Activity-Based Protein Profiling: New Developments and Directions in Functional Proteomics

Insights into functional divergence, catalytic versatility and specificity of small molecule glycosyltransferases

Glycosylation is one of the most fundamental biochemical processes in cells. It plays crucial roles in diversifying plant natural products for structures, bioavailability and bioactivity, and thus, renders the glycosylated compounds valuable as food additives, nutraceuticals and pharmaceuticals. Moreover, glycosylated compounds impact plant growth, development and stress response. Therefore, understanding the biochemical function of the glycosyltransferases (GTs) is crucial to the elucidation of natural product biosynthetic pathways, improving plant traits and development of processes for industrially-important compounds. UDP-dependent glycosyltransferases (UGTs) that belong to the glycosyltransferase family-1 (GT1) and catalyze the transfer of glycosyl moieties from UDP-sugars to various small molecules, are the key players in natural product glycosylation. Recent studies also found the involvement of non-canonical cellulose synthase-like (CesAs) and glycosyl hydrolase (GH) family enzymes in the glycosylation of plant specialized metabolites. Decades of research on GTs provided critical insights into catalytic mechanism, substrate/product specificity and catalytic promiscuity, but biochemical function and physiological roles of GTs in majority of the natural product biosynthetic pathways remain to be understood. It is also important to redefine high-throughput strategies of GT mining to uncover novel biochemical function, considering that GTs are the large superfamily members in plants and other organisms. This review underscores the roles of GTs in small molecule glycosylation, plant development and stress responses, highlighting the catalytic versatility and substrate/product specificity of GTs in shaping plant metabolic diversity, and discusses the emerging strategies for mining of uncharacterized GTs to unravel biochemical and physiological functions and to elucidate natural product biosynthetic pathways.

Quinone Methide Based Self-Immobilizing Molecular Fluorescent Probes for In Situ Imaging of Enzymes

Enzymes play important roles not only in normal physiological processes but in the development of many diseases. In situ imaging of enzymes with high-resolution in living systems would helpful for clinical diagnosis and treatment. However, many molecular fluorescent probes suffer from the drawback of diffusing away from the reaction site of enzymes even out of the cells, losing the in situ information and resulting in poor imaging resolution. Quinone methide (QM) based self-immobilizing probes allow the fluorescent signal to be immobilized near the target for an extended period without deactivating the target enzymes, ensuring that it will provide amplified signals and in situ information of the target with high resolution. In this review, we summarized the recent progress of QM-based self-immobilizing probes including their design strategies, working mechanisms, classifications and applications in in situ enzyme imaging. This review calls for the development of more activatable QM-based probe with the advantages of high stability in the absence of the target but very high labeling efficiency after activation.

Recent progress of self-immobilizing and self-precipitating molecular fluorescent probes for higher-spatial-resolution imaging

Flourished in the past two decades, fluorescent probe technology provides researchers with accurate and efficient tools for in situ imaging of biomarkers in living cells and tissues and may play a significant role in clinical diagnosis and treatment such as biomarker detection, fluorescence imaging-guided surgery, and photothermal/photodynamic therapy. In situ imaging of biomarkers depends on the spatial resolution of molecular probes. Nevertheless, the majority of currently available molecular fluorescent probes suffer from the drawback of diffusing from the target region. This leads to a rapid attenuation of the fluorescent signal over time and a reduction in spatial resolution. Consequently, the diffused fluorescent signal cannot accurately reflect the in situ information of the target. Self-immobilizing and self-precipitating molecular fluorescent probes can be used to overcome this problem. These probes ensure that the fluorescent signal remains at the location where the signal is generated for a long time. In this review, we introduce the development history of the two types of probes and classify them in detail according to different design strategies. In addition, we compare their advantages and disadvantages, summarize some representative studies conducted in recent years, and propose prospects for this field.

