Activity-based protein profiling in bacteria: Applications for identification of therapeutic targets and characterization of microbial communities

Activity-based metaproteomics driven discovery and enzymological characterization of potential α-galactosidases in the mouse gut microbiome

The gut microbiota offers an extensive resource of enzymes, but many remain uncharacterized. To distinguish the activities of similar annotated proteins and mine the potentially applicable ones in the microbiome, we applied an effective Activity-Based Metaproteomics (ABMP) strategy using a specific activity-based probe (ABP) to screen the entire gut microbiome for directly discovering active enzymes and their potential applications, not for exploring host-microbiome interactions. By using an activity-based cyclophellitol aziridine probe specific to α-galactosidases (AGAL), we successfully identified and characterized several gut microbiota enzymes possessing AGAL activities. Cryo-electron microscopy analysis of a newly characterized enzyme (AGLA5) revealed the covalent binding conformations between the AGAL5 active site and the cyclophellitol aziridine ABP, which could provide insights into the enzyme's catalytic mechanism. The four newly characterized AGALs have diverse potential activities, including raffinose family oligosaccharides (RFOs) hydrolysis and enzymatic blood group transformation. Collectively, we present a ABMP platform that facilitates gut microbiota AGALs discovery, biochemical activity annotations and potential industrial or biopharmaceutical applications.

Environmental activity-based protein profiling for function-driven enzyme discovery from natural communities

BACKGROUND

Microbial communities are important drivers of global biogeochemical cycles, xenobiotic detoxification, as well as organic matter decomposition. Their major metabolic role in ecosystem functioning is ensured by a unique set of enzymes, providing a tremendous yet mostly hidden enzymatic potential. Exploring this enzymatic repertoire is therefore not only relevant for a better understanding of how microorganisms function in their natural environment, and thus for ecological research, but further turns microbial communities, in particular from extreme habitats, into a valuable resource for the discovery of novel enzymes with potential applications in biotechnology. Different strategies for their uncovering such as bioprospecting, which relies mainly on metagenomic approaches in combination with sequence-based bioinformatic analyses, have emerged; yet accurate function prediction of their proteomes and deciphering the in vivo activity of an enzyme remains challenging.

RESULTS

Here, we present environmental activity-based protein profiling (eABPP), a multi-omics approach that extends genome-resolved metagenomics with mass spectrometry-based ABPP. This combination allows direct profiling of environmental community samples in their native habitat and the identification of active enzymes based on their function, even without sequence or structural homologies to annotated enzyme families. eABPP thus bridges the gap between environmental genomics, correct function annotation, and in vivo enzyme activity. As a showcase, we report the successful identification of active thermostable serine hydrolases from eABPP of natural microbial communities from two independent hot springs in Kamchatka, Russia.

CONCLUSIONS

By reporting enzyme activities within an ecosystem in their native state, we anticipate that eABPP will not only advance current methodological approaches to sequence homology-guided enzyme discovery from environmental ecosystems for subsequent biocatalyst development but also contributes to the ecological investigation of microbial community interactions by dissecting their underlying molecular mechanisms.

Monitoring host-pathogen interactions using chemical proteomics

With the rapid emergence and the dissemination of microbial resistance to conventional chemotherapy, the shortage of novel antimicrobial drugs has raised a global health threat. As molecular interactions between microbial pathogens and their mammalian hosts are crucial to establish virulence, pathogenicity, and infectivity, a detailed understanding of these interactions has the potential to reveal novel therapeutic targets and treatment strategies. Bidirectional molecular communication between microbes and eukaryotes is essential for both pathogenic and commensal organisms to colonise their host. In particular, several devastating pathogens exploit host signalling to adjust the expression of energetically costly virulent behaviours. Chemical proteomics has emerged as a powerful tool to interrogate the protein interaction partners of small molecules and has been successfully applied to advance host-pathogen communication studies. Here, we present recent significant progress made by this approach and provide a perspective for future studies.

Covalent Targeting of Splicing in T Cells

Despite significant interest in therapeutic targeting of splicing, few chemical probes are available for the proteins involved in splicing. Here, we show that elaborated stereoisomeric acrylamide chemical probe EV96 and its analogues lead to a selective T cell state-dependent loss of interleukin 2-inducible T cell kinase (ITK) by targeting one of the core splicing factors SF3B1. Mechanistic investigations suggest that the state-dependency stems from a combination of differential protein turnover rates and availability of functional mRNA pools that can be depleted due to extensive alternative splicing. We further introduce a comprehensive list of proteins involved in splicing and leverage both cysteine- and protein-directed activity-based protein profiling (ABPP) data with electrophilic scout fragments to demonstrate covalent ligandability for many classes of splicing factors and splicing regulators in primary human T cells. Taken together, our findings show how chemical perturbation of splicing can lead to immune state-dependent changes in protein expression and provide evidence for the broad potential to target splicing factors with covalent chemistry.

