Azaphilones as Activation-Free Primary-Amine-Specific Bioconjugation Reagents for Peptides, Proteins and Lipids

Substrate-Selective Catalysis Enabled Synthesis of Azaphilone Natural Products

Achieving substrate-selectivity is a central element of nature's approach to synthesis. By relying on the ability of a catalyst to discriminate between components in a mixture, control can be exerted over which molecules will move forward in a synthesis. This approach can be powerful when realized but can be challenging to duplicate in the laboratory. In this work, substrate-selective catalysis is leveraged to discriminate between two intermediates that exist in equilibrium, subsequently directing the final cyclization to arrive at either the linear or angular tricyclic core common to subsets of azaphilone natural products. By using a flavin-dependent monooxygenase (FDMO) in sequence with an acyl transferase (AT), the conversion of several orcinaldehyde substrates directly to the corresponding linear tricyclic azaphilones in a single reaction vessel was achieved. Further, mechanistic studies support that a substrate equilibrium together with enzyme substrate selectivity play an import role in the selectivity of the final cyclization step. Using this strategy, five azaphilone natural products were synthesized for the first time as well as a number of unnatural derivatives thereof.

3-Acyl-4-Pyranone as a Lysine Residue-Selective Bioconjugation Reagent for Peptide and Protein Modification

Chemoselective protein modification plays extremely important roles in various biological, medical, and pharmaceutical investigations. Mimicking the mechanism of the chemoselective reaction between natural azaphilones and primary amines, this work successfully simplified the azaphilone scaffold into much simpler 3-acyl-4-pyranones. Examinations confirmed that these slim-size mimics perfectly kept the unique reactivity for selective conjugation with the primary amines including lysine residues of peptides and proteins. The newly developed pyranone tool presents remarkably increased aqueous solubility and compatible second-order rate constant by comparison with the original azaphilone. Additional advantages also include the ease of biorthogonal combinative use with a copper-catalyzed azide-alkyne Click reaction, which was conveniently applied to decorate lysozyme with neutral-, positive- and negative-charged functionalities in parallel. Moderate-degree modification of lysozyme with positively charged quaternary ammoniums was revealed to increase the enzymatic activities.

Research progress on chemical modifications of tyrosine residues in peptides and proteins

The chemical modifications (CMs) of protein is an important technique in chemical biology, protein-based therapy, and material science. In recent years, there has been rapid advances in the development of CMs of peptides and proteins, providing new approaches for peptide and protein functionalization, as well as drug discovery. In this review, we highlight the methods for chemically modifying tyrosine (Tyr) residues in different regions, offering a comprehensive exposition of the research content related to Tyr modification. This review summarizes and provides an outlook on Tyr residue modification, aiming to offer readers assistance in the site-selective modification of macromolecules and to facilitate application research in this field.

Chemical technology principles for selective bioconjugation of proteins and antibodies

Proteins are multifunctional large organic compounds that constitute an essential component of a living system. Hence, control over their bioconjugation impacts science at the chemistry-biology-medicine interface. A chemical toolbox for their precision engineering can boost healthcare and open a gateway for directed or precision therapeutics. Such a chemical toolbox remained elusive for a long time due to the complexity presented by the large pool of functional groups. The precise single-site modification of a protein requires a method to address a combination of selectivity attributes. This review focuses on guiding principles that can segregate them to simplify the task for a chemical method. Such a disintegration systematically employs a multi-step chemical transformation to deconvolute the selectivity challenges. It constitutes a disintegrate (DIN) theory that offers additional control parameters for tuning precision in protein bioconjugation. This review outlines the selectivity hurdles faced by chemical methods. It elaborates on the developments in the perspective of DIN theory to demonstrate simultaneous regulation of reactivity, chemoselectivity, site-selectivity, modularity, residue specificity, and protein specificity. It discusses the progress of such methods to construct protein and antibody conjugates for biologics, including antibody-fluorophore and antibody-drug conjugates (AFCs and ADCs). It also briefs how this knowledge can assist in developing small molecule-based covalent inhibitors. In the process, it highlights an opportunity for hypothesis-driven routes to accelerate discoveries of selective methods and establish new targetome in the precision engineering of proteins and antibodies.

Recent Advances in Bioorthogonal Ligation and Bioconjugation

The desire to create biomolecules modified with functionalities that go beyond nature's toolbox has resulted in the development of biocompatible and selective methodologies and reagents, each with different scope and limitations. In this overview, we highlight recent advances in the field of bioconjugation from 2016 to 2023. First, (metal-mediated) protein functionalization by exploiting the specific reactivity of amino acids will be discussed, followed by novel bioorthogonal reagents for bioconjugation of modified biomolecules.

