Targeting deubiquitinases enabled by chemical synthesis of proteins

Using Förster Resonance Energy Transfer (FRET) to Understand the Ubiquitination Landscape

Förster resonance energy transfer (FRET) is a fluorescence technique that allows quantitative measurement of protein interactions, kinetics and dynamics. This review covers the use of FRET to study the structures and mechanisms of ubiquitination and related proteins. We survey FRET assays that have been developed where donor and acceptor fluorophores are placed on E1, E2 or E3 enzymes and ubiquitin (Ub) to monitor steady-state and real-time transfer of Ub through the ubiquitination cascade. Specialized FRET probes placed on Ub and Ub-like proteins have been developed to monitor Ub removal by deubiquitinating enzymes (DUBs) that result in a loss of a FRET signal upon cleavage of the FRET probes. FRET has also been used to understand conformational changes in large complexes such as multimeric E3 ligases and the proteasome, frequently using sophisticated single molecule methods. Overall, FRET is a powerful tool to help unravel the intricacies of the complex ubiquitination system.

Detection of Bacterial Neutral Ceramidase in Diabetic Foot Ulcers with an Optimized Substrate and Chemoenzymatic Probes

Ceramidases (CDases) are important in controlling skin barrier integrity by regulating ceramide composition and affording downstream signal molecules. While the functions of epidermal CDases are known, roles of neutral CDases secreted by skin-residing microbes are undefined. Here, we developed a one-step fluorogenic substrate, S-B, for specific detection of bacterial CDase activity and inhibitor screening. We identified a non-hydrolyzable substrate mimic, C6, as the best hit. Based on C6, we designed a photoaffinity probe, JX-1, which efficiently detects bacterial CDases. Using JX-1, we identified endogenous low-abundance PaCDase in a P. aeruginosa monoculture and in a mixed skin bacteria culture. Harnessing both S-B and JX-1, we found that CDase activity positively correlates with the relative abundance of P. aeruginosa and is negatively associated with wound area reduction in clinical diabetic foot ulcer patient samples. Overall, our study demonstrates that bacterial CDases are important regulators of skin ceramides and potentially play a role in wound healing.

Genetic Encoding of a Fluorescent Noncanonical Amino Acid as a FRET Donor for the Analysis of Deubiquitinase Activities

The genetic code expansion technology enables the genetic encoding of fluorescent noncanonical amino acids (ncAAs) for site-specific fluorescent labeling of proteins. These co-translational and internal fluorescent tags have been harnessed to establish genetically encoded Förster resonance energy transfer (FRET) probes for studying protein structural changes and interactions. Here, we describe the protocols for site-specific incorporation of an aminocoumarin-derived fluorescent ncAA into proteins in E. coli and preparation of a fluorescent ncAA-based FRET probe for assaying the activities of deubiquitinases, a key class of enzymes involved in ubiquitination. We also describe the deployment of an in vitro fluorescence assay to screen and analyze small-molecule inhibitors against deubiquitinases.

Genetically encoded fluorescent unnatural amino acids and FRET probes for detecting deubiquitinase activities

Herein we present the genetic encoding of 7-aminocoumarin-based lysine derivatives, ACouK and AFCouK, into proteins in both bacterial and mammalian cells and the characterization of FRET pairs comprising ACouK or AFCouK as the donor and GFP as the acceptor. We further report the application of the FRET pairs to construct fully genetically encoded ratiometric probes for detecting deubiquitinases and screening for inhibitors.

UCH-L3 structure and function: Insights about a promising drug target

In the past few years, researchers have shed light on the immense importance of ubiquitin in numerous regulatory pathways. The post-translational addition of mono or poly-ubiquitin molecules namely "ubiquitinoylation" is therefore pivotal to maintain the cell's vitality, maturation, differentiation, and division. Part of conserving homeostasis stems from maintaining the ubiquitin pool in the vicinity of the cell's intracellular environment; this crucial role is played by deubiquitylating enzymes (DUBs) that cleave ubiquitin molecules from target molecules. To date, they are categorized into 7 families with ubiquitin carboxyl c-terminal de-hydrolase family (UCH) as the most common and well-studied. Ubiquitin C-terminal hydrolase L (UCH-L3) is a significant protein in this family as it has been implicated in many molecular and cellular processes with its mRNA identified in a range of body tissues including the brain. It goes without saying that it manifests in maintaining health and when abnormally regulated in disease. As it is an attractive small molecule drug target, scientists have used high throughput screening (HTS) and other drug discovery methods to discover inhibitors for this enzyme for the treatment of cancer and neurodegenerative diseases. In this review we present an overview of UCH-L3 catalytic mechanism, structure, its role in DNA repair and cancer along with the inhibitors discovered so far to halt its activity.