Activity-based protein profiling in microbes and the gut microbiome

Activity-based protein profiling (ABPP) is a powerful chemical approach for probing protein function and enzymatic activity in complex biological systems. This strategy typically utilizes activity-based probes that are designed to bind a specific protein, amino acid residue, or protein family and form a covalent bond through a reactivity-based warhead. Subsequent analysis by mass spectrometry-based proteomic platforms that involve either click chemistry or affinity-based labeling to enrich for the tagged proteins enables identification of protein function and enzymatic activity. ABPP has facilitated elucidation of biological processes in bacteria, discovery of new antibiotics, and characterization of host-microbe interactions within physiological contexts. This review will focus on recent advances and applications of ABPP in bacteria and complex microbial communities.

Selection and enrichment of microbial species with an increased lignocellulolytic phenotype from a native soil microbiome by activity-based probing

Multi-omic analyses can provide information on the potential for activity within a microbial community but often lack specificity to link functions to cell, primarily offer potential for function or rely on annotated databases. Functional assays are necessary for understanding in situ microbial activity to better describe and improve microbiome biology. Targeting enzyme activity through activity-based protein profiling enhances the accuracy of functional studies. Here, we introduce a pipeline of coupling activity-based probing with fluorescence-activated cell sorting, culturing, and downstream activity assays to isolate and examine viable populations of cells expressing a function of interest. We applied our approach to a soil microbiome using two activity-based probes to enrich for communities with elevated activity for lignocellulose-degradation phenotypes as determined by four fluorogenic kinetic assays. Our approach efficiently separated and identified microbial members with heightened activity for glycosyl hydrolases, and by expanding this workflow to various probes for other function, this process can be applied to unique phenotype targets of interest.

Biosynthesis of ansamitocin P-3 incurs stress on the producing strain Actinosynnema pretiosum at multiple targets

Microbial bioactive natural products mediate ecologically beneficial functions to the producing strains, and have been widely used in clinic and agriculture with clearly defined targets and underlying mechanisms. However, the physiological effects of their biosynthesis on the producing strains remain largely unknown. The antitumor ansamitocin P-3 (AP-3), produced by Actinosynnema pretiosum ATCC 31280, was found to repress the growth of the producing strain at high concentration and target the FtsZ protein involved in cell division. Previous work suggested the presence of additional cryptic targets of AP-3 in ATCC 31280. Herein we use chemoproteomic approach with an AP-3-derived photoaffinity probe to profile the proteome-wide interactions of AP-3. AP-3 exhibits specific bindings to the seemingly unrelated deoxythymidine diphosphate glucose-4,6-dehydratase, aldehyde dehydrogenase, and flavin-dependent thymidylate synthase, which are involved in cell wall assembly, central carbon metabolism and nucleotide biosynthesis, respectively. AP-3 functions as a non-competitive inhibitor of all three above target proteins, generating physiological stress on the producing strain through interfering diverse metabolic pathways. Overexpression of these target proteins increases strain biomass and markedly boosts AP-3 titers. This finding demonstrates that identification and engineering of cryptic targets of bioactive natural products can lead to in-depth understanding of microbial physiology and improved product titers.

Chemical approaches to unraveling the biology of mycobacteria

Mycobacterium tuberculosis (Mtb), perhaps more than any other organism, is intrinsically appealing to chemical biologists. Not only does the cell envelope feature one of the most complex heteropolymers found in nature1 but many of the interactions between Mtb and its primary host (we humans) rely on lipid and not protein mediators.2,3 Many of the complex lipids, glycolipids, and carbohydrates biosynthesized by the bacterium still have unknown functions, and the complexity of the pathological processes by which tuberculosis (TB) disease progress offers many opportunities for these molecules to influence the human response. Because of the importance of TB in global public health, chemical biologists have applied a wide-ranging array of techniques to better understand the disease and improve interventions.

Targeting the human gut microbiome with small-molecule inhibitors

The human gut microbiome is a complex microbial community that is strongly linked to both host health and disease. However, the detailed molecular mechanisms underlying the effects of these microorganisms on host biology remain largely uncharacterized. The development of non-lethal, small-molecule inhibitors that target specific gut microbial activities enables a powerful but underutilized approach to studying the gut microbiome and a promising therapeutic strategy. In this Review, we will discuss the challenges of studying this microbial community, the historic use of small-molecule inhibitors in microbial ecology, and recent applications of this strategy. We also discuss the evidence suggesting that host-targeted drugs can affect the growth and metabolism of gut microbes. Finally, we address the issues of developing and implementing microbiome-targeted small-molecule inhibitors and define important future directions for this research.

