The Bump-and-Hole Tactic: Expanding the Scope of Chemical Genetics

Late-Stage Diversification of Natural Products

Late-stage diversification of natural products is an efficient way to generate natural product derivatives for drug discovery and chemical biology. Benefiting from the development of site-selective synthetic methodologies, late-stage diversification of natural products has achieved notable success. This outlook will outline selected examples of novel methodologies for site-selective transformations of reactive functional groups and inert C-H bonds that enable late-stage diversification of complex natural products. Accordingly, late-stage diversification provides an opportunity to rapidly access various derivatives for modifying lead compounds, identifying cellular targets, probing protein-protein interactions, and elucidating natural product biosynthetic relationships.

Chemical biology of noncanonical G protein-coupled receptor signaling: Toward advanced therapeutics

G protein-coupled receptors (GPCRs), the largest family of signaling membrane proteins, are the target of more than 30% of the drugs on the market. Recently, it has become clear that GPCR functions are far more multidimensional than previously thought, with multiple noncanonical aspects coming to light, including biased, oligomeric, and compartmentalized signaling. These additional layers of functional selectivity greatly expand opportunities for advanced therapeutic interventions, but the development of new chemical biology tools is absolutely required to improve our understanding of noncanonical GPCR regulation and pave the way for future drugs. In this opinion, we highlight the most notable examples of chemical and chemogenetic tools addressing new paradigms in GPCR signaling, discuss their promises and limitations, and explore future directions.

Avoiding Drug Resistance by Substrate Envelope-Guided Design: Toward Potent and Robust HCV NS3/4A Protease Inhibitors

Hepatitis C virus (HCV) infects millions of people worldwide, causing chronic liver disease that can lead to cirrhosis, hepatocellular carcinoma, and liver transplant. In the last several years, the advent of direct-acting antivirals, including NS3/4A protease inhibitors (PIs), has remarkably improved treatment outcomes of HCV-infected patients. However, selection of resistance-associated substitutions and polymorphisms among genotypes can lead to drug resistance and in some cases treatment failure. A proactive strategy to combat resistance is to constrain PIs within evolutionarily conserved regions in the protease active site. Designing PIs using the substrate envelope is a rational strategy to decrease the susceptibility to resistance by using the constraints of substrate recognition. We successfully designed two series of HCV NS3/4A PIs to leverage unexploited areas in the substrate envelope to improve potency, specifically against resistance-associated substitutions at D168. Our design strategy achieved better resistance profiles over both the FDA-approved NS3/4A PI grazoprevir and the parent compound against the clinically relevant D168A substitution. Crystallographic structural analysis and inhibition assays confirmed that optimally filling the substrate envelope is critical to improve inhibitor potency while avoiding resistance. Specifically, inhibitors that enhanced hydrophobic packing in the S4 pocket and avoided an energetically frustrated pocket performed the best. Thus, the HCV substrate envelope proved to be a powerful tool to design robust PIs, offering a strategy that can be translated to other targets for rational design of inhibitors with improved potency and resistance profiles.IMPORTANCE Despite significant progress, hepatitis C virus (HCV) continues to be a major health problem with millions of people infected worldwide and thousands dying annually due to resulting complications. Recent antiviral combinations can achieve >95% cure, but late diagnosis, low access to treatment, and treatment failure due to drug resistance continue to be roadblocks against eradication of the virus. We report the rational design of two series of HCV NS3/4A protease inhibitors with improved resistance profiles by exploiting evolutionarily constrained regions of the active site using the substrate envelope model. Optimally filling the S4 pocket is critical to avoid resistance and improve potency. Our results provide drug design strategies to avoid resistance that are applicable to other quickly evolving viral drug targets.

Methionine Adenosyltransferase Engineering to Enable Bioorthogonal Platforms for AdoMet-Utilizing Enzymes

The structural conservation among methyltransferases (MTs) and MT functional redundancy is a major challenge to the cellular study of individual MTs. As a first step toward the development of an alternative biorthogonal platform for MTs and other AdoMet-utilizing enzymes, we describe the evaluation of 38 human methionine adenosyltransferase II-α (hMAT2A) mutants in combination with 14 non-native methionine analogues to identify suitable bioorthogonal mutant/analogue pairings. Enabled by the development and implementation of a hMAT2A high-throughput (HT) assay, this study revealed hMAT2A K289L to afford a 160-fold inversion of the hMAT2A selectivity index for a non-native methionine analogue over the native substrate l-Met. Structure elucidation of K289L revealed the mutant to be folded normally with minor observed repacking within the modified substrate pocket. This study highlights the first example of exchanging l-Met terminal carboxylate/amine recognition elements within the hMAT2A active-site to enable non-native bioorthgonal substrate utilization. Additionally, several hMAT2A mutants and l-Met substrate analogues produced AdoMet analogue products with increased stability. As many AdoMet-producing (e.g., hMAT2A) and AdoMet-utlizing (e.g., MTs) enzymes adopt similar active-site strategies for substrate recognition, the proof of concept first generation hMAT2A engineering highlighted herein is expected to translate to a range of AdoMet-utilizing target enzymes.

Recent Advances in Plant Chemical Biology of Jasmonates

Lipid-derived plant hormone jasmonates are implicated in plant growth, reproductive performance, senescence, secondary metabolite productions, and defense against both necrotrophic pathogens and feeding insects. A major jasmonate is (+)-7-iso-jasmonoyl-l-isoleucine (JA-Ile), which is perceived by the unique COI1-JAZ coreceptor system. Recent advances in plant chemical biology have greatly informed the bioscience of jasmonate, including the development of chemical tools such as the antagonist COR-MO; the agonist NOPh; and newly developed jasmonates, including JA-Ile-macrolactone and 12-OH-JA-Ile. This review article summarizes the current status of plant chemical biology as it pertains to jasmonates, and offers some perspectives for the future.