Recent Advances about the Applications of Click Reaction in Chemical Proteomics

Despite significant advances in biological and analytical approaches, a comprehensive portrait of the proteome and its dynamic interactions and modifications remains a challenging goal. Chemical proteomics is a growing area of chemical biology that seeks to design small molecule probes to elucidate protein composition, distribution, and relevant physiological and pharmacological functions. Click chemistry focuses on the development of new combinatorial chemical methods for carbon heteroatom bond (C-X-C) synthesis, which have been utilized extensively in the field of chemical proteomics. Click reactions have various advantages including high yield, harmless by-products, and simple reaction conditions, upon which the molecular diversity can be easily and effectively obtained. This paper reviews the application of click chemistry in proteomics from four aspects: (1) activity-based protein profiling, (2) enzyme-inhibitors screening, (3) protein labeling and modifications, and (4) hybrid monolithic column in proteomic analysis.

Exploration of the Reactivity of Multivalent Electrophiles for Affinity Labeling: Sulfonyl Fluoride as a Highly Efficient and Selective Label

Here we explored the reactivity of a set of multivalent electrophiles cofunctionalized with a carbohydrate ligand on gold nanoparticles to achieve efficient affinity labeling for target protein analysis. Evaluation of the reactivity and selectivity of the electrophiles against three different cognate binding proteins identified arylsulfonyl fluoride as the most efficient protein-reactive group in this study. We demonstrated that multivalent arylsulfonyl fluoride probe 4 at 50 nm concentration achieved selective affinity labeling and enrichment of a model protein PNA in cell lysate, which was more effective than photoaffinity probe 1 with arylazide group. Labeling site analysis by LC-MS/MS revealed that the nanoparticle-immobilized arylsulfonyl fluoride group can target multiple amino acid residues around the ligand binding site of the target proteins. Our study highlights the utility of arylsulfonyl fluoride as a highly effective multivalent affinity label suitable for covalently capturing unknown target proteins.

The number of catalytic cycles in an enzyme's lifetime and why it matters to metabolic engineering

The continuous replacement of enzymes and other proteins appropriates up to half the maintenance energy budget in microorganisms and plants. High enzyme replacement rates therefore cut the productivity of biosystems ranging from microbial fermentations to crops. However, yardsticks to assess what drives enzyme protein replacement and guidelines on how to reduce it are lacking. Accordingly, we compared enzymes’ life spans across kingdoms using a new yardstick (catalytic cycles until replacement [CCR]) and related CCR to enzyme reaction chemistry. We concluded that 1) many enzymes fail due to collateral damage from the reaction they catalyze, and 2) such damage and its attendant enzyme replacement costs are mitigable by engineering and are therefore promising targets for synthetic biology.

Metabolic engineering uses enzymes as parts to build biosystems for specified tasks. Although a part’s working life and failure modes are key engineering performance indicators, this is not yet so in metabolic engineering because it is not known how long enzymes remain functional in vivo or whether cumulative deterioration (wear-out), sudden random failure, or other causes drive replacement. Consequently, enzymes cannot be engineered to extend life and cut the high energy costs of replacement. Guided by catalyst engineering, we adopted catalytic cycles until replacement (CCR) as a metric for enzyme functional life span in vivo. CCR is the number of catalytic cycles that an enzyme mediates in vivo before failure or replacement, i.e., metabolic flux rate/protein turnover rate. We used estimated fluxes and measured protein turnover rates to calculate CCRs for ∼100–200 enzymes each from Lactococcus lactis, yeast, and Arabidopsis. CCRs in these organisms had similar ranges (<10³ to >10⁷) but different median values (3–4 × 10⁴ in L. lactis and yeast versus 4 × 10⁵ in Arabidopsis). In all organisms, enzymes whose substrates, products, or mechanisms can attack reactive amino acid residues had significantly lower median CCR values than other enzymes. Taken with literature on mechanism-based inactivation, the latter finding supports the proposal that 1) random active-site damage by reaction chemistry is an important cause of enzyme failure, and 2) reactive noncatalytic residues in the active-site region are likely contributors to damage susceptibility. Enzyme engineering to raise CCRs and lower replacement costs may thus be both beneficial and feasible.

Chemoproteomic Profiling of Cobalamin-Independent Methionine Synthases in Plants with a Covalent Probe

Cobalamin-independent methionine synthases (MS) are zinc-binding methyltransferases that catalyze de novo methionine biosynthesis in higher plants, which are enzymes critically involved in seed germination and plant growth. Here, we report a highly selective sulfonyl fluoride-based probe for chemoproteomic profiling of MS enzymes in living systems of the model plant Arabidopsis thaliana, as implemented in in-gel-, mass spectrometry-, and imaging-based platforms. This probe holds promise for facilitating and accelerating fundamental research and industrial application of MS enzymes, particularly in the contexts of MS1/2-targeting herbicide screening and design.