Sequence and Structural Motifs Controlling the Broad Substrate Specificity of the Mycobacterial Hormone-Sensitive Lipase LipN

Mycobacterium tuberculosis has a complex life cycle transitioning between active and dormant growth states depending on environmental conditions. LipN (Rv2970c) is a conserved mycobacterial serine hydrolase with regulated catalytic activity at the interface between active and dormant growth conditions. LipN also catalyzes the xenobiotic degradation of a tertiary ester substrate and contains multiple conserved motifs connected with the ability to catalyze the hydrolysis of difficult tertiary ester substrates. Herein, we expanded a library of fluorogenic ester substrates to include more tertiary and constrained esters and screened 33 fluorogenic substrates for activation by LipN, identifying its unique substrate signature. LipN preferred short, unbranched ester substrates, but had its second highest activity against a heteroaromatic five-membered oxazole ester. Oxazole esters are present in multiple mycobacterial serine hydrolase inhibitors but have not been tested widely as ester substrates. Combined structural modeling, kinetic measurements, and substitutional analysis of LipN showcased a fairly rigid binding pocket preorganized for catalysis of short ester substrates. Substitution of diverse amino acids across the binding pocket significantly impacted the folded stability and catalytic activity of LipN with two conserved motifs (HGGGW and GDSAG) playing interconnected, multidimensional roles in regulating its substrate specificity. Together this detailed substrate specificity profile of LipN illustrates the complex interplay between structure and function in mycobacterial hormone-sensitive lipase homologues and indicates oxazole esters as promising inhibitor and substrate scaffolds for mycobacterial hydrolases.

Direct capture, inhibition and crystal structure of HsaD (Rv3569c) from M. tuberculosis

A hallmark of Mycobacterium tuberculosis (M. tb), the aetiologic agent of tuberculosis, is its ability to metabolise host-derived lipids. However, the enzymes and mechanisms underlying such metabolism are still largely unknown. We previously reported that the Cyclophostin & Cyclipostins (CyC) analogues, a new family of potent antimycobacterial molecules, react specifically and covalently with (Ser/Cys)-based enzymes mostly involved in bacterial lipid metabolism. Here, we report the synthesis of new CyC alkyne-containing inhibitors (CyCyne ) and their use for the direct fishing of target proteins in M. tb culture via bio-orthogonal click-chemistry activity-based protein profiling (CC-ABPP). This approach led to the capture and identification of a variety of enzymes, and many of them involved in lipid or steroid metabolisms. One of the captured enzymes, HsaD (Rv3569c), is required for the survival of M. tb within macrophages and is thus a potential therapeutic target. This prompted us to further explore and validate, through a combination of biochemical and structural approaches, the specificity of HsaD inhibition by the CyC analogues. We confirmed that the CyC bind covalently to the catalytic Ser114 residue, leading to a total loss of enzyme activity. These data were supported by the X-ray structures of four HsaD-CyC complexes, obtained at resolutions between 1.6 and 2.6 Å. The identification of mycobacterial enzymes directly captured by the CyCyne probes through CC-ABPP paves the way to better understand and potentially target key players at crucial stages of the bacilli life cycle.

Activity-based Tools for Interrogating Host Biology During Infection

Host cells sense and respond to pathogens by dynamically regulating cell signaling. The rapid modulation of signaling pathways is achieved by post-translational modifications (PTMs) that can alter protein structure, function, and/or binding interactions. By using chemical probes to broadly profile changes in enzyme function or side-chain reactivity, activity-based protein profiling (ABPP) can reveal PTMs that regulate host-microbe interactions. While ABPP has been widely utilized to uncover microbial mechanisms of pathogenesis, in this review, we focus on more recent applications of this technique to the discovery of host PTMs and enzymes that modulate signaling within infected cells. Collectively, these advances underscore the importance of ABPP as a tool for interrogating the host response to infection and identifying potential targets for host-directed therapies.

Using click chemistry to study microbial ecology and evolution

Technological advances have largely driven the revolution in our understanding of the structure and function of microbial communities. Culturing, long the primary tool to probe microbial life, was supplanted by sequencing and other -omics approaches, which allowed detailed quantitative insights into species composition, metabolic potential, transcriptional activity, secretory responses and more. Although the ability to characterize "who's there" has never been easier or cheaper, it remains technically challenging and expensive to understand what the diverse species and strains that comprise microbial communities are doing in situ, and how these behaviors change through time. Our aim in this brief review is to introduce a developing toolkit based on click chemistry that can accelerate and reduce the expense of functional analyses of the ecology and evolution of microbial communities. After first outlining the history of technological development in this field, we will discuss key applications to date using diverse labels, including BONCAT, and then end with a selective (biased) view of areas where click-chemistry and BONCAT-based approaches stand to have a significant impact on our understanding of microbial communities.