Synthesis and Evaluation of Ubiquitin-Dioxetane Conjugate as a Chemiluminescent Probe for Monitoring Deubiquitinase Activity

The removal of ubiquitin (Ub) from a modified protein or Ub chain is a process that occurs regularly by the ubiquitin-proteasome system. This process is known to be mediated by various deubiquitinating enzymes (DUBs) in order to control the protein's half-life and its expression levels among many other signaling processes. Since the function of DUBs is also involved in numerous human diseases, such as cancer, there is an obvious need for an effective diagnostic probe that can monitor the activity of these enzymes. We have developed the first chemiluminescence probe for detection of DUBs activity. The probe was prepared by conjugation of the chemically synthesized C-terminally activated Ub(1-75) with a Gly-enolether precursor. Subsequent oxidation, under aqueous conditions, of the enolether conjuagate with singlet-oxygen furnished the dioxetane probe Ub-CL. This synthesis provides the first example of a dioxetane-luminophore protein conjugate. The probe's ability to detect deubiquitinating activity was successfully validated with three different DUBs. In order to demonstrate the advantage of our new probe, comparison measurements for detection of DUB UCH-L3 activity were performed between the chemiluminescent probe Ub-CL and the well-known Ub-AMC probe. The obtained data showed significantly higher S/N, for probe Ub-CL (>93-fold) in comparison to that observed for Ub-AMC (1.5-fold). We anticipate that the successful design and synthesis of the turn-ON protein-dioxetane conjugate probe, demonstrated in this work, will provide the insight and motivation for preparation of other relevant protein-dioxetane conjugates.

Traceless Click-Assisted Native Chemical Ligation Enabled by Protecting Dibenzocyclooctyne from Acid-Mediated Rearrangement with Copper(I)

The scope of proteins accessible to total chemical synthesis via native chemical ligation (NCL) is often limited by slow ligation kinetics. Here we describe Click-Assisted NCL (CAN), in which peptides are incorporated with traceless "helping hand" lysine linkers that enable addition of dibenzocyclooctyne (DBCO) and azide handles. The resulting strain-promoted alkyne-azide cycloaddition (SPAAC) increases their effective concentration to greatly accelerate ligations. We demonstrate that copper(I) protects DBCO from acid-mediated rearrangement during acidic peptide cleavage, enabling direct production of DBCO synthetic peptides. Excitingly, triazole-linked model peptides ligated rapidly and accumulated little side product due to the fast reaction time. Using the E. coli ribosomal subunit L32 as a model protein, we further demonstrate that SPAAC, ligation, desulfurization, and linker cleavage steps can be performed in one pot. CAN is a useful method for overcoming challenging ligations involving sterically hindered junctions. Additionally, CAN is anticipated to be an important stepping stone toward a multisegment, one-pot, templated ligation system.

Advances in the Development Ubiquitin-Specific Peptidase (USP) Inhibitors

Ubiquitylation and deubiquitylation are reversible protein post-translational modification (PTM) processes involving the regulation of protein degradation under physiological conditions. Loss of balance in this regulatory system can lead to a wide range of diseases, such as cancer and inflammation. As the main members of the deubiquitinases (DUBs) family, ubiquitin-specific peptidases (USPs) are closely related to biological processes through a variety of molecular signaling pathways, including DNA damage repair, p53 and transforming growth factor-β (TGF-β) pathways. Over the past decade, increasing attention has been drawn to USPs as potential targets for the development of therapeutics across diverse therapeutic areas. In this review, we summarize the crucial roles of USPs in different signaling pathways and focus on advances in the development of USP inhibitors, as well as the methods of screening and identifying USP inhibitors.