Differently Tagged Probes for Protein Profiling of Mitochondria

The mitochondrion is one of the most important organelles in the eukaryotic cell. Characterization of the mitochondrial proteome is a prerequisite for understanding its cellular functions at the molecular level. Here we report a proteomics method based on mitochondrion-targeting groups and click chemistry. In our strategy, three different mitochondrion-targeting moieties were each augmented with a clickable handle and a cysteine-reactive group. Fluorescence-based bioimaging and fractionation experiments clearly showed that most signals arising from the labels were localized in the mitochondria of cells, as a result of covalent attachment between probe and target proteins. The three probes had distinct profiling characteristics. Furthermore, we successfully identified more than two hundred mitochondrial proteins. The results showed that different mitochondrion-targeting groups targeted distinct proteins with partial overlap. Most of the labeled proteins were localized in the mitochondrial matrix and inner mitochondrial membrane. Our results provide a tool for chemoproteomic analysis of mitochondrion-related proteins.

Current advances of carbene-mediated photoaffinity labeling in medicinal chemistry

Photoaffinity labeling (PAL) in combination with a chemical probe to covalently bind its target upon UV irradiation has demonstrated considerable promise in drug discovery for identifying new drug targets and binding sites. In particular, carbene-mediated photoaffinity labeling (cmPAL) has been widely used in drug target identification owing to its excellent photolabeling efficiency, minimal steric interference and longer excitation wavelength. Specifically, diazirines, which are among the precursors of carbenes and have higher carbene yields and greater chemical stability than diazo compounds, have proved to be valuable photolabile reagents in a diverse range of biological systems. This review highlights current advances of cmPAL in medicinal chemistry, with a focus on structures and applications for identifying small molecule-protein and macromolecule-protein interactions and ligand-gated ion channels, coupled with advances in the discovery of targets and inhibitors using carbene precursor-based biological probes developed in recent decades.

Chemical Glycoproteomics

Chemical tools have accelerated progress in glycoscience, reducing experimental barriers to studying protein glycosylation, the most widespread and complex form of posttranslational modification. For example, chemical glycoproteomics technologies have enabled the identification of specific glycosylation sites and glycan structures that modulate protein function in a number of biological processes. This field is now entering a stage of logarithmic growth, during which chemical innovations combined with mass spectrometry advances could make it possible to fully characterize the human glycoproteome. In this review, we describe the important role that chemical glycoproteomics methods are playing in such efforts. We summarize developments in four key areas: enrichment of glycoproteins and glycopeptides from complex mixtures, emphasizing methods that exploit unique chemical properties of glycans or introduce unnatural functional groups through metabolic labeling and chemoenzymatic tagging; identification of sites of protein glycosylation; targeted glycoproteomics; and functional glycoproteomics, with a focus on probing interactions between glycoproteins and glycan-binding proteins. Our goal with this survey is to provide a foundation on which continued technological advancements can be made to promote further explorations of protein glycosylation.

Arylfluorosulfates Inactivate Intracellular Lipid Binding Protein(s) through Chemoselective SuFEx Reaction with a Binding Site Tyr Residue

Arylfluorosulfates have appeared only rarely in the literature and have not been explored as probes for covalent conjugation to proteins, possibly because they were assumed to possess high reactivity, as with other sulfur(VI) halides. However, we find that arylfluorosulfates become reactive only under certain circumstances, e.g., when fluoride displacement by a nucleophile is facilitated. Herein, we explore the reactivity of structurally simple arylfluorosulfates toward the proteome of human cells. We demonstrate that the protein reactivity of arylfluorosulfates is lower than that of the corresponding aryl sulfonyl fluorides, which are better characterized with regard to proteome reactivity. We discovered that simple hydrophobic arylfluorosulfates selectively react with a few members of the intracellular lipid binding protein (iLBP) family. A central function of iLBPs is to deliver small-molecule ligands to nuclear hormone receptors. Arylfluorosulfate probe 1 reacts with a conserved tyrosine residue in the ligand-binding site of a subset of iLBPs. Arylfluorosulfate probes 3 and 4, featuring a biphenyl core, very selectively and efficiently modify cellular retinoic acid binding protein 2 (CRABP2), both in vitro and in living cells. The X-ray crystal structure of the CRABP2-4 conjugate, when considered together with binding site mutagenesis experiments, provides insight into how CRABP2 might activate arylfluorosulfates toward site-specific reaction. Treatment of breast cancer cells with probe 4 attenuates nuclear hormone receptor activity mediated by retinoic acid, an endogenous client lipid of CRABP2. Our findings demonstrate that arylfluorosulfates can selectively target single iLBPs, making them useful for understanding iLBP function.