A Nucleophilic Activity-Based Probe Enables Profiling of PLP-Dependent Enzymes

PLP-dependent enzymes represent an important class of highly "druggable" enzymes that perform a wide array of critical reactions to support all organisms. Inhibition of individual members of this family of enzymes has been validated as a therapeutic target for pathologies ranging from infection with Mycobacterium tuberculosis to epilepsy. Given the broad nature of the activities within this family of enzymes, we envisioned a universally acting probe to characterize existing and putative members of the family that also includes the necessary chemical moieties to enable activity-based protein profiling experiments. Hence, we developed a probe that contains an N-hydroxyalanine warhead that acts as a covalent inhibitor of PLP-dependent enzymes, a linear diazirine for UV crosslinking, and an alkyne moiety to enable enrichment of crosslinked proteins. Our molecule was used to study PLP-dependent enzymes in vitro as well as look at whole-cell lysates of M. tuberculosis and assess inhibitory activity. The probe was able to enrich and identify LysA, a PLP-dependent enzyme crucial for lysine biosynthesis, through mass spectrometry. Overall, our study shows the utility of this trifunctional first-generation probe. We anticipate further optimization of probes for PLP-dependent enzymes will enable the characterization of rationally designed covalent inhibitors of PLP-dependent enzymes, which will expedite the preclinical characterization of these important therapeutic targets.

A Small-Molecule Probe with a Dual Function of miRNA Inhibition and Target identification

By virtue of their key roles in pathologies, miRNAs represent a promising class of therapeutic targets. While high-fidelity small-molecule modulators of miRNAs can be identified via high-throughput screening using cellular reporter systems, their modes of action are elusive due to the lack of proper tools. Here, we report a small-molecule probe, 1 a, that is capable of elucidating its biological target along miRNA inhibition. Derived from norathyriol, a nature product, 1 a possessed a bioorthogonal alkyne moiety for subsequent labeling via copper-catalyzed azide-alkyne cycloaddition chemistry. We demonstrated that 1 a inhibited a panel of different miRNAs by blocking their loading onto argonaute 2 (AGO2), which is the key protein responsible for miRNA function. With the alkyne handle, we successfully identified AGO2 as an intracellular target of 1 a. Therefore, this work presents a novel small-molecule tool for suppressing and probing miRNA regulatory pathways.

Chemoproteomic Analysis of Microbiota Metabolite-Protein Targets and Mechanisms

The microbiota have emerged as an important factor in host physiology, disease, and response to therapy. These diverse microbes (bacteria, virus, fungi, and protists) encode unique functions and metabolites that regulate intraspecies and interspecies interactions. While the mechanisms of some microbiota species and metabolites have been elucidated, the diversity and abundance of different microbiota species and their associated pathways suggest many more metabolites and mechanisms of action remain to be discovered. In this Perspective, we highlight how the advances in chemical proteomics have provided new opportunities to elucidate the molecular targets of specific microbiota metabolites and reveal new mechanisms of action. The continued development of specific microbiota metabolite reporters and more precise proteomic methods should reveal new microbiota mechanisms of action, therapeutic targets, and biomarkers for a variety of human diseases.

Diagnostic and therapeutic potential of protease inhibition

Proteases are enzymes that hydrolyze peptide bonds in proteins and peptides; thus, they control virtually all biological processes. Our understanding of protease function has advanced considerably from nonselective digestive enzymes to highly specialized molecular scissors that orchestrate complex signaling networks through a limited proteolysis. The catalytic activity of proteases is tightly regulated at several levels, ranging from gene expression through trafficking and maturation to posttranslational modifications. However, when this delicate balance is disturbed, many diseases develop, including cancer, inflammatory disorders, diabetes, and neurodegenerative diseases. This new understanding of the role of proteases in pathologic physiology indicates that these enzymes represent excellent molecular targets for the development of therapeutic inhibitors, as well as for the design of chemical probes to visualize their redundant activity. Recently, numerous platform technologies have been developed to identify and optimize protease substrates and inhibitors, which were further used as lead structures for the development of chemical probes and therapeutic drugs. Due to this considerable success, the clinical potential of proteases in therapeutics and diagnostics is rapidly growing and is still not completely explored. Therefore, small molecules that can selectively target aberrant protease activity are emerging in diseases cells. In this review, we describe modern trends in the design of protease drugs as well as small molecule activity-based probes to visualize selected proteases in clinical settings.

Identification of fungal lignocellulose-degrading biocatalysts secreted by Phanerochaete chrysosporium via activity-based protein profiling

Activity-based protein profiling (ABPP) has emerged as a versatile biochemical method for studying enzyme activity under various physiological conditions, with applications so far mainly in biomedicine. Here, we show the potential of ABPP in the discovery of biocatalysts from the thermophilic and lignocellulose-degrading white rot fungus Phanerochaete chrysosporium. By employing a comparative ABPP-based functional screen, including a direct profiling of wood substrate-bound enzymes, we identify those lignocellulose-degrading carbohydrate esterase (CE1 and CE15) and glycoside hydrolase (GH3, GH5, GH16, GH17, GH18, GH25, GH30, GH74 and GH79) enzymes specifically active in presence of the substrate. As expression of fungal enzymes remains challenging, our ABPP-mediated approach represents a preselection procedure for focusing experimental efforts on the most promising biocatalysts. Furthermore, this approach may also allow the functional annotation of domains-of-unknown functions (DUFs). The ABPP-based biocatalyst screening described here may thus allow the identification of active enzymes in a process of interest and the elucidation of novel biocatalysts that share no sequence similarity to known counterparts.