Targeting homologous recombination (HR) repair mechanism for cancer treatment: discovery of new potential UCHL-3 inhibitors via virtual screening, molecular dynamics and binding mode analysis

UCHL3 (ubiquitin C-terminal hydrolase-L3) is a de-ubiquitinating enzyme involved in the homologous recombination repair mechanism of double-strand breaks (DBS) of the DNA. Multiple studies indicated that UCHL3 inhibitors could be used in combination therapy with high therapeutic efficacy against cancer thus highlighting the validity of directing research against UCHL3 as a druggable target in oncology. In this study, a combination of virtual screening methods was utilized to identify new potential UCHL3 inhibitors. A series of UCHL3 ligands were identified by applying a combination of cheminformatics and molecular modeling filtration techniques to a ChemBl database of over two million small molecules viz. Lipinski's Rule of Five, Veber's rule, pharmacophore model, Hierarchical molecular docking, Pan-assay Interference Compounds (PAINS) alerts, toxicity filter, and single-point Molecular mechanics Poisson/Boltzmann surface area (MM/PBSA) docking pose rescoring. This multi-layer filtration strategy led to the identification of twenty-one compounds as potential UCHL3 inhibitors that were subsequently subjected to a 50 ns molecular dynamics (MD) simulations predict the stability of their ligand-protein complexes. Furthermore, MM/PBSA calculations based on MD trajectories were performed, and the energy contribution per residue to the binding energy was calculated. Three compounds, 1, 2 and 3, were finally recognized as having the highest potential of being UCHL3 inhibitors. Therefore, those were used for binding mode analysis to the UCHL3 active site, leading to identification of four residues as key for binding viz. Pro8, Leu55, Val166, and Leu168.Communicated by Ramaswamy H. Sarma.

Fluorescence resonance energy transfer (FRET)-based nanoarchitecture for monitoring deubiquitinating enzyme activity

A novel nanoarchitecture (MSN-Tb-UbR) was prepared by modifying rhodamine B-labelled Ubs (Ub-Rs) on the surface of mesoporous silica nanoparticles (MSNs) loaded with Tb³⁺-complexes. The MSN-Tb-UbR exhibits ratiometric sensing ability for DUB (UCH-L1) with good sensitivity and selectivity.

The Journey for the Total Chemical Synthesis of a 53 kDa Protein

Chemical protein synthesis has been proved as an efficient way to afford medium-sized proteins with high homogeneity in workable quantities for various biochemical, structural, and functional studies. In particular, chemical protein synthesis has enabled access to proteins that are difficult or impossible to prepare by molecular biology approaches, such as those with post-translational modifications and mirror-image proteins. One prominent example is related to ubiquitination, a well-known modification that mediates a variety of cellular processes (e.g., proteasomal degradation). Ubiquitination is considered as a modification that is difficult to introduce into proteins in a test tube to generate ubiquitin (Ub) conjugates with high homogeneity with respect to the chain length and the anchored Lys residue in workable quantities to perform the biochemical and biophysical studies. Chemical protein synthesis has emerged as a powerful approach to prepare Ub conjugates for studies aiming to understand ubiquitination in great detail and decipher its roles in cell processes. Nevertheless, in order to answer more challenging questions in this field, it has been clear that researchers must also prepare Ub conjugates with increased size and complexity. Employing solid-phase peptide synthesis and chemoselective ligation, chemical protein synthesis offers a powerful way to furnish polypeptides composed of 100-200 residues. However, to synthesize larger proteins such as Ub conjugates, longer and more segments are required. This on the other hand leads to difficulties related to solubility, purification, ligation, and late-stage modifications. These challenges have encouraged us to explore more practical synthetic tools to facilitate the synthesis of complex Ub conjugates. In this Account, we summarize the synthetic tools that we have developed to achieve these goals. These include (1) δ-mercaptolysine-mediated isopeptide chemical ligation, (2) chemical synthesis of Ub building blocks, (3) palladium-mediated deprotection of key side chains during protein synthesis, (4) one-pot ligation and desulfurization, and (5) improving the solubility of peptide segments. The developed chemical toolbox has been a key for our successes in the synthesis of diverse and complex Ub conjugates. In this Account, we describe our approaches for generating various Ub conjugates, including (1) the K48 tetra-Ub chain composed of 304 amino acids, (2) the ubiquitinated histones and their analogues made of >200 amino acids, (3) the di-Ub-SUMO-2 hybrid chain composed of 245 amino acids, and (4) the 53 kDa tetra-Ub-α-globin composed of 472 amino acids, which represents the largest protein composed of natural amino acids ever made using chemical protein synthesis. The last target, Flag-Ub-Ub-Ub-Myc-Ub-(HA-α-globin), was prepared in the labeled form where the proximal Ub and distal Ub in the chain were labeled with Myc and Flag tags, respectively, while the α-globin was labeled with the HA tag. Applying the tetra-Ub-α-globin in proteasomal degradation studies assisted us to shed light on the proteolytic signal and the fates of the Ub moieties in the chains. Although these developments have contributed to the synthesis of interesting and challenging targets related to Ub signaling, several other targets may enforce new synthetic challenges. Hence, there is still a need to optimize the current synthetic tools and explore novel synthetic approaches to facilitate this process.