Two-Photon Small Molecule Enzymatic Probes

Enzymes are essential for life, especially in the development of disease and on drug effects, but as we cannot yet directly observe the inside interactions and only partially observe biochemical outcomes, tools "translating" these processes into readable information are essential for better understanding of enzymes as well as for developing effective tools to fight against diseases. Therefore, sensitive small molecule probes suitable for direct in vivo monitoring of enzyme activities are ultimately desirable. For fulfilling this desire, two-photon small molecule enzymatic probes (TSMEPs) producing amplified fluorescent signals based on enzymatic conversion with better photophysical properties and deeper penetration in intact tissues and whole animals have been developed and demonstrated to be powerful in addressing the issues described above. Nonetheless, currently available TSMEPs only cover a small portion of enzymes despite the distinct advantages of two-photon fluorescence microscopy. In this Account, we would like to share design principles for TSMEPs as potential indicators of certain pathology-related biomarkers together with their applications in disease models to inspire more elegant work to be done in this area. Highlights will be addressed on how to equip two-photon fluorescent probes with features amenable for direct assessment of enzyme activities in complex pathological environments. We give three recent examples from our laboratory and collaborations in which TSMEPs are applied to visualize the distribution and activity of enzymes at cellular and organism levels. The first example shows that we could distinguish endogenous phosphatase activity in different organelles; the second illustrates that TSMEP is suitable for specific and sensitive detection of a potential Parkinson's disease marker (monoamine oxidase B) in a variety of biological systems from cells to patient samples, and the third identifies that TSMEPs can be applied to other enzyme families (proteases). Indeed, TSMEPs have helped to uncover new biological roles and functions of a series of enzymes; therefore, we hope to encourage more TSMEPs to be developed for diverse enzymes. Meanwhile, improvements in the TSMEP properties (such as new two-photon fluorophores with longer excitation and emission wavelengths and strategies allowing high specificity) are also indispensable for producing high-fidelity information inside biological systems. We are enthusiastic however that, with these efforts and wider applications of TSMEPs in both research studies and further clinical diagnoses, comprehensive knowledge of enzyme contributions to various physiologies will be obtained.

Target identification of covalently binding drugs by activity-based protein profiling (ABPP)

The characterization of the target proteins of drug molecules has become an important goal in understanding its mode of action and origin of side effects due to off-target binding. This is especially important for covalently binding drugs usually containing electrophilic moieties, which potentially can react with nucleophilic residues found in many proteins. This review gives a comprehensive overview of the use of activity-based protein profiling (ABPP) as an efficient tool for the target identification of covalently binding drugs.

Activity profiling reveals changes in the diversity and activity of proteins in Arabidopsis roots in response to nematode infection

Cyst nematodes are obligate, sedentary endoparasites with a highly specialised biology and a huge economic impact in agriculture. Successful parasitism involves morphological and physiological modifications of the host cells which lead to the formation of specialised syncytial feeding structures in roots. The development of the syncytium is aided by a cocktail of nematode effectors that manipulate the host plant activities in a complex network of interactions through post-translational modifications. Traditional transcriptomic and proteomic approaches cannot display this functional proteomic information. Activity-based protein profiling (ABPP) is a powerful technology that can be used to investigate the activity of the proteome through activity-based probes. To better understand the functional proteomics of syncytium, ABPP was conducted on syncytia induced by the beet cyst nematode Heterodera schachtii in Arabidopsis roots. Our results demonstrated that the activity of several enzymes is differentially regulated in the syncytium compared to the control roots. Among those specifically activated in the syncytium are a putative S-formyl-glutathione hydrolase (SFGH), a putative methylesterase (MES) and two unidentified enzymes. In contrast, the activities of vacuolar processing enzymes (VPEs) are specifically suppressed in the syncytium. Competition labelling, quantitative gene expression and T-DNA knock-out mutants were used to further characterise the roles of the differentially regulated enzymes during plant-nematode interaction. In conclusion, our study will open the door to generate a comprehensive and integrated view of the host-pathogen warfare that results in the formation of long-term feeding sites for pathogens.

