Capture and Recycling of Sortase A through Site‐Specific Labeling with Lithocholic Acid

N-Terminal-Specific Dual Modification of Peptides through Copper-Catalyzed [3+2] Cycloaddition

Site-specific introduction of multiple components into peptides is greatly needed for the preparation of densely functionalized and structurally uniform peptides. In this regard, N-terminal-specific peptide modification is attractive, but it can be difficult due to the presence of highly nucleophilic lysine ϵ-amine. In this work, we developed a method for the N-terminal-specific dual modification of peptides through a three-component [3+2] cycloaddition with aldehydes and maleimides under mild copper catalysis. This approach enables exclusive functionalization at the glycine N-terminus of iminopeptides, regardless of the presence of lysine ϵ-amine, thus affording the cycloadducts in excellent yields. Tolerating a broad range of functional groups and molecules, the present method provides the opportunity to rapidly construct doubly functionalized peptides using readily accessible aldehyde and maleimide modules.

Affinity Bioorthogonal Chemistry (ABC) Tags for Site-Selective Conjugation, On-Resin Protein-Protein Coupling, and Purification of Protein Conjugates

The site-selective functionalization of proteins has broad application in chemical biology, but can be limited when mixtures result from incomplete conversion or the formation of protein containing side products. It is shown here that when proteins are covalently tagged with pyridyl-tetrazines, the nickel-iminodiacetate (Ni-IDA) resins commonly used for His-tags can be directly used for protein affinity purification. These Affinity Bioorthogonal Chemistry (ABC) tags serve a dual role by enabling affinity-based protein purification while maintaining rapid kinetics in bioorthogonal reactions. ABC-tagging works with a range of site-selective bioconjugation methods with proteins tagged at the C-terminus, N-terminus or at internal positions. ABC-tagged proteins can also be purified from complex mixtures including cell lysate. The combination of site-selective conjugation and clean-up with ABC-tagged proteins also allows for facile on-resin reactions to provide protein-protein conjugates.

Strong and Reversible Covalent Double Network Hydrogel Based on Force-Coupled Enzymatic Reactions

Biological load-bearing tissues are strong, tough, and recoverable under periodic mechanical loads. However, such features have rarely been achieved simultaneously in the same synthetic hydrogels. Here, we use a force-coupled enzymatic reaction to tune a strong covalent peptide linkage to a reversible bond. Based on this concept we engineered double network hydrogels that combine high mechanical strength and reversible mechanical recovery in the same hydrogels. Specifically, we found that a peptide ligase, sortase A, can promote the proteolysis of peptides under force. The peptide bond can be re-ligated by the same enzyme in the absence of force. This allows the sacrificial network in the double-network hydrogels to be ruptured and rebuilt reversibly. Our results demonstrate a general approach for precisely controlling the mechanical and dynamic properties of hydrogels at the molecular level.

When Supramolecular Chemistry Meets Chemical Biology: New Strategies to Target Proteins through Host-Guest Interactions

Supramolecular chemistry for targeting proteins is of great interest for the development of novel approaches to recognize, isolate and control proteins. Taking advantage of chemical biology approaches, such as genetic-code expansion and enzyme-mediated ligation, guest recognition elements can be built into proteins of interest, allowing supramolecular control of protein function and regulation. In this viewpoint article, we will discuss the methods, applications, limitations, and future perspectives of supramolecular chemistry for targeting proteins in a site-specific manner.

In Situ Cyclization of Native Proteins: Structure-Based Design of a Bicyclic Enzyme

Increased tolerance of enzymes towards thermal and chemical stress is required for many applications and can be achieved by macrocyclization of the enzyme resulting in the stabilizing of its tertiary structure. Thus far, macrocyclization approaches utilize a very limited structural diversity, which complicates the design process. Herein, we report an approach that enables cyclization through the installation of modular crosslinks into native proteins composed entirely of proteinogenic amino acids. Our stabilization procedure involves the introduction of three surface-exposed cysteine residues, which are reacted with a triselectrophile, resulting in the in situ cyclization of the protein (INCYPRO). A bicyclic version of sortase A was designed that exhibits increased tolerance towards thermal as well as chemical denaturation, and proved to be efficient in protein labeling under denaturing conditions. In addition, we applied INCYPRO to the KIX domain, resulting in up to 24 °C increased thermal stability.

Proximity-Based Sortase-Mediated Ligation

Protein bioconjugation has been a crucial tool for studying biological processes and developing therapeutics. Sortase A (SrtA), a bacterial transpeptidase, has become widely used for its ability to site-specifically label proteins with diverse functional moieties, but a significant limitation is its poor reaction kinetics. In this work, we address this by developing proximity-based sortase-mediated ligation (PBSL), which improves the ligation efficiency to over 95 % by linking the target protein to SrtA using the SpyTag-SpyCatcher peptide-protein pair. By expressing the target protein with SpyTag C-terminal to the SrtA recognition motif, it can be covalently captured by an immobilized SpyCatcher-SrtA fusion protein during purification. Following the ligation reaction, SpyTag is cleaved off, rendering PBSL traceless, and only the labeled protein is released, simplifying target protein purification and labeling to a single step.