Palladium-Mediated Cleavage of Proteins with Thiazolidine-Modified Backbone in Live Cells

Chemical protein synthesis and biorthogonal modification chemistries allow production of unique proteins for a range of biological studies. Bond-forming reactions for site-selective protein labeling are commonly used in these endeavors. Selective bond-cleavage reactions, however, are much less explored and still pose a great challenge. In addition, most of studies with modified proteins prepared by either total synthesis or semisynthesis have been applied mainly for in vitro experiments with very limited extension to live cells. Reported here is an approach for studying uniquely modified proteins containing a traceless cell delivery unit and palladium-based cleavable element for chemical activation, and monitoring the effect of these proteins in live cells. This approach is demonstrated for the synthesis of a caged ubiquitin-aldehyde, which was decaged for the inhibition of deubiquitinases in live cells.

Direct measurement of deubiquitinating enzyme activity in intact cells using a protease-resistant, cell-permeable, peptide-based reporter

Deubiquitinating enzymes (DUBs) regulate the removal of the polyubiquitin chain from proteins targeted for degradation. Current approaches to quantify DUB activity are limited to test tube-based assays that incorporate enzymes or cell lysates, but not intact cells. The goal of this work was to develop a novel peptide-based biosensor of DUB activity that is cell permeable, protease-resilient, fluorescent, and specific to DUBs. The biosensor consists of an N-terminal β-hairpin motif that acts as both a 'protectide' to increase intracellular stability and a cell penetrating peptide (CPP) to facilitate the uptake into intact cells. The β-hairpin was conjugated to a C-terminal substrate consisting of the last four amino acids in ubiquitin (LRGG) to facilitate DUB mediated cleavage of a C-terminal fluorophore (AFC). The kinetics of the peptide reporter were characterized in cell lysates by dose response and inhibition enzymology studies. Inhibition studies with an established DUB inhibitor (PR-619) confirmed the specificity of both reporters to DUBs. Fluorometry and fluorescent microscopy experiments followed by mathematical modeling established the capability of the biosensor to measure DUB activity in intact cells while maintaining cellular integrity. The novel reporter introduced here is compatible with high-throughput single cell analysis platforms such as FACS and droplet microfluidics facilitating direct quantification of DUB activity in single intact cells with direct application in point-of-care cancer diagnostics and drug discovery.

Targeting the signalling pathways regulated by deubiquitinases for prostate cancer therapeutics

Prostate cancer (PCa) is the most common cancer diagnosed and the second most common cause of cancer-related death in men worldwide. The current androgen deprivation therapy for PCa cannot fully cure this disease. Moreover, androgen receptor gene amplification and mutation are associated with PCa to develop castration-resistant prostate cancer (CRPC). This review focuses on the deubiquitinases (DUBs) involved in PCa development and progression. For PCa development and progression, several cellular pathways are regulated by specific DUBs which are also highlighted in here. The ubiquitin-specific proteases (USPs), a family member of DUBs mostly involved in the regulation of cellular pathways for PCa development, and the ubiquitin C-terminal hydrolases (UCHs), another family member of DUBs, are responsible for PCa metastasis. Small molecular inhibitors against DUBs can inhibit or reduce the level of specific DUBs through the regulation of cellular pathway to treat this disease. Some small molecular inhibitors are already identified against some of the DUBs, but very few of them are clinically proved in PCa. So, to find out other DUBs involving in the regulation of PCa-related pathways and to develop more effective small molecule inhibitors with greater potency would be a great idea to target PCa cells for future therapeutics and drug development with or without the combination of other anticancer drugs. SIGNIFICANCE OF THE STUDY: This review is targeting DUB proteins which are responsible for PCa induction, proliferation, and metastasis by highlighting their signalling pathway so that the readers can get information about other mechanisms for PCa besides androgen receptor pathway and helps to find other oncogenic DUBs involving in these signalling pathways. This review also hopes to find other oncogenic DUBs involving in PCa-related signalling pathways or to find the DUBs that can regulate multiple oncogenic signalling pathways which might be a good target for PCa therapeutics. In addition, there are some small molecule inhibitors that can inhibit the oncogenic DUBs and thus able to control the oncogenic pathways which would be a novel strategy to treat CRPC by using DUB inhibitor combined with or without other anticancer drugs.