Small Molecule Active Site Directed Tools for Studying Human Caspases

Caspases are proteases of clan CD and were described for the first time more than two decades ago. They play critical roles in the control of regulated cell death pathways including apoptosis and inflammation. Due to their involvement in the development of various diseases like cancer, neurodegenerative diseases, or autoimmune disorders, caspases have been intensively investigated as potential drug targets, both in academic and industrial laboratories. This review presents a thorough, deep, and systematic assessment of all technologies developed over the years for the investigation of caspase activity and specificity using substrates and inhibitors, as well as activity based probes, which in recent years have attracted considerable interest due to their usefulness in the investigation of biological functions of this family of enzymes.

Activity-based probes as molecular tools for biomarker discovery

Activity-based protein profiling has emerged as an exceptional tool for biomarker discovery and validation.

A small azide-modified thiazole-based reporter molecule for fluorescence and mass spectrometric detection

Molecular probes are widely used tools in chemical biology that allow tracing of bioactive metabolites and selective labeling of proteins and other biomacromolecules. A common structural motif for such probes consists of a reporter that can be attached by copper(I)-catalyzed 1,2,3-triazole formation between terminal alkynes and azides to a reactive headgroup. Here we introduce the synthesis and application of the new thiazole-based, azide-tagged reporter 4-(3-azidopropoxy)-5-(4-bromophenyl)-2-(pyridin-2-yl)thiazole for fluorescence, UV and mass spectrometry (MS) detection. This small fluorescent reporter bears a bromine functionalization facilitating the automated data mining of electrospray ionization MS runs by monitoring for its characteristic isotope signature. We demonstrate the universal utility of the reporter for the detection of an alkyne-modified small molecule by LC–MS and for the visualization of a model protein by in-gel fluorescence. The novel probe advantageously compares with commercially available azide-modified fluorophores and a brominated one. The ease of synthesis, small size, stability, and the universal detection possibilities make it an ideal reporter for activity-based protein profiling and functional metabolic profiling.

Cellular biochemistry methods for investigating protein tyrosine phosphatases

SIGNIFICANCE

The protein tyrosine phosphatases (PTPs) are a family of proteins that play critical roles in cellular signaling and influence many aspects of human health and disease. Although a wealth of information has been collected about PTPs since their discovery, many questions regarding their regulation and function still remain.

CRITICAL ISSUES

Of particular importance are the elucidation of the biological substrates of individual PTPs and understanding of the chemical and biological basis for temporal and spatial resolution of PTP activity within a cell.

RECENT ADVANCES

Drawing from recent advances in both biology and chemistry, innovative approaches have been developed to study the intracellular biochemistry and physiology of PTPs. We provide a summary of PTP-tailored techniques and approaches, emphasizing methodologies to study PTP activity within a cellular context. We first provide a discussion of methods for identifying PTP substrates, including substrate-trapping mutants and synthetic peptide libraries for substrate selectivity profiling. We next provide an overview of approaches for monitoring intracellular PTP activity, including a discussion of mechanistic-based probes, gel-based assays, substrates that can be used intracellularly, and assays tied to cell growth. Finally, we review approaches used for monitoring PTP oxidation, a key regulatory pathway for these enzymes, discussing the biotin switch method and variants of this approach, along with affinity trapping techniques and probes designed to detect PTP oxidation.

FUTURE DIRECTIONS

Further development of approaches to investigate the intracellular PTP activity and functions will provide specific insight into their mechanisms of action and control of diverse signaling pathways.

