic properties, although not comparable to those of natural proteins. Previous reviews have focused on the design of stably folded and non-aggregating β-sheet sequences. In our review, we now also address design strategies to obtain functional miniproteins from β-sheet folding motifs.

5-Hydroxypyrroloindoline Affords Tryptathionine and 2,2'-bis-Indole Peptide Staples: Application to Melanotan-II

With peptides increasingly favored as drugs, natural product motifs, namely the tryptathionine staple, found in amatoxins and phallotoxins, and the 2,2'-bis-indole found in staurosporine represent unexplored staples for unnatural peptide macrocycles. We disclose the efficient condensation of a 5-hydroxypyrroloindoline with either a cysteine-thiol or a tryptophan-indole to form a tryptathionine or 2-2'-bis-indole staple. Judicious use of protecting groups provides for chemoselective stapling using α-MSH, which provides a basis for investigating both chemoselectivity and affinity. Both classes of stapled peptides show nanomolar Ki's, with one showing a sub-nanomolar Ki value.

Inhibition of Aurora-A/N-Myc Protein-Protein Interaction Using Peptidomimetics: Understanding the Role of Peptide Cyclization

Using N-Myc61-89 as a starting template we showcase the systematic use of truncation and maleimide constraining to develop peptidomimetic inhibitors of the N-Myc/Aurora-A protein-protein interaction (PPI); a potential anticancer drug discovery target. The most promising of these - N-Myc73-94-N85C/G89C-mal - is shown to favour a more Aurora-A compliant binding ensemble in comparison to the linear wild-type sequence as observed through fluorescence anisotropy competition assays, circular dichroism (CD) and nuclear magnetic resonance (NMR) experiments. Further in silico investigation of this peptide in its Aurora-A bound state, by molecular dynamics (MD) simulations, imply (i) the bound conformation is more stable as a consequence of the constraint, which likely suppresses dissociation and (ii) the constraint may make further stabilizing interactions with the Aurora-A surface. Taken together this work unveils the first orthosteric N-Myc/Aurora-A inhibitor and provides useful insights on the biophysical properties and thus design of constrained peptides, an attractive therapeutic modality.

2-Pyridone Formation: An Efficient Method for the Solid-Phase Synthesis of Homodimers

This study presents an efficient method for on-resin dimer generation through self-condensation of 3,3-dimethoxypropionic acid-modified molecules, resulting in 2-pyridones. The approach demonstrated remarkable versatility by producing homodimers of peptides, peptoids, and non-peptidic ligands. Its ease of application, broad utility, and mild reaction conditions not only hold significance for peptide and peptoid research but also offer potential for the on-resin development of a wide range of bivalent ligands.

Stabilized cyclic peptides as modulators of protein-protein interactions: promising strategies and biological evaluation

Protein-protein interactions (PPIs) control many essential biological pathways which are often misregulated in disease. As such, selective PPI modulators are desirable to unravel complex functions of PPIs and thus expand the repertoire of therapeutic targets. However, the large size and relative flatness of PPI interfaces make them challenging molecular targets for conventional drug modalities, rendering most PPIs "undruggable". Therefore, there is a growing need to discover innovative molecules that are able to modulate crucial PPIs. Peptides are ideal candidates to deliver such therapeutics attributed to their ability to closely mimic structural features of protein interfaces. However, their inherently poor proteolysis resistance and cell permeability inevitably hamper their biomedical applications. The introduction of a constraint (i.e., peptide cyclization) to stabilize peptides' secondary structure is a promising strategy to address this problem as witnessed by the rapid development of cyclic peptide drugs in the past two decades. Here, we comprehensively review the recent progress on stabilized cyclic peptides in targeting challenging PPIs. Technological advancements and emerging chemical approaches for stabilizing active peptide conformations are categorized in terms of α-helix stapling, β-hairpin mimetics and macrocyclization. To discover potent and selective ligands, cyclic peptide library technologies were updated based on genetic, biochemical or synthetic methodologies. Moreover, several advances to improve the permeability and oral bioavailability of biologically active cyclic peptides enable the de novo development of cyclic peptide ligands with pharmacological properties. In summary, the development of cyclic peptide-based PPI modulators carries tremendous promise for the next generation of therapeutic agents to target historically "intractable" PPI systems.