Halogen Substituents in the Isoquinoline Scaffold Switches the Selectivity of Inhibition between USP2 and USP7

Deubiquitinases are important components of the protein regulatory network and, hence, constitute a tempting drug target. We report herein structure-activity relationship studies to develop halogen-substituted isoquionoline-1,3-dione-based inhibitors of the deubiquitinase USP2. In contrast to our previous reports, the best compound discovered was found to act through a reactive oxygen species independent, uncompetitive mechanism with an IC₅₀ of 250 nm. We show the crucial role of halogens in the common scaffold to provide potency and selectivity of our compound, where the introduction of the fluorine atom completely switches the selectivity of the inhibitor between USP2 and USP7. Our cellular studies highlight the potential applicability of the reported compound for in vivo experiments. The discovery of the isoquinoline-1,3-dione core and the knowledge obtained with regard to halogen substituents provide a platform towards understanding USP2 inhibition and the development of highly selective next-generation deubiquitinase inhibitors.

How to Target Viral and Bacterial Effector Proteins Interfering with Ubiquitin Signaling

Ubiquitination is a frequently occurring and very diverse posttranslational modification influencing a wide scope of cellular processes. Ubiquitin (Ub) has the unique ability to form eight different lysine-linked polymeric chains, mixed chains and engages with ubiquitin-like (Ubl) molecules. The distinct signals evoked by specific enzymes play a crucial role in, for instance, proteasome-mediated protein degradation, cell cycle regulation, and DNA damage responses. Due to the large variety of cellular functions that this posttranslational modification influences, the enzymes that construct such Ub modifications, and subsequently controle and degrade these signals, is enormous. In this chapter, we will discuss the current state-of-the-art of activity-based probes, reporter substrates, and other relevant tools based on Ub as recognition element, to study the enzymes involved in the complex system of ubiquitination.

Chemical and semisynthetic approaches to study and target deubiquitinases

Ubiquitination is a key posttranslational modification, which affects numerous biological processes and is reversed by a class of enzymes known as deubiquitinases (DUBs). This family of enzymes cleaves mono-ubiquitin or poly-ubiquitin chains from a target protein through different mechanisms and mode of interactions with their substrates. Studying the role of DUBs in health and diseases has been a major goal for many laboratories both in academia and in industry. However, the field has been challenged by the difficulties in obtaining native substrates and novel reagents using traditional enzymatic and molecular biology approaches. Recent advancements in the synthesis and semisynthesis of proteins made it possible to prepare several unique ubiquitin conjugates to study various aspects of DUBs such as their specificities and structures. Moreover, these approaches enable the preparation of novel activity based probes and assays to monitor DUB activities in vitro and in cellular contexts. Efforts made to bring new chemical entities for the selective inhibition of DUBs based on these tools are also highlighted with selected examples.

The roles of ubiquitin modifying enzymes in neoplastic disease

The initial experiments performed by Rose, Hershko, and Ciechanover describing the identification of a specific degradation signal in short-lived proteins paved the way to the discovery of the ubiquitin mediated regulation of numerous physiological functions required for cellular homeostasis. Since their discovery of ubiquitin and ubiquitin function over 30years ago it has become wholly apparent that ubiquitin and their respective ubiquitin modifying enzymes are key players in tumorigenesis. The human genome encodes approximately 600 putative E3 ligases and 80 deubiquitinating enzymes and in the majority of cases these enzymes exhibit specificity in sustaining either pro-tumorigenic or tumour repressive responses. In this review, we highlight the known oncogenic and tumour suppressive effects of ubiquitin modifying enzymes in cancer relevant pathways with specific focus on PI3K, MAPK, TGFβ, WNT, and YAP pathways. Moreover, we discuss the capacity of targeting DUBs as a novel anticancer therapeutic strategy.