Small molecule probe suitable for in situ profiling and inhibition of protein disulfide isomerase

Proper folding of cellular proteins is assisted by protein disulfide isomerases (PDIs) in the endoplasmic reticulum of mammalian cells. Of the at least 21 PDI family members known in humans, the 57-kDa PDI has been found to be a potential therapeutic target for a variety of human diseases including cancer and neurodegenerative diseases. Consequently, small molecule PDI-targeting inhibitors have been actively pursued in recent years, and thus far, compounds possessing moderate inhibitory activities (IC50 between 0.1 and 100 μM against recombinant PDI) have been discovered. In this article, by using in situ proteome profiling experiments in combination with in vitro PDI enzymatic inhibition assays, we have discovered a phenyl vinyl sulfonate-containing small molecule (P1; shown) as a relatively potent and specific inhibitor of endogenous human PDI in several mammalian cancer cells (e.g., GI50 ∼ 4 μM). It also possesses an IC50 value of 1.7 ± 0.4 μM in an in vitro insulin aggregation assay. Our results indicate P1 is indeed a novel, cell-permeable small molecule PDI inhibitor, and the electrophilic vinyl sulfonate scaffold might serve as a starting point for future development of next-generation PDI inhibitors and probes.

Single-stranded DNA as a cleavable linker for bioorthogonal click chemistry-based proteomics

In this communication, we report a new class of cleavable linker based on automatically synthesized, single-stranded DNAs. We incorporated a DNA oligo into an azide-functionalized biotin (biotin-DNA-N3) and used the probe to enrich for alkyne-tagged glycoproteins from mammalian cell lysates. Highly efficient and selective release of the captured proteins from streptavidin agarose resins was achieved using DNase treatment under very mild conditions. A total of 36 sialylated glycoproteins were identified from the lysates of HL60 cells, an acute human promyeloid leukemia cell line. These sialylated glycoproteins were involved in many different biological processes ranging from glycan biosynthesis to cell adhesion events.

Chemistry-based functional proteomics for drug target deconvolution

Drug target deconvolution, a process that identifies targets to small molecules in complex biological samples, which underlie the biological responses that are observed when a drug is administered, plays an important role in current drug discovery. Despite the fact that genomics and proteomics have provided a flood of information that contributes to the progress of drug target identification and validation, the current approach to drug target deconvolution still poses dilemmas. Chemistry-based functional proteomics, a multidisciplinary strategy, has become the preferred method of choice to deconvolute drug target pools, based on direct interactions between small molecules and their protein targets. This approach has already identified a broad panel of previously undefined enzymes with potential as drug targets and defined targets that can rationalize side effects and toxicity for new drug candidates and existing therapeutics. Herein, the authors discuss both activity-based protein profiling and compound-centric chemical proteomics approaches used in chemistry-based functional proteomics and their applications for the identification and characterization of small molecular targets.

Chemical proteomics and its impact on the drug discovery process

Despite the rapid growth of postgenomic data and fast-paced technology advancement, drug discovery is still a lengthy and difficult process. More effective drug design requires a better understanding of the interaction between drug candidates and their targets/off-targets in various situations. The ability of chemical proteomics to integrate a multiplicity of disciplines enables the direct analysis of protein activities on a proteome-wide scale, which has enormous potential to facilitate drug target elucidation and lead drug verification. Over recent years, chemical proteomics has experienced rapid growth and provided a valuable method for drug target identification and inhibitor discovery. This review introduces basic concepts and technologies of different popular chemical proteomic approaches. It also covers the essential features and recent advances of each approach while underscoring their potentials in drug discovery and development.

Qualitative analysis of the fluorophosphonate-based chemical probes using the serine hydrolases from mouse liver and poly-3-hydroxybutyrate depolymerase (PhaZ) from Bacillus thuringiensis

The serine hydrolase family consists of more than 200 members and is one of the largest enzyme families in the human genome. Although up to 50 % of this family remains unannotated, there are increasing evidences that activities of certain serine hydrolases are associated with diseases like cancer neoplasia, invasiveness, etc. By now, several activity-based chemical probes have been developed and are applied to profile the global activity of serine hydrolases in diverse proteomes. In this study, two fluorophosphonate (FP)-based chemical probes were synthesized. Further examination of their abilities to label and pull down serine hydrolases was conducted. In addition, the poly-3-hydroxybutyrate depolymerase (PhaZ) from Bacillus thuringiensis was demonstrated as an appropriate standard serine hydrolase, which can be applied to measure the labeling ability and pull-down efficiency of FP-based probes. Furthermore, mass spectrometry (MS) was used to identify the serine residue that covalently bonded to the active probes. Finally, these FP-based probes were shown capable of establishing the serine hydrolase profiles in diverse mouse tissues; the serine hydrolases pulled down from mouse liver organ were further identified by MS. In summary, our study provides an adequate method to evaluate the reactivity of FP-based probes targeting serine hydrolases.