Chemoproteomic Profiling Reveals the Mechanism of Bile Acid Tolerance in Bacteria

Bile acids (BAs) are a class of endogenous metabolites with important functions. As amphipathic molecules, BAs have strong antibacterial effects, preventing overgrowth of the gut microbiota and defending the invasion of pathogens. However, some disease-causing pathogens can survive the BA stress and knowledge is limited about how they develop BA tolerance. In this work, we applied a quantitative chemoproteomic strategy to profile BA-interacting proteins in bacteria, aiming to discover the sensing pathway of BAs. Using a clickable and photo-affinity BA probe with quantitative mass spectrometry, we identified a list of histidine kinases (HKs) of the two-component systems (TCS) in bacteria as the novel binding targets of BA. Genetic screening revealed that knocking out one specific HK, EnvZ, renders bacteria with significant sensitivity to BA. Further biochemical and genetic experiments demonstrated that BA binds to a specific pocket in EnvZ and activates a downstream signaling pathway to help efflux of BA from bacteria, resulting in BA tolerance. Collectively, our data revealed that EnvZ is a novel sensor of BA in bacteria and its associated TCS signaling pathway plays a critical role in mediating bacterial BA tolerance, which opens new opportunities to combat BA-tolerating pathogens.

Functionalised Cofactor Mimics for Interactome Discovery and Beyond

Cofactors are required for almost half of all enzyme reactions, but their functions and binding partners are not fully understood even after decades of research. Functionalised cofactor mimics that bind in place of the unmodified cofactor can provide answers, as well as expand the scope of cofactor activity. Through chemical proteomics approaches such as activity-based protein profiling, the interactome and localisation of the native cofactor in its physiological environment can be deciphered and previously uncharacterised proteins annotated. Furthermore, cofactors that supply functional groups to substrate biomolecules can be hijacked by mimics to site-specifically label targets and unravel the complex biology of post-translational protein modification. The diverse activity of cofactors has inspired the design of mimics for use as inhibitors, antibiotic therapeutics, and chemo- and biosensors, and cofactor conjugates have enabled the generation of novel enzymes and artificial DNAzymes.

Integration of bioinformatic and chemoproteomic tools for the study of enzyme conservation in closely related bacterial species

Activity-based protein profiling (ABPP) is a commonly utilized technique to globally characterize the endogenous activity of multiple enzymes within a related family. While it has been used extensively to identify enzymes that are differentially active across various mammalian tissues, recent efforts have expanded this technique to studying bacteria. As ABPP is applied to diverse sets of bacterial strains found in microbial communities, there is also an increasing need for robust tools for assessing the conservation of enzymes across closely related bacterial species and strains. In this chapter, we detail the integration of gel-based ABPP with basic bioinformatic tools to enable the analysis of enzyme activity, distribution, and homology. We use as an example the family of serine hydrolases identified in the skin commensal bacterium Staphylococcus epidermidis.

Activity-Based Protein Profiling for the Identification of Novel Carbohydrate-Active Enzymes Involved in Xylan Degradation in the Hyperthermophilic Euryarchaeon Thermococcus sp. Strain 2319x1E

Activity-based protein profiling (ABPP) has so far scarcely been applied in Archaea in general and, especially, in extremophilic organisms. We herein isolated a novel Thermococcus strain designated sp. strain 2319x1E derived from the same enrichment culture as the recently reported Thermococcus sp. strain 2319x1. Both strains are able to grow with xylan as the sole carbon and energy source, and for Thermococcus sp. strain 2319x1E (optimal growth at 85°C, pH 6–7), the induction of xylanolytic activity in the presence of xylan was demonstrated. Since the solely sequence-based identification of xylanolytic enzymes is hardly possible, we established a complementary approach by conducting comparative full proteome analysis in combination with ABPP using α- or β-glycosidase selective probes and subsequent mass spectrometry (MS)-based analysis. This complementary proteomics approach in combination with recombinant protein expression and classical enzyme characterization enabled the identification of a novel bifunctional maltose-forming α-amylase and deacetylase (EGDIFPOO_00674) belonging to the GH57 family and a promiscuous β-glycosidase (EGIDFPOO_00532) with β-xylosidase activity. We thereby further substantiated the general applicability of ABPP in archaea and expanded the ABPP repertoire for the identification of glycoside hydrolases in hyperthermophiles.

UCHL1 as a novel target in breast cancer: emerging insights from cell and chemical biology

Breast cancer has the highest incidence and death rate among cancers in women worldwide. In particular, metastatic estrogen receptor negative (ER-) breast cancer and triple-negative breast cancer (TNBC) subtypes have very limited treatment options, with low survival rates. Ubiquitin carboxyl terminal hydrolase L1 (UCHL1), a ubiquitin C-terminal hydrolase belonging to the deubiquitinase (DUB) family of enzymes, is highly expressed in these cancer types, and several key reports have revealed emerging and important roles for UCHL1 in breast cancer. However, selective and potent small-molecule UCHL1 inhibitors have been disclosed only very recently, alongside chemical biology approaches to detect regulated UHCL1 activity in cancer cells. These tools will enable novel insights into oncogenic mechanisms driven by UCHL1, and identification of substrate proteins deubiquitinated by UCHL1, with the ultimate goal of realising the potential of UCHL1 as a drug target in breast cancer.