Emerging role of DUBs in tumor metastasis and apoptosis: Therapeutic implication

Malfunction of ubiquitin-proteasome system is tightly linked to tumor formation and tumor metastasis. Targeting the ubiquitin-pathway provides a new strategy for anti-cancer therapy. Despite the parts played by ubiquitin modifiers, removal of ubiquitin from the functional proteins by the deubiquitinating enzymes (DUBs) plays an important role in governing the multiple steps of the metastatic cascade, including local invasion, dissemination, and eventual colonization of the tumor to distant organs. Both deregulated ubiquitination and deubiquitination could lead to dysregulation of various critical events and pathways such as apoptosis and epithelial-mesenchymal transition (EMT). Recent TCGA study has further revealed the connection between mutations of DUBs and various types of tumors. In addition, emerging drug design targeting DUBs provides a new strategy for anti-cancer therapy. In this review, we will summarize the role of deubiquitination and highlight the recent discoveries of DUBs with regards to multiple metastatic events including anti-apoptosis pathway and EMT. We will further discuss the regulation of deubiquitination as a novel strategy for anti-cancer therapy.

Development of a highly reliable assay for ubiquitin-specific protease 2 inhibitors

The dynamic modification of proteins with ubiquitin plays crucial roles in major celluar functions, and is associated with a number of pathological conditions. Ubiquitin-specific proteases (USPs) cleave ubiquitin from substrate proteins, and rescue them from proteasomal degradation. Among them, USP2 is overexpressed and plays important roles in various cancers including prostate cancer. Thus, it represents an attractive target for drug discovery. In order to develop potent and selective USP2 inhibitors, a highly reliable assay is needed for in-depth structure-activity relationship study. We report the cloning, expression, and purification of USP2 and UBA52, and the development of a highly reliable assay based on readily available SDS-PAGE-Coomassie systeme using UBA52 as the substrate protein. A number of effective USP2 inhibitors were also identified using this assay.

Ubiquitin Signaling: Chemistry Comes to the Rescue

Posttranslational modification of proteins by ubiquitin (Ub), i.e., ubiquitination, mediates a variety of cellular processes, including protein homeostasis, cell cycle, DNA repair, and viral infections. Understanding the molecular mechanism of ubiquitination in these events is the basis for unraveling its precise role in health and disease. However, the inherent complexity of Ub signaling due to the high atomic complexity of Ub conjugates, where Ub is attached to other Ub molecules and to protein substrates in various forms, imposes a major challenge for these studies. In this regard, the enzymatic approaches employed for the preparation of important Ub conjugates have severe limitations to deliver them in high homogeneity and in adequate amounts for the desired study. Recent developments in the area of chemical synthesis and semisynthesis of proteins offer great solutions to the enzymatic limitations and enabling the preparation of various Ub conjugates with precise control over the atomic structure. These conjugates significantly contribute to deciphering Ub signaling at the molecular level, and with the synthetic tools in hand, chemical biologists have become key players in efforts toward understanding the complexity of the Ub code. In this Perspective, we highlight the key contributions of these synthetic approaches and how the development of novel Ub-based reagents is greatly assisting in uncovering unknown aspects of Ub signaling. We also discuss future aspirations to address unresolved questions in this exciting area of research.

Entropic stabilization of a deubiquitinase provides conformational plasticity and slow unfolding kinetics beneficial for functioning on the proteasome

Human ubiquitin C-terminal hydrolyase UCH-L5 is a topologically knotted deubiquitinase that is activated upon binding to the proteasome subunit Rpn13. The length of its intrinsically disordered cross-over loop is essential for substrate recognition. Here, we showed that the catalytic domain of UCH-L5 exhibits higher equilibrium folding stability with an unfolding rate on the scale of 10⁻⁸ s⁻¹, over four orders of magnitudes slower than its paralogs, namely UCH-L1 and -L3, which have shorter cross-over loops. NMR relaxation dynamics analysis confirmed the intrinsic disorder of the cross-over loop. Hydrogen deuterium exchange analysis further revealed a positive correlation between the length of the cross-over loop and the degree of local fluctuations, despite UCH-L5 being thermodynamically and kinetically more stable than the shorter UCHs. Considering the role of UCH-L5 in removing K48-linked ubiquitin to prevent proteasomal degradation of ubiquitinated substrates, our findings offered mechanistic insights into the evolution of UCH-L5. Compared to its paralogs, it is entropically stabilized to withstand mechanical unfolding by the proteasome while maintaining structural plasticity. It can therefore accommodate a broad range of substrate geometries at the cost of unfavourable entropic loss.