Design, synthesis and biological evaluation of potent azadipeptide nitrile inhibitors and activity-based probes as promising anti-Trypanosoma brucei agents

Trypanosoma cruzi and Trypanosoma brucei are parasites that cause Chagas disease and African sleeping sickness, respectively. There is an urgent need for the development of new drugs against both diseases due to the lack of adequate cures and emerging drug resistance. One promising strategy for the discovery of small-molecule therapeutics against parasitic diseases has been to target the major cysteine proteases such as cruzain for T. cruzi, and rhodesain/TbCatB for T. brucei. Azadipeptide nitriles belong to a novel class of extremely potent cysteine protease inhibitors against papain-like proteases. We herein report the design, synthesis, and evaluation of a series of azanitrile-containing compounds, most of which were shown to potently inhibit both recombinant cruzain and rhodesain at low nanomolar/picomolar ranges. A strong correlation between the potency of rhodesain inhibition (i.e., target-based screening) and trypanocidal activity (i.e., whole-organism-based screening) of the compounds was observed. To facilitate detailed studies of this important class of inhibitors, selected hit compounds from our screenings were chemically converted into activity-based probes (ABPs), which were subsequently used for in situ proteome profiling and cellular localization studies to further elucidate potential cellular targets (on and off) in both the disease-relevant bloodstream form (BSF) and the insect-residing procyclic form (PCF) of Trypanosoma brucei. Overall, the inhibitors presented herein show great promise as a new class of anti-trypanosome agents, which possess better activities than existing drugs. The activity-based probes generated from this study could also serve as valuable tools for parasite-based proteome profiling studies, as well as bioimaging agents for studies of cellular uptake and distribution of these drug candidates. Our studies therefore provide a good starting point for further development of these azanitrile-containing compounds as potential anti-parasitic agents.

Ugi reaction-assisted rapid assembly of affinity-based probes against potential protein tyrosine phosphatases

The multi-component Ugi reaction has been employed to assemble a small library of affinity-based probes (AfBPs) that target potential protein tyrosine phosphatases. The probes showed good labelling of PTP1B and MptpB, and were subsequently used to label endogenous PTP1B in MCF-7 cell lysates.

Probing proteomes with benzophenone photoprobes

Benzophenone photoprobes represent powerful tools for chemical proteomics. Upon UV irradiation, a benzophenone photoprobe can selectively form a covalent bond with its target protein in complex protein mixtures. Thus, photoprobes can be used to profile a wide variety of proteins in complex proteomes. This chapter describes simple protocols to derivatize fluorenylmethyloxycarbonyl (Fmoc)-protected peptide-nucleic-acid adenine (PNA adenine) into a benzophenone photoprobe and its application in photolabeling its target proteins. The method as described does not require specialized equipment for probe synthesis and photolabeling. In addition, the strategy is applicable to recognition motifs other than PNA adenine, such as peptides, to profile their target proteins in complex proteomes.

Cell-based proteome profiling of potential dasatinib targets by use of affinity-based probes