Bioorthogonal Chemistry and Its Applications

Bioorthogonal chemistry is a set of methods using the chemistry of non-native functional groups to explore and understand biology in living organisms. In this review, we summarize the most common reactions used in bioorthogonal methods, their relative advantages and disadvantages, and their frequency of occurrence in the published literature. We also briefly discuss some of the less common but potentially useful methods. We then analyze the bioorthogonal-related publications in the CAS Content Collection to determine how often different types of biomolecules such as proteins, carbohydrates, glycans, and lipids have been studied using bioorthogonal chemistry. The most prevalent biological and chemical methods for attaching bioorthogonal functional groups to these biomolecules are elaborated. We also analyze the publication volume related to different types of bioorthogonal applications in the CAS Content Collection. The use of bioorthogonal chemistry for imaging, identifying, and characterizing biomolecules and for delivering drugs to treat disease is discussed at length. Bioorthogonal chemistry for the surface attachment of proteins and in the use of modified carbohydrates is briefly noted. Finally, we summarize the state of the art in bioorthogonal chemistry and its current limitations and promise for its future productive use in chemistry and biology.

Secretory Proteases of the Human Skin Microbiome

The human skin is our outermost layer and serves as a protective barrier against external insults. Advances in next generation sequencing have enabled the discoveries of a rich and diverse community of microbes - bacteria, fungi and viruses that are residents of this surface. The genomes of these microbes also revealed the presence of many secretory enzymes. In particular, proteases which are hydrolytic enzymes capable of protein cleavage and degradation are of special interest in the skin environment which is enriched in proteins and lipids. In this minireview, we will focus on the roles of these skin-relevant microbial secreted proteases, both in terms of their widely studied roles as pathogenic agents in tissue invasion and host immune inactivation, and their recently discovered roles in inter-microbial interactions and modulation of virulence factors. From these studies, it has become apparent that while microbial proteases are capable of a wide range of functions, their expression is tightly regulated and highly responsive to the environments the microbes are in. With the introduction of new biochemical and bioinformatics tools to study protease functions, it will be important to understand the roles played by skin microbial secretory proteases in cutaneous health, especially the less studied commensal microbes with an emphasis on contextual relevance.

Spatiotemporally-resolved mapping of RNA binding proteins via functional proximity labeling reveals a mitochondrial mRNA anchor promoting stress recovery

Proximity labeling (PL) with genetically-targeted promiscuous enzymes has emerged as a powerful tool for unbiased proteome discovery. By combining the spatiotemporal specificity of PL with methods for functional protein enrichment, we show that it is possible to map specific protein subclasses within distinct compartments of living cells. In particular, we develop a method to enrich subcompartment-specific RNA binding proteins (RBPs) by combining peroxidase-catalyzed PL with organic-aqueous phase separation of crosslinked protein-RNA complexes (“APEX-PS”). We use APEX-PS to generate datasets of nuclear, nucleolar, and outer mitochondrial membrane (OMM) RBPs, which can be mined for novel functions. For example, we find that the OMM RBP SYNJ2BP retains specific nuclear-encoded mitochondrial mRNAs at the OMM during translation stress, facilitating their local translation and import of protein products into the mitochondrion during stress recovery. Functional PL in general, and APEX-PS in particular, represent versatile approaches for the discovery of proteins with novel function in specific subcellular compartments.

Proximity labeling is used to map and discover proteins in specific subcellular compartments. Here the authors combine APEX-mediated proximity labeling with organic-aqueous phase separation to identify nuclear, nucleolar, and outer mitochondrial membrane RNA binding proteins.

Discovery of a Cell-Active SuTEx Ligand of Prostaglandin Reductase 2

Sulfonyl-triazoles have emerged as a new reactive group for covalent modification of tyrosine sites on proteins through sulfur-triazole exchange (SuTEx) chemistry. The extent to which this sulfur electrophile can be tuned for developing ligands with cellular activity remains largely underexplored. Here, we performed fragment-based ligand discovery in live cells to identify SuTEx compounds capable of liganding tyrosine sites on diverse protein targets. We verified our quantitative chemical proteomic findings by demonstrating concentration-dependent activity of SuTEx ligands, but not inactive counterparts, against recombinant protein targets directly in live cells. Our structure-activity relationship studies identified the SuTEx ligand HHS-0701 as a cell-active inhibitor capable of blocking prostaglandin reductase 2 (PTGR2) biochemical activity.