The emerging role of deubiquitinating enzymes in genomic integrity, diseases, and therapeutics

The addition of mono-ubiquitin or poly-ubiquitin chain to signaling proteins in response to DNA damage signal is thought to be a critical event that facilitates the recognition of DNA damage lesion site, the activation of checkpoint function, termination and checkpoint response and the recruitment of DNA repair proteins. Despite the ubiquitin modifiers, removal of ubiquitin from the functional proteins by the deubiquitinating enzymes (DUBs) plays an important role in orchestrating DNA damage response as well as DNA repair processes. Deregulated ubiquitination and deubiquitination could lead to genome instability that in turn causes tumorigenesis. Recent TCGA study has further revealed the connection between mutations in alteration of DUBs and various types of tumors. In addition, emerging drug design based on DUBs provides a new avenue for anti-cancer therapy. In this review, we will summarize the role of deubiquitination and specificity of DUBs, and highlight the recent discoveries of DUBs in the modulation of ubiquitin-mediated DNA damage response and DNA damage repair. We will furthermore discuss the DUBs involved in the tumorigenesis as well as interception of deubiquitination as a novel strategy for anti-cancer therapy.

Understanding and predicting the potency of ROS-based enzyme inhibitors, exemplified by naphthoquinones and ubiquitin specific protease-2

Recent studies have suggested that selective targeting of overexpressed enzymes in cancer cells by small molecules that induce the formation of reactive oxygen species (ROS) could be a viable approach in cancer therapy. One such example is the inactivation of ubiquitin specific protease-2 (USP2)-an emerging drug target to combat prostate cancer-by β-lapachone, which has been identified to involve oxidation of the catalytic cysteine's thiol residue to sulfinic acid. A rational design of β-lapachone analogs with improved activity requires a much better understanding of the variables that determine ROS production by this class of molecules. This crucial aspect was addressed via modulation of its 1,2-naphthoquinone scaffold and establishment of a structure/activity relationship, regarding its ability to reduce molecular oxygen to a ROS. The same series of compounds was also examined in terms of their inhibitory effect on the enzymatic activity of USP2. One deduction from these investigations was that the ortho-quinone motif in β-lapachone is much better suited for the catalytic reduction of oxygen than the para-quinone motif and some approved quinone based drugs. A broader conclusion, obtained from the series of compounds with ortho-quinone motifs, is that only the agents whose redox potential is in the narrow range of -0.3 ± 0.1 V (vs. Ag/AgCl in pH 7.5 aqueous buffer) induce the formation of ROS. The excellent correlation between the ROS production ability and the USP2 inhibition potency emphasizes that the relatively easy, fast, and reliable testing of electrocatalytic oxygen reduction by small molecules might be applied to screening and evaluating new drug candidates for similar targets.

Dual-labeling of ubiquitin proteins by chemoselective reactions for sensing UCH-L3

A site-specific dual-color labeled ubiquitin for sensing deubiquitinase's activity was prepared by consecutively using chemoselective native chemical ligation reactions in a facile and efficient way. The prepared sensor was applied to establish a sensitive FRET-based assay for UCH-L3.

How Chemical Synthesis of Ubiquitin Conjugates Helps To Understand Ubiquitin Signal Transduction

Ubiquitin (Ub) is a small post-translational modifier protein involved in a myriad of biochemical processes including DNA damage repair, proteasomal proteolysis, and cell cycle control. Ubiquitin signaling pathways have not been completely deciphered due to the complex nature of the enzymes involved in ubiquitin conjugation and deconjugation. Hence, probes and assay reagents are important to get a better understanding of this pathway. Recently, improvements have been made in synthesis procedures of Ub derivatives. In this perspective, we explain various research reagents available and how chemical synthesis has made an important contribution to Ub research.