Protein kinases (PKs) play an important role in the development and progression of cancer by regulating cell growth, survival, invasion, metastasis, and angiogenesis. Dasatinib (BMS-354825), a dual Src/Abl inhibitor, is a promising therapeutic agent with oral bioavailability. It has been used for the treatment of imatinib-resistant chronic myelogenous leukemia (CML). Most kinase inhibitors, including Dasatinib, inhibit multiple cellular targets and do not possess exquisite cellular specificity. Recent efforts in kinase research thus focus on the development of large-scale, proteome-wide chemical profiling methods capable of rapid identification of potential cellular (on- and off-) targets of kinase inhibitors. Most existing approaches, however, are still problematic and in many cases not compatible with live-cell studies. In this work, we have successfully developed a cell-permeable kinase probe (DA-2) capable of proteome-wide profiling of potential cellular targets of Dasatinib. In this way, highly regulated, compartmentalized kinase-drug interactions were maintained. By comparing results obtained from different proteomic setups (live cells, cell lysates, and immobilized affinity matrix), we found DA-2 was able to identify significantly more putative kinase targets. In addition to Abl and Src family tyrosine kinases, a number of previously unknown Dasatinib targets have been identified, including several serine/threonine kinases (PCTK3, STK25, eIF-2A, PIM-3, PKA C-α, and PKN2). They were further validated by pull-down/immunoblotting experiments as well as kinase inhibition assays. Further studies are needed to better understand the exact relevance of Dasatinib and its pharmacological effects in relation to these newly identified cellular targets. The approach developed herein should be amenable to the study of many of the existing reversible drugs/drug candidates.

Current chemical biology tools for studying protein phosphorylation and dephosphorylation

Amongst different posttranslational events involved in cellular-signaling pathways, phosphorylation and dephosphorylation of proteins are the most prevalent. Aberrant regulations in the cellular phosphoproteome network are implicated in most major human diseases. Consequently, kinases and phosphatases are two of the most important groups of drug targets in medicinal research today. A major challenge in the understanding of protein phosphorylation and dephosphorylation is the sheer complexity of the phosphoproteome network and the lack of tools capable of studying protein phosphorylation and dephosphorylation as they occur in cells. We highlight herein various chemical biology tools that have emerged in the last decade for such studies. First, we discuss the use of small-molecule mimics of phosphoamino acids and their use in elucidating the function of protein phosphorylation and dephosphorylation. We also introduce recent advances in the field of activity-based protein profiling (ABPP) for proteome-wide detection of protein phosphorylation and dephosphorylation. We next discuss the key concepts in the design of peptide- and protein-based biosensors capable of real-time reporting of phosphorylation/dephosphorylation events. Finally, we highlight the application of peptide and small-molecule microarrays (SMMs), and their applications in high-throughput screening and discovery of new compounds related to phosphorylation/dephosphorylation.

Multicolor, one- and two-photon imaging of enzymatic activities in live cells with fluorescently Quenched Activity-Based Probes (qABPs)

Fluorescence imaging provides an indispensable way to locate and monitor biological targets within complex and dynamic intracellular environments. Of the various imaging agents currently available, small molecule-based probes provide a powerful tool for live cell imaging, primarily due to their desirable properties, including cell permeability (as a result of their smaller sizes), chemical tractability (e.g., different molecular structures/designs can be installed), and amenability to imaging a wide variety of biological events. With a few exceptions, most existing small molecule probes are however not suitable for in vivo bioimaging experiments in which high-resolution studies of enzyme activity and localization are necessary. In this article, we reported a new class of fluorescently Quenched Activity-Based Probes (qABPs) which are highly modular, and can sensitively image (through multiple enzyme turnovers leading to fluorescence signal amplification) different types of enzyme activities in live mammalian cells with good spatial and temporal resolution. We have also incorporated two-photon dyes into our modular probe design, enabling for the first time activity-based, fluorogenic two-photon imaging of enzyme activities. This, hence, expands the repertoire of 'smart', responsive probes currently available for live cell bioimaging experiments.

Design and synthesis of ATP-based nucleotide analogues and profiling of nucleotide-binding proteins

Two nucleotide-based probes were designed and synthesized in order to enrich samples for specific classes of proteins by affinity-based protein profiling. We focused on the profiling of adenine nucleotide-binding proteins. Two properties were considered in the design of the probes: the bait needs to bind adenine nucleotide-binding proteins with high affinity and carry a second functional group suitable and easily accessible for coupling to a chromatography resin. For this purpose, we synthesized p-biotinyl amidobenzoic acid-ATP (p-BABA-ATP) and p-biotinyl aminomethylbenzoic acid-ATP (p-BAMBA-ATP). p-BABA-ATP and p-BAMBA-ATP both bind to ATP-binding cassette (ABC) proteins with at least 10-fold higher affinity than ATP. Several ABC transporters could be enriched using p-BABA-ATP or p-BAMBA-ATP.