Using a safe and effective fixative to improve the immunofluorescence staining of bacteria

The emerging and development of green chemistry has once again drawn the researchers' attention to eliminating the use and generation of hazardous materials. Here we report the use of a safe and effective fixative, chlorine dioxide (ClO₂), instead of traditional hazardous fixatives for the cross-linking of cellular proteins to improve immunofluorescence staining of bacteria. The concentration of ClO₂needed for 100% fixation is 50μg ml^-1, which is much lower than that of traditional fixatives (1000-10000μg ml^-1). The ClO₂mediated cross-linking can preserve the integrity of bacterial cells and prevent cell loss through lysis. Meanwhile, lysozyme can permeabilize the bacterial cells, allowing the labelled antibodies to diffuse to their intracellular target molecules. By usingE. coliO157:H7/RP4 as a gram-negative bacteria model, immunofluorescence staining assays for both intracellular protein and surface polysaccharide were carried out to investigate the effect of ClO₂fixation on the staining. The results demonstrated that ClO₂fixation could prevent the target antigens from cracking off the bacteria without damage on the interaction between the antibodies and antigens (either for polysaccharide or protein). As a safe and effective fixative, ClO₂has potential practical applications in immunofluorescence staining and fluorescencein situhybridization for single bacteria/cell analysis.

Activity-Based Protein Profiling (ABPP) of Oxidoreductases

Over the last two decades, activity-based protein profiling (ABPP) has been established as a tremendously useful proteomic tool for measuring the activity of proteins in their cellular context, annotating the function of uncharacterized proteins, and investigating the target profile of small-molecule inhibitors. Unlike hydrolases and other enzyme classes, which exhibit a characteristic nucleophilic residue, oxidoreductases have received much less attention in ABPP. In this minireview, the state of the art of ABPP of oxidoreductases is described and the scope and limitations of the existing approaches are discussed. It is noted that several ABPP probes have been described for various oxidases, but none so far for a reductase, which gives rise to opportunities for future research.

Lipolytic enzymes inhibitors: A new way for antibacterial drugs discovery

Tuberculosis (TB) caused by Mycobacterium tuberculosis (M. tb) still remains the deadliest infectious disease worldwide with 1.5 million deaths in 2018, of which about 15% are attributed to resistant strains. Another significant example is Mycobacterium abscessus (M. abscessus), a nontuberculous mycobacteria (NTM) responsible for cutaneous and pulmonary infections, representing up to 95% of NTM infections in cystic fibrosis (CF) patients. M. abscessus is a new clinically relevant pathogen and is considered one of the most drug-resistant mycobacteria for which standardized chemotherapeutic regimens are still lacking. Together the emergence of M. tb and M. abscessus multi-drug resistant strains with ineffective and expensive therapeutics, have paved the way to the development of new classes of anti-mycobacterial agents offering additional therapeutic options. In this context, specific inhibitors of mycobacterial lipolytic enzymes represent novel and promising antibacterial molecules to address this challenging issue. The results highlighted here include a complete overview of the antibacterial activities, either in broth medium or inside infected macrophages, of two families of promising and potent anti-mycobacterial multi-target agents, i.e. oxadiazolone-core compounds (OX) and Cyclophostin & Cyclipostins analogs (CyC); the identification and biochemical validation of their effective targets (e.g., the antigen 85 complex and TesA playing key roles in mycolic acid metabolism) together with their respective crystal structures. To our knowledge, these are the first families of compounds able to target and impair replicating as well as intracellular bacteria. We are still impelled in deciphering their mode of action and finding new potential therapeutic targets against mycobacterial-related diseases.

Combining experimental strategies for successful target deconvolution

Investment in phenotypic drug discovery has led to increased demand for rapid and robust target deconvolution to aid successful drug development. Although methods for target identification and mechanism of action (MoA) discovery are flourishing, they typically lead to lists of putative targets. Validating which target(s) are involved in the therapeutic mechanism of a compound poses a significant challenge, requiring direct binding, target engagement, and functional studies in relevant physiological contexts. A combination of orthogonal approaches can allow target identification beyond the proteome as well as aid prioritisation for resource-intensive target validation studies.

Targeting gut microbiota for precision medicine: Focusing on the efficacy and toxicity of drugs

Intra- and interindividual variation in drug responses is one major reason for the failure of drug therapy, drug toxicity, and even the death of patients. Precision medicine, or personalized medicine, is a field of medicine that customizes an individual's medical diagnosis and treatment based on his/her genes, microbiomes, environments, etc. Over the past decade, a large number of studies have demonstrated that gut microbiota can modify the efficacy and toxicity of drugs, and the extent of the modification varies greatly from person to person because of the variability of the gut microbiota. Personalized manipulation of gut microbiota is an important approach to rectify the abnormal drug response. In this review, we aim to improve drug efficacy and reduce drug toxicity by combining precision medicine and gut microbiota. After describing the interactions between gut microbiota and xenobiotics, we discuss (1) the effects of gut microbiota on drug efficacy and toxicity and the corresponding mechanisms, (2) the variability of gut microbiota, which leads to variation in drug responses, (3) the biomarkers used for the patient stratification and treatment decisions before the use of drugs, and (4) the methods used for the personalized manipulation of gut microbiota to improve drug outcomes. Overall, we hope to improve the drug response by incorporating the knowledge of gut microbiota into clinical practice.