Alkynone β-trifluoroborates: A new class of amine-specific biocompatible click reagents

Amine-targeting reactions that work under biocompatible conditions or in water are green processes that are extremely useful for the synthesis of functional materials and biotherapeutics. Unfortunately, despite the usefulness of this reaction, there are very few good amine-specific click methods reported thus far. Here, we report an amine-specific click reagent using alkynone β-trifluoroborates as the electrophiles. These boron-containing alkynyl reagents exhibit extremely high chemoselectivity toward amines even in the presence of thiols. The resulting oxaboracycle products are bench-stable, displaying the reactivities of both organoborates and enaminones. Intrinsic advantages of this methodology include benign reaction conditions, operational simplicity, remarkable product stability, and excellent chemoselectivity, which satisfy the criteria of click chemistry and demonstrate the high potential in bioconjugation. Hence, this water-based chemical approach is also applicable to the modification of native amino acids, peptides, and proteins. Ultimately, the essential role of water during the reaction was elucidated.

Deciphering the evolution of flavin-dependent monooxygenase stereoselectivity using ancestral sequence reconstruction

Controlling the selectivity of a reaction is critical for target-oriented synthesis. Accessing complementary selectivity profiles enables divergent synthetic strategies, but is challenging to achieve in biocatalytic reactions given enzymes' innate preferences of a single selectivity. Thus, it is critical to understand the structural features that control selectivity in biocatalytic reactions to achieve tunable selectivity. Here, we investigate the structural features that control the stereoselectivity in an oxidative dearomatization reaction that is key to making azaphilone natural products. Crystal structures of enantiocomplementary biocatalysts guided the development of multiple hypotheses centered on the structural features that control the stereochemical outcome of the reaction; however, in many cases, direct substitutions of active site residues in natural proteins led to inactive enzymes. Ancestral sequence reconstruction (ASR) and resurrection were employed as an alternative strategy to probe the impact of each residue on the stereochemical outcome of the dearomatization reaction. These studies suggest that two mechanisms are active in controlling the stereochemical outcome of the oxidative dearomatization reaction: one involving multiple active site residues in AzaH and the other dominated by a single Phe to Tyr switch in TropB and AfoD. Moreover, this study suggests that the flavin-dependent monooxygenases (FDMOs) adopt simple and flexible strategies to control stereoselectivity, which has led to stereocomplementary azaphilone natural products produced by fungi. This paradigm of combining ASR and resurrection with mutational and computational studies showcases sets of tools for understanding enzyme mechanisms and provides a solid foundation for future protein engineering efforts.

Development and Recent Advances in Lysine and N-Terminal Bioconjugation for Peptides and Proteins

The demand for creation of protein diversity and regulation of protein function through native protein modification and post-translational modification has ignited the development of selective chemical modification methods for peptides and proteins. Chemical bioconjugation offers selective functionalization providing bioconjugates with desired properties and functions for diverse applications in chemical biology, medicine, and biomaterials. The amino group existing at the lysine residue and N-terminus of peptides and proteins has been extensively studied in bioconjugation because of its good nucleophilicity and high surface exposure. Herein, we review the development of chemical methods for modification of the amino groups on lysine residue and N-terminus featuring excellent selectivity, mild reaction conditions, short reaction time, high conversion, biocompatibility, and preservation of protein integrity. This review is organized based on the chemoselectivity and site-selectivity of the chemical bioconjugation reagents to the amino acid residues aiming to provide guidance for the selection of appropriate bioconjugation methods.

Bioconjugation via Hetero-Selective Clamping of Two Different Amines with ortho-Phthalaldehyde

Amino groups are common in both natural and synthetic compounds and offer a very attractive class of endogenous handles for bioconjugation. However, the ability to differentiate two types of amino groups and join them with high hetero-selectivity and efficiency in a complex setting remains elusive. Herein, we report a new method for bioconjugation via one-pot chemoselective clamping of two different amine nucleophiles using a simple ortho-phthalaldehyde (OPA) reagent. Various α-amino acids, aryl amines, and secondary amines can be crosslinked to the ϵ-amino side chain of lysine on peptides or proteins with high efficiency and hetero-selectivity. This method offers a simple and powerful means to crosslink small molecule drugs, imaging probes, peptides, proteins, carbohydrates, and even virus particles without any pre-functionalization.

Synthesis of Isoquinolone, 1,2-Benzothiazine, and Naphtho[1',2':4,5]imidazo[1,2-a]pyridine Derivatives via Rhodium(III)-Catalyzed (4 + 2) Annulation

In this study, we report a novel and efficient synthetic method to construct isoquinolone scaffold via the Rh(III)-catalyzed (4 + 2) annulation of benzamide with an unreported coupling reagent methyl 2-chloroacrylate. Accordingly, other valuable 1,2-benzothiazine and naphtho[1',2':4,5]imidazo[1,2-a]pyridine derivatives are also obtained through a similar synthetic protocol. Thus, our developed method is highlighted by high yield and reaction versatility.