Phosphine-Activated Lysine Analogues for Fast Chemical Control of Protein Subcellular Localization and Protein SUMOylation

The Staudinger reduction and its variants have exceptional compatibility with live cells but can be limited by slow kinetics. Herein we report new small-molecule triggers that turn on proteins through a Staudinger reduction/self-immolation cascade with substantially improved kinetics and yields. We achieved this through site-specific incorporation of a new set of azidobenzyloxycarbonyl lysine derivatives in mammalian cells. This approach allowed us to activate proteins by adding a nontoxic, bioorthogonal phosphine trigger. We applied this methodology to control a post-translational modification (SUMOylation) in live cells, using native modification machinery. This work significantly improves the rate, yield, and tunability of the Staudinger reduction-based activation, paving the way for its application in other proteins and organisms.

Developments in drug design strategies for bromodomain protein inhibitors to target Plasmodium falciparum parasites

Introduction: Bromodomains (BRDs) bind to acetylated lysine residues, often on histones. The BRD proteins can contribute to gene regulation either directly through enzymatic activity or indirectly through recruitment of chromatin-modifying complexes or transcription factors. There is no evidence of direct orthologues of the Plasmodium falciparum BRD proteins (PfBDPs) outside the apicomplexans. PfBDPs are expressed during the parasite's life cycle in both the human host's blood and in the mosquito. PfBDPs could also prove to be promising targets for novel antimalarials, which are urgently required to address increasing drug resistance.Areas covered: This review discusses recent studies of the biology of PfBDPs, current target-based strategies for PfBDP inhibitor discovery, and different approaches to the important step of validating the specificity of hit compounds for PfBDPs.Expert opinion: The novelty of Plasmodium BRDs suggests that they could be targeted by selective compounds. Chemical series that showed promise in screens against human BRDs could be leveraged to create targeted compound libraries, as could hits from P. falciparum phenotypic screens. These targeted libraries and hits could be screened in target-based strategies aimed at discovery and optimization of novel inhibitors of PfBDPs. A key task for the field is to generate parasite assays to validate the hit compounds' specificity for PfBDPs.

Engineering Methyllysine Writers and Readers for Allele-Specific Regulation of Protein-Protein Interactions

Protein-protein interactions mediated by methyllysine are ubiquitous in biological systems. Specific perturbation of such interactions has remained a challenging endeavor. Herein, we describe an allele-specific strategy toward an engineered protein-protein interface orthogonal to the human proteome. We develop a methyltransferase (writer) variant that installs aryllysine moiety on histones that can only be recognized by an engineered chromodomain (reader). We establish biochemical integrity of the engineered interface, provide structural evidence for orthogonality and validate its applicability to identify transcriptional regulators. Our approach provides an unprecedented strategy for specific manipulation of the methyllysine interactome.

Engineering Orthogonal Polypeptide GalNAc-Transferase and UDP-Sugar Pairs

O-Linked α-N-acetylgalactosamine (O-GalNAc) glycans constitute a major part of the human glycome. They are difficult to study because of the complex interplay of 20 distinct glycosyltransferase isoenzymes that initiate this form of glycosylation, the polypeptide N-acetylgalactosaminyltransferases (GalNAc-Ts). Despite proven disease relevance, correlating the activity of individual GalNAc-Ts with biological function remains challenging due to a lack of tools to probe their substrate specificity in a complex biological environment. Here, we develop a "bump-hole" chemical reporter system for studying GalNAc-T activity in vitro. Individual GalNAc-Ts were rationally engineered to contain an enlarged active site (hole) and probed with a newly synthesized collection of 20 (bumped) uridine diphosphate N-acetylgalactosamine (UDP-GalNAc) analogs to identify enzyme-substrate pairs that retain peptide specificities but are otherwise completely orthogonal to native enzyme-substrate pairs. The approach was applicable to multiple GalNAc-T isoenzymes, including GalNAc-T1 and -T2 that prefer nonglycosylated peptide substrates and GalNAcT-10 that prefers a preglycosylated peptide substrate. A detailed investigation of enzyme kinetics and specificities revealed the robustness of the approach to faithfully report on GalNAc-T activity and paves the way for studying substrate specificities in living systems.

Enabling drug discovery for the PARP protein family through the detection of mono-ADP-ribosylation

Poly-ADP-ribose polymerases (PARPs) are a family of enzymes responsible for transferring individual or chains of ADP-ribose subunits to substrate targets as a type of post-translational modification. PARPs regulate a wide variety of important cellular processes, ranging from DNA damage repair to antiviral response. However, most research to date has focused primarily on the polyPARPs, which catalyze the formation of ADP-ribose polymer chains, while the monoPARPs, which transfer individual ADP-ribose monomers, have not been studied as thoroughly. This is partially due to the lack of robust assays to measure mono-ADP-ribosylation in the cell. In this study, the recently developed MAR/PAR antibody has been shown to detect mono-ADP-ribosylation in cells, enabling the field to investigate the function and therapeutic potential of monoPARPs. In this study, the antibody was used in conjunction with engineered cell lines that overexpress various PARPs to establish a panel of assays to evaluate the potencies of literature-reported PARP inhibitors. These assays should be generally applicable to other PARP family members for future compound screening efforts. A convenient and generalizable workflow to identify and validate PARP substrates has been established. As an initial demonstration, aryl hydrocarbon receptor was verified as a direct PARP7 substrate and other novel substrates for this enzyme were also identified and validated. This workflow takes advantage of commercially available detection reagents and conventional mass spectrometry instrumentation and methods. Ultimately, these assays and methods will help drive research in the PARP field and benefit future therapeutics development.