Photoradical-Mediated Catalyst-Independent Protein Cross-Link with Unusual Fluorescence Properties

Photo-actively modified natural amino acids have served as lucrative probes for precise mapping of the dynamics, interaction networks, and turnover of cytosolic proteins both in vivo and ex vivo. In our attempts to extend the utility of photoreactive reporters to map the molecular characteristics of vital membrane proteins, we carried out site-selective incorporation of 7-fluoro-indole in the human mitochondrial outer membrane protein VDAC2 (voltage-dependent anion channel isoform 2), with the aim of generating Trp-Phe/Tyr cross-links. Prolonged irradiation at 282 nm provided us with a surprisingly unusual fluorophore that displayed sizably red-shifted excitation (λex-max =280 nm→360 nm) and emission (λem-max =330 nm→430 nm) spectra that was reversible with organic solvents. By measuring the kinetics of the photo-activated cross-linking with a library of hVDAC2 variants, we demonstrate that formation of this unusual fluorophore is kinetically retarded, independent of tryptophan, and is site-specific. Using other membrane (Tom40 and Sam50) and cytosolic (MscR and DNA Pol I) proteins, we additionally show that formation of this fluorophore is protein-independent. Our findings reveal the photoradical-mediated accumulation of reversible tyrosine cross-links, with unusual fluorescent properties. Our findings have immediate applications in protein biochemistry and UV-mediated protein aggregation and cellular damage, opening avenues for formulating therapeutics that prolong cell viability in humans.

An Engineered β-Hairpin Peptide Forming Thermostable Complexes with ZnII , NiII , and CuII through a His3 Site

The three-dimensional structure of a peptide, which determines its function, can denature at elevated temperatures, in the presence of chaotropic reagents, or in organic solvents. These factors limit the applicability of peptides. Herein, we present an engineered β-hairpin peptide containing a His₃ site that forms complexes with Zn^II , Ni^II , and Cu^II . Circular dichroism spectroscopy shows that the peptide-metal complexes exhibit melting temperatures up to 80 °C and remain folded in 6 M guanidine hydrochloride as well as in organic solvents. Intrinsic fluorescence titration experiments were used to determine the dissociation constants of metal binding in the nano- to sub-nanomolar range. The coordination geometry of the peptide-Cu^II complex was studied by EPR spectroscopy, and a distorted square planar coordination geometry with weak interactions to axial ligands was revealed. Due to their impressive stability, the presented peptide-metal complexes open up interesting fields of application, such as the development of a new class of peptide-metal catalysts for stereoselective organic synthesis or the directed design of extremophilic β-sheet peptides.

Reactivity and degradation products of tryptophan in solution and proteins

Tryptophan is one of the essential mammalian amino acids and is thus a required component in human nutrition, animal feeds, and cell culture media. However, this aromatic amino acid is highly susceptible to oxidation and is known to degrade into multiple products during manufacturing, storage, and processing. Many physical and chemical processes contribute to the degradation of this compound, primarily via oxidation or cleavage of the highly reactive indole ring. The central contributing factors are reactive oxygen species, such as singlet oxygen, hydrogen peroxide, and hydroxyl radicals; light and photosensitizers; metals; and heat. In a multi-component mixture, tryptophan also commonly reacts with carbonyl-containing compounds, leading to a wide variety of products. The purpose of this review is to summarize the current state of knowledge regarding the degradation and interaction products of tryptophan in complex liquid solutions and in proteins. For the purposes of context, a brief summary of the key pathways in tryptophan metabolism will be included, along with common methods and issues in tryptophan manufacturing. The review will focus on the conditions that lead to tryptophan degradation, the products generated in these processes, their known biological effects, and methods which may be applied to stabilize the amino acid.

Stereoisomerism of stapled peptide inhibitors of the p53-Mdm2 interaction: an assessment of synthetic strategies and activity profiles

Staple composition can influence target binding and bioactivity of peptides. We present strategies to modulate E/Z ratios and access saturated analogues.

All-hydrocarbon, i, i+7 stapled peptide inhibitors of the p53-Mdm2 interaction have emerged as promising new leads for cancer therapy. Typical chemical synthesis via olefin metathesis results in the formation of both E- and Z-isomers, an observation that is rarely disclosed but may be of importance in targeting PPI. In this study, we evaluated the effect of staple geometry on the biological activity of five p53-reactivating peptides. We also present strategies for the modulation of the E/Z ratio and attainment of the hydrogenated adduct through repurposing of the metathesis catalyst.

Aromatic interactions in β-hairpin scaffold stability: A historical perspective

Non-covalent interactions between naturally occurring aromatic residues have been widely exploited as scaffold stabilizing agents in de novo designed peptides and in Nature - inspired structures. Our understanding of the factors driving aromatic interactions and their observed interaction geometries have advanced remarkably with improvements in conventional structural studies, availability of novel molecular methods and in silico studies, which have together provided atomistic information on aromatic interactions and interaction strengths. This review attempts to recapitulate the early advances in our understanding of aromatic interactions as stabilizing agents of peptide β-hairpins.

An intramolecular tryptophan-condensation approach for peptide stapling

Stapled peptides are gaining tremendous interest as next-generation therapeutic agents to target protein–protein interactions. Herein, we report an intramolecular peptide stapling method which links two tryptophan residues at C2 position of the indole moieties via acid-mediated condensation with an aldehyde.