Protein structure similarity clustering (PSSC) and natural product structure as inspiration sources for drug development and chemical genomics

Finding small molecules that modulate protein function is of primary importance in drug development and in the emerging field of chemical genomics. To facilitate the identification of such molecules, we developed a novel strategy making use of structural conservatism found in protein domain architecture and natural product inspired compound library design. Domains and proteins identified as being structurally similar in their ligand-sensing cores are grouped in a protein structure similarity cluster (PSSC). Natural products can be considered as evolutionary pre-validated ligands for multiple proteins and therefore natural products that are known to interact with one of the PSSC member proteins are selected as guiding structures for compound library synthesis. Application of this novel strategy for compound library design provided enhanced hit rates in small compound libraries for structurally similar proteins.     

An ultraefficient affinity-based high-throughout screening process: application to bacterial cell wall biosynthesis enzyme MurF

The authors describe the discovery of a new class of inhibitors to an essential Streptococcus pneumoniae cell wall biosyn-thesis enzyme, MurF, by a novel affinity screening method. The strategy involved screening very large mixtures of diverse small organic molecules against the protein target on the basis of equilibrium binding, followed by iterative ultrafiltration steps and ligand identification by mass spectrometry. Hits from any affinity-based screening method often can be relatively nonselective ligands, sometimes referred to as "nuisance" or "promiscuous" compounds. Ligands selective in their binding affinity for the MurF target were readily identified through electronic subtraction of an empirically determined subset of promiscuous compounds in the library without subsequent selectivity panels. The complete strategy for discovery and identification of novel specific ligands can be applied to all soluble protein targets and a wide variety of ligand libraries.     

Proteomic methods for drug target discovery

The field of drug target discovery is currently very popular with a great potential for advancing biomedical research and chemical genomics. Innovative strategies have been developed to aid the process of target identification, either by elucidating the primary mechanism-of-action of a drug, by understanding side effects involving unanticipated 'off-target' interactions, or by finding new potential therapeutic value for an established drug. Several promising proteomic methods have been introduced for directly isolating and identifying the protein targets of interest that are bound by active small molecules or for visualizing enzyme activities affected by drug treatment. Significant progress has been made in this rapidly advancing field, speeding the clinical validation of drug candidates and the discovery of the novel targets for lead compounds developed using cell-based phenotypic screens. Using these proteomic methods, further insight into drug activity and toxicity can be ascertained.     

The use of chemical design tools to transform proteomics data into drug candidates

The Human Genome Project has fueled the massive information-driven growth of genomics and proteomics and promises to deliver new insights into biology and medicine. Since proteins represent the majority of drug targets, these molecules are the focus of activity in pharmaceutical and biotechnology organizations. In this article, we describe the processes by which computational drug design may be used to exploit protein structural information to create virtual small molecules that may become novel medicines. Experimental protein structure determination, site exploration, and virtual screening provide a foundation for small molecule generation in silico, thus creating the bridge between proteomics and drug discovery.     

Leveraging structure determination with fragment screening for infectious disease drug targets: MECP synthase from Burkholderia pseudomallei

As part of the Seattle Structural Genomics Center for Infectious Disease, we seek to enhance structural genomics with ligand-bound structure data which can serve as a blueprint for structure-based drug design. We have adapted fragment-based screening methods to our structural genomics pipeline to generate multiple ligand-bound structures of high priority drug targets from pathogenic organisms. In this study, we report fragment screening methods and structure determination results for 2C-methyl-D-erythritol-2,4-cyclo-diphosphate (MECP) synthase from Burkholderia pseudomallei, the gram-negative bacterium which causes melioidosis. Screening by nuclear magnetic resonance spectroscopy as well as crystal soaking followed by X-ray diffraction led to the identification of several small molecules which bind this enzyme in a critical metabolic pathway. A series of complex structures obtained with screening hits reveal distinct binding pockets and a range of small molecules which form complexes with the target. Additional soaks with these compounds further demonstrate a subset of fragments to only bind the protein when present in specific combinations. This ensemble of fragment-bound complexes illuminates several characteristics of MECP synthase, including a previously unknown binding surface external to the catalytic active site. These ligand-bound structures now serve to guide medicinal chemists and structural biologists in rational design of novel inhibitors for this enzyme.     

Genomics, systems biology and drug development for infectious diseases

Although a variety of drugs are available for many infectious diseases that predominantly affect the developing world reasons remain for continuing to search for new chemotherapeutics. First, the development of microbial resistance has made some of the most effective and inexpensive drug regimes unreliable and dangerous to use on severely ill patients. Second, many existing antimicrobial drugs show toxicity or are too expensive for countries where the per capita income is in the order of hundreds of dollars per year. In recognition of this, new publicly and privately financed drug discovery efforts have been established to identify and develop new therapies for diseases such as tuberculosis, malaria and AIDS. This in turn, has intensified the need for tools to facilitate drug identification for those microbes whose molecular biology is poorly understood, or which are difficult to grow in the laboratory. While much has been written about how functional genomics can be used to find novel protein targets for chemotherapeutics this review will concentrate on how genome-wide, systems biology approaches may be used following whole organism, cell-based screening to understand the mechanism of drug action or to identify biological targets of small molecules. Here we focus on protozoan parasites, however, many of the approaches can be applied to pathogenic bacteria or parasitic helminths, insects or disease-causing fungi.     

Novel protein science enabled by total chemical synthesis

Chemical synthesis is a well‐established method for the preparation in the research laboratory of multiple‐tens‐of‐milligram amounts of correctly folded, high purity protein molecules. Chemically synthesized proteins enable a broad spectrum of novel protein science. Racemic mixtures consisting of d ‐protein and l ‐protein enantiomers facilitate crystallization and determination of protein structures by X‐ray diffraction. d ‐Proteins enable the systematic development of unnatural mirror image protein molecules that bind with high affinity to natural protein targets. The d ‐protein form of a therapeutic target can also be used to screen natural product libraries to identify novel small molecule leads for drug development. Proteins with novel polypeptide chain topologies including branched, circular, linear‐loop, and interpenetrating polypeptide chains can be constructed by chemical synthesis. Medicinal chemistry can be applied to optimize the properties of therapeutic protein molecules. Chemical synthesis has