Enzyme-targeted fluorescent small-molecule probes for bacterial imaging

Molecular imaging methods to visualize myriad biochemical processes in bacteria have traditionally been dependent upon molecular biology techniques to incorporate fluorescent biomolecules (e.g., fusion proteins). Such methods have been instrumental in our understanding of how bacteria function but are not without drawbacks, including potential perturbation to native protein expression and function. To overcome these limitations, the use of fluorescent small-molecule probes has gained much attention. Here, we highlight examples from the recent literature that showcase the utility of small-molecule probes for the fluorescence imaging of bacterial cells, including electrophilic, metabolic, and enzyme-activated probes. Although the use of these types of compounds for bacterial imaging is still relatively new, the selected examples demonstrate the exciting potential of these critical tools in the exploration of bacterial physiology.

Activity- and reactivity-based proteomics: Recent technological advances and applications in drug discovery

Activity-based protein profiling (ABPP) is recognized as a powerful and versatile chemoproteomic technology in drug discovery. Central to ABPP is the use of activity-based probes to report the activity of specific enzymes or reactivity of amino acid types in complex biological systems. Over the last two decades, ABPP has facilitated the identification of new drug targets and discovery of lead compounds in human and infectious disease. Furthermore, as part of a sustained global effort to illuminate the druggable proteome, the repertoire of target classes addressable with activity-based probes has vastly expanded in recent years. Here, we provide an overview of ABPP and summarise the major technological advances with an emphasis on probe development.

Who Is Metabolizing What? Discovering Novel Biomolecules in the Microbiome and the Organisms Who Make Them

Even as the field of microbiome research has made huge strides in mapping microbial community composition in a variety of environments and organisms, explaining the phenotypic influences on the host by microbial taxa-both known and unknown-and their specific functions still remain major challenges. A pressing need is the ability to assign specific functions in terms of enzymes and small molecules to specific taxa or groups of taxa in the community. This knowledge will be crucial for advancing personalized therapies based on the targeted modulation of microbes or metabolites that have predictable outcomes to benefit the human host. This perspective article advocates for the combined use of standards-free metabolomics and activity-based protein profiling strategies to address this gap in functional knowledge in microbiome research via the identification of novel biomolecules and the attribution of their production to specific microbial taxa.

Characterization of Serine Hydrolases Across Clinical Isolates of Commensal Skin Bacteria Staphylococcus epidermidis Using Activity-Based Protein Profiling

The bacterial genus Staphylococcus comprises diverse species that colonize the skin as commensals but can also cause infection. Previous work identified a family of serine hydrolases termed fluorophoshonate-binding hydrolases (Fphs) in the pathogenic bacteria Staphylococcus aureus, one of which, FphB, functions as a virulence factor. Using a combination of bioinformatics and activity-based protein profiling (ABPP), we identify homologues of these enzymes in the related commensal bacteria Staphylococcus epidermidis. Two of the S. aureus Fph enzymes were not identified in S. epidermidis. Using ABPP, we identified several candidate hydrolases that were not previously identified in S. aureus that may be functionally related to the Fphs. Interestingly, the activity of the Fphs vary across clinical isolates of S. epidermidis. Biochemical characterization of the FphB homologue in S. epidermidis (SeFphB) suggests it is a functional homologue of FphB in S. aureus, but our preliminary studies suggest it may not have a role in colonization in vivo. This potential difference in biological function between the Fphs of closely related staphylococcal species may provide mechanisms for specific inhibition of S. aureus infection without perturbing commensal communities of related bacteria.

Chemical tools for selective activity profiling of bacterial penicillin-binding proteins

Penicillin-binding proteins (PBPs) are membrane-associated proteins involved in the biosynthesis of peptidoglycan (PG), the main component of bacterial cell walls. These proteins were discovered and named for their affinity to bind the β-lactam antibiotic penicillin. The importance of the PBPs has long been appreciated; however, specific roles of individual family members in each bacterial strain, as well as their protein-protein interactions, are yet to be understood. The apparent functional redundancy of the 4-18 PBPs that most eubacteria possess makes determination of their individual roles difficult. Existing techniques to study PBPs are not ideal because they do not directly visualize protein activity and can suffer from artifacts and perturbations of native PBP function. Therefore, development of new methods for studying the roles of individual PBPs in cell wall synthesis is required. We recently generated a library of fluorescent chemical probes containing a β-lactone scaffold that specifically targets the PBPs, enabling the visualization of their catalytic activity. Herein, we describe a general protocol to label and detect the activity of individual PBPs in Streptococcus pneumoniae using our fluorescent β-lactone probes.