Switchable Zinc(II)-Responsive Globular β-Sheet Peptide

Thermostable WW-Domain Scaffold to Design Functional β-Sheet Miniproteins

There has been a recent surge in the design of miniproteins for medicinal chemistry, biomaterial design, or synthetic biology. In particular, there is an interest in peptide scaffolds that fold reliably, predictably, and with solid stability. In this article, we present the design of a highly thermostable WW domain, a three-stranded β-sheet motif, with a superior melting temperature of about 90 °C to serve as a core scaffold onto which receptor-like properties can be grafted. We have performed specific rounds of sequence iteration on a WW-domain consensus sequence to decipher sequence positions that affect structural and, thus, thermal stability. We identified a sequence-structure relationship that yields a highly thermostable WW-domain scaffold. High-resolution NMR spectroscopy was applied, which enabled the identification of structural features at the atomic scale that contribute to this high thermostability. Finally, we grafted the binding motifs of the three WW-domain groups─Group I, Group II/III, and Group IV─and organophosphate and metal binding onto the highly thermostable WW-domain scaffold and obtained thermostable de novo WW domains that indeed display the different binding modes that were intended. The organophosphate-binding WW domains exhibit melting temperatures that are up to 34 K higher than previously reported top-down designs. These results impressively demonstrate that the highly thermostable WW-domain core scaffold is a solid platform for the design of discrete and reliably folding functional β-sheet peptide miniproteins, providing an essential addition to the toolbox of peptide scaffolds previously used in synthetic biology and material design.

Synthesis and Biological Activities of Some Metal Complexes of Peptides: A Review

Peptides, both natural and synthetic, are well suited for a wide range of purposes and offer versatile applications in different fields such as biocatalysts, injectable hydrogels, tumor treatment, and drug delivery. The research of the better part of the cited papers was conducted using various database platforms such as MetalPDB. The rising prominence of therapeutic peptides encompasses anticancer, antiviral, antimicrobial, and anti-neurodegenerative properties. The metals Na, K, Mg, Ca, Fe, Mn, Co, Cu, Zn, and Mo are ten of the twenty elements that are considered essential for life. Crucial for understanding the biological role of metals is the exploration of metal-bound proteins and peptides. Aside from essential metals, there are other non-essential metals that also interact biologically, exhibiting either therapeutic or toxic effects. Irregularities in metal binding contribute to diseases like Alzheimer's, neurodegenerative disorders, Wilson's, and Menkes disease. Certain metal complexes have potential applications as radiopharmaceuticals. The examination of these complexes was achieved by preforming UV-Vis, IR, EPR, NMR spectroscopy, and X-ray analysis. This summary, although unable to cover all of the studies in the field, offers a review of the ongoing experimentation and is a basis for new ideas, as well as strategies to explore and gain knowledge from the extensive realm of peptide-chelated metals and biotechnologies.

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.

Relationship of Thermostability and Binding Affinity in Metal-binding WW-Domain Minireceptors

The design of metallo-miniproteins advances our understanding of the structural and functional roles of metals in proteins. We recently designed a metal-binding WW domain, WW-CA-Nle, which displays three histidine residues on its surface for coordination of divalent metals Ni(II), Zn(II) and Cu(II). However, WW-CA-Nle is a molten globule in the apo state and thus showed only moderate binding affinities with Kd values in the μM regime. In this report, we hypothesize that improved thermal stability of the apo state of the metal binding WW-domain scaffold should lead to improved preorganization of the metal-binding site and consequently to higher metal-binding affinities. By redesigning WW-CA-Nle, we obtained WW-CA variants, WW-CA-min and WW-CA-ANG, which were fully folded in the apo states and displayed moderate to excellent thermostabilities in the apo and holo states. We were able to show that the improved thermal stabilities led to improved metal binding, which was reflected in Kd values that were at least one order of magnitude lower compared to WW-CA-Nle. EPR spectroscopy and ITC measurements revealed a better defined and predisposed metal binding site in WW-CA-ANG.

Rapid On-Resin N-Formylation of Peptides as One-Pot Reaction

N-formylation is a common pre- and post-translational modification of the N-terminus or the lysine side chain of peptides and proteins that plays a role in the initiation of immune responses, gene expression, or epigenetics. Despite its high biological relevance, protocols for the chemical N-formylation of synthetic peptides are scarce. The few available methods are elaborate in their execution and the yields are highly sequence-dependent. We present a rapid, easy-to-use one-pot procedure that runs at room temperature and can be used to formylate protected peptides at both the N-terminus and the lysine side chain on the resin in near-quantitative yields. Only insensitive, storage-stable standard chemicals - formic acid, acetic anhydride, pyridine and DMF - are used. Formylation works for both short and long peptides of up to 34 amino acids and over the spectrum of canonical amino acids.

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.

Studying Peptide-Metal Ion Complex Structures by Solution-State NMR

Metal chelation can provide structural stability and form reactive centers in metalloproteins. Approximately one third of known protein structures are metalloproteins, and metal binding, or the lack thereof, is often implicated in disease, making it necessary to be able to study these systems in detail. Peptide-metal complexes are both present in nature and can provide a means to focus on the binding region of a protein and control experimental variables to a high degree. Structural studies of peptide complexes with metal ions by nuclear magnetic resonance (NMR) were surveyed for all the essential metal complexes and many non-essential metal complexes. The various methods used to study each metal ion are presented together with examples of recent research. Many of these metal systems have been individually reviewed and this current overview of NMR studies of metallopeptide complexes aims to provide a basis for inspiration from structural studies and methodology applied in the field.

Late‐Stage Functionalisation of Peptides on the Solid Phase by an Iodination‐Substitution Approach

The functionalisation of peptides at a late synthesis stage holds great potential, for example, for the synthesis of peptide pharmaceuticals, fluorescent biosensors or peptidomimetics. Here we describe an on‐resin iodination‐substitution reaction sequence on homoserine that is also suitable for peptide modification in a combinatorial format. The reaction sequence is accessible to a wide range of sulfur nucleophiles with various functional groups including boronic acids, hydroxy groups or aromatic amines. In this way, methionine‐like thioethers or thioesters and thiosulfonates are accessible. Next to sulfur nucleophiles, selenols, pyridines and carboxylic acids were successfully used as nucleophiles, whereas phenols did not react. The late‐stage iodination‐substitution approach is not only applicable to short peptides but also to the more complex 34‐amino‐acid WW domains. We applied this strategy to introduce 7‐mercapto‐4‐methylcoumarin into a switchable Zn^II responsive WW domain to design an iFRET‐based Zn^II sensor.

Identification of novel functional mini-receptors by combinatorial screening of split-WW domains

A combinatorial approach toward novel functional WW domains based on coiled-coil-mediated reconstitution of split WW domains is presented. As such, an ATP-binding WW domain was found from a 4-by-6 library of N- and C-terminal WW domain fragments.