Development of Diubiquitin-Based FRET Probes To Quantify Ubiquitin Linkage Specificity of Deubiquitinating Enzymes

Deubiquitinating enzymes (DUBs) are proteases that fulfill crucial roles in the ubiquitin (Ub) system, by deconjugation of Ub from its targets and disassembly of polyUb chains. The specificity of a DUB towards one of the polyUb chain linkages largely determines the ultimate signaling function. We present a novel set of diubiquitin FRET probes, comprising all seven isopeptide linkages, for the absolute quantification of chain cleavage specificity of DUBs by means of Michaelis–Menten kinetics. Each probe is equipped with a FRET pair consisting of Rhodamine110 and tetramethylrhodamine to allow the fully synthetic preparation of the probes by SPPS and NCL. Our synthetic strategy includes the introduction of N,N′‐Boc‐protected 5‐carboxyrhodamine as a convenient building block in peptide chemistry. We demonstrate the value of our probes by quantifying the linkage specificities of a panel of nine DUBs in a high‐throughput manner.

Recent Advances in the Discovery of Deubiquitinating Enzyme Inhibitors

This review examines the small molecules described over the past decade as inhibitors of any of the approximately 100 human deubiquitinating enzymes (DUBs). Structures from patent publications as well as from the primary literature are included. Inhibitors of two viral DUBs are also described since these proteases share structural similarity with one of the human DUB sub-families. The structure, function and disease associations of certain DUBs are presented. The evolution of the screening assays used to identify and characterise new inhibitors is discussed. Several emerging trends in the series are highlighted and the ‘drug-likeness’ of the various inhibitors is analysed. Large pharmaceutical company collaborations have drawn attention to this field, and these recent advances are discussed in the context of the wider range of therapeutically important DUB targets.

Rapid optimization of labeled ubiquitinated peptides for monitoring deubiquitinases activities

A new synthetic approach is reported which enables the rapid synthesis of labeled-ubiquitinated peptides to facilitate optimization of deubiquitinases substrates.

Inhibiting the deubiquitinating enzymes (DUBs)

The diverse roles of deubiquitinating enzymes, or DUBs, in determining the fate of specific proteins continue to unfold. Concurrent with the revelation of DUBs as potential therapeutic targets are publications of small molecule inhibitors of these enzymes. In this review, we summarize these molecules and their associated data and suggest additional experiments to further validate and characterize these compounds. We believe the field of drug discovery against DUBs is still in its infancy, but advances in assay development, biophysical techniques, and screening libraries hold promise for identifying suitable agents that could advance into the clinic.

Harnessing the oxidation susceptibility of deubiquitinases for inhibition with small molecules

Deubiquitinases (DUBs) counteract ubiquitination by removing or trimming ubiquitin chains to alter the signal. Their diverse role in biological processes and involvement in diseases have recently attracted great interest with regard to their mechanism and inhibition. It has been shown that some DUBs are regulated by reactive oxygen species (ROS) in which the catalytic Cys residue undergoes reversible oxidation, hence modulating DUBs activity under oxidative stress. Reported herein for the first time, the observation that small molecules, which are capable of generating ROS efficiently, inhibit DUBs by selective and nonreversible oxidation of the catalytic Cys residue. Interestingly, the small molecule beta-lapachone, which is currently in clinical trials for cancer, is among the potent inhibitors, thus suggesting possible new cellular targets for its therapeutic effects. Our study describes a novel mechanism of DUBs inhibition and opens new opportunities in exploiting them for cancer therapy.

Small-molecule control of intracellular protein levels through modulation of the ubiquitin proteasome system

Traditionally, biological probes and drugs have targeted the activities of proteins (such as enzymes and receptors) that can be readily controlled by small molecules. The remaining majority of the proteome has been deemed "undruggable". By using small-molecule modulators of the ubiquitin proteasome, protein levels, rather than protein activity, can be targeted instead, thus increasing the number of druggable targets. Whereas targeting of the proteasome itself can lead to a global increase in protein levels, the targeting of other components of the UPS (e.g., the E3 ubiquitin ligases) can lead to an increase in protein levels in a more targeted fashion. Alternatively, multiple strategies for inducing protein degradation with small-molecule probes are emerging. With the ability to induce and inhibit the degradation of targeted proteins, small-molecule modulators of the UPS have the potential to significantly expand the druggable portion of the proteome beyond traditional targets, such as enzymes and receptors.

One-pot native chemical ligation of peptide hydrazides enables total synthesis of modified histones

One of the rising demands in the field of protein chemical synthesis is the development of facile strategies that yield the protein in workable quantities and homogeneity, with fewer handling steps.