Human Behavior-Inspired Linchpin-Directed Catalysis for Traceless Precision Labeling of Lysine in Native Proteins.

The complex social ecosystem regulates the spectrum of human behavior. However, it becomes relatively easier to understand if we disintegrate the contributing factors, such as locality and interacting partners. Interestingly, it draws remarkable similarity with the behavior of a residue placed in a social setup of functional groups in a protein. Can it inspire principles for creating a unique environment for the precision engineering of proteins? We demonstrate that localization-regulated interacting partner(s) could render precise and traceless single-site modification of structurally diverse native proteins. The method targets a combination of high-frequency Lys residues through an array of reversible and irreversible reactions. However, excellent simultaneous control over chemoselectivity, site selectivity, and modularity ensures that the user-friendly protocol renders acyl group installation, including post-translational modifications (PTMs), on a single Lys. Besides, it offers a chemically orthogonal handle for the installation of probes. Also, a purification protocol integration delivers analytically pure single-site tagged protein bioconjugates. The precise labeling of a surface Lys residue ensures that the structure and enzymatic activities remain conserved post-bioconjugation. For example, the precise modification of insulin does not affect its uptake and downstream signaling pathway. Further, the method enables the synthesis of homogeneous antibody-fluorophore and antibody-drug conjugates (AFC and ADC; K183 and K249 labeling). The trastuzumab-rhodamine B conjugate displays excellent serum stability along with antigen-specific cellular imaging. Further, the trastuzumab-emtansine conjugate offers highly specific antiproliferative activity toward HER-2 positive SKBR-3 breast cancer cells. This work validates that disintegrate theory can create a comprehensive platform to enrich the chemical toolbox to meet the technological demands at the chemistry, biology, and medicine interface.

Linchpins empower promiscuous electrophiles to enable site-selective modification of histidine and aspartic acid in proteins.

The conservation of chemoselectivity becomes invalid for multiple electrophilic warheads during protein bioconjugation. Consequently, it leads to unpredictable heterogeneous labeling of proteins. Here, we report that a linchpin can create a unique chemical space to enable site-selectivity for histidine and aspartic acid modifications overcoming the pre-requisite of chemoselectivity.

Serine-Selective Bioconjugation.

This Communication reports the first general method for rapid, chemoselective, and modular functionalization of serine residues in native polypeptides, which uses a reagent platform based on the P(V) oxidation state. This redox-economical approach can be used to append nearly any kind of cargo onto serine, generating a stable, benign, and hydrophilic phosphorothioate linkage. The method tolerates all other known nucleophilic functional groups of naturally occurring proteinogenic amino acids. A variety of applications can be envisaged by this expansion of the toolbox of site-selective bioconjugation methods.

Chemoselective and Site-Selective Lysine-Directed Lysine Modification Enables Single-Site Labeling of Native Proteins.

The necessity for precision labeling of proteins emerged during the efforts to understand and regulate their structure and function. It demands selective attachment of tags such as affinity probes, fluorophores, and potent cytotoxins. Here, we report a method that enables single-site labeling of a high-frequency Lys residue in the native proteins. At first, the enabling reagent forms stabilized imines with multiple solvent-accessible Lys residues chemoselectively. These linchpins create the opportunity to regulate the position of a second Lys-selective electrophile connected by a spacer. Consequently, it enables the irreversible single-site labeling of a Lys residue independent of its place in the reactivity order. The user-friendly protocol involves a series of steps to deconvolute and address chemoselectivity, site-selectivity, and modularity. Also, it delivers ordered immobilization and analytically pure probe-tagged proteins. Besides, the methodology provides access to antibody-drug conjugate (ADC), which exhibits highly selective anti-proliferative activity towards HER-2 expressing SKBR-3 breast cancer cells.

A single amino acid Gly-tag enables metal-free protein purification.

Analytically pure proteins are indispensable for diverse applications, including therapeutics. Here, we report a methodology where a single amino acid, glycine, enables metal-free protein purification. This robust platform is enabled by a Gly-tag resin for site-specific capture, enrichment, and release through chemically triggered C-C bond dissociation by resonance-assisted electron density polarization.

Single-site glycine-specific labeling of proteins.

Labeling of native proteins invites interest from diverse segments of science. However, there remains the significant unmet challenge in precise labeling at a single site of a protein. Here, we report the site-specific labeling of natural or easy-to-engineer N-terminus Gly in proteins with remarkable efficiency and selectivity. The method generates a latent nucleophile from N-terminus imine that reacts with an aldehyde to deliver an aminoalcohol under physiological conditions. It differentiates N-Gly as a unique target amongst other proteinogenic amino acids. The method allows single-site labeling of proteins in isolated form and extends to lysed cells. It administers an orthogonal aldehyde group primed for late-stage tagging with an affinity tag, 19F NMR probe, and a fluorophore. A user-friendly protocol delivers analytically pure tagged proteins. The mild reaction conditions do not alter the structure and function of the protein. The cellular uptake of fluorophore-tagged insulin and its ability to activate the insulin-receptor mediated signaling remains unperturbed.

Aldehydes can switch the chemoselectivity of electrophiles in protein labeling.

We show that the chemoselectivity of an electrophile in protein labeling can be promiscuous. An aldehyde enables switching of chemoselectivity of an epoxide and a sulfonate ester along with an enhanced rate of reaction. The chemical technology renders single-site installation of diverse probes on a protein and delivers analytically pure tagged proteins.

Single-Site Labeling of Native Proteins Enabled by a Chemoselective and Site-Selective Chemical Technology.

Chemical biology research often requires precise covalent attachment of labels to the native proteins. Such methods are sought after to probe, design, and regulate the properties of proteins. At present, this demand is largely unmet due to the lack of empowering chemical technology. Here, we report a chemical platform that enables site-selective labeling of native proteins. Initially, a reversible intermolecular reaction places the "chemical linchpins" globally on all the accessible Lys residues. These linchpins have the capability to drive site-selective covalent labeling of proteins. The linchpin detaches within physiological conditions and capacitates the late-stage installation of various tags. The chemical platform is modular, and the reagent design regulates the site of modification. The linchpin is a multitasking group and facilitates purification of the labeled protein eliminating the requirement of additional chromatography tag. The methodology allows the labeling of a single protein in a mixture of proteins. The precise modification of an accessible residue in protein ensures that their structure remains unaltered. The enzymatic activity of myoglobin, cytochrome C, aldolase, and lysozyme C remains conserved after labeling. Also, the cellular uptake of modified insulin and its downstream signaling process remain unperturbed. The linchpin directed modification (LDM) provides a convenient route for the conjugation of a fluorophore and drug to a Fab and monoclonal antibody. It delivers trastuzumab-doxorubicin and trastuzumab-emtansine conjugates with selective antiproliferative activity toward Her-2 positive SKBR-3 breast cancer cells.

Chemoselective and site-selective peptide and native protein modification enabled by aldehyde auto-oxidation.

We report a chemoselective and site-selective formylation of ε-amine in native proteins. The aldehyde auto-oxidation re-routing, regulated generation of formate, and reversible N-terminus protection drive the transformation. It labels a single ε-amine in a pool of its copies, other nucleophilic residues, and α-amine. The extension of the methodology leads to site-selective acylation.

A phthalimidation protocol that follows protein defined parameters.

This work outlines the first phthalimidation protocol suitable for protein labeling and performed in aqueous media at room temperature and neutral pH with no catalyst or co-reagent required. The methodology is suitable for a range of amines and its efficiency was determined with chemoselective and site-selective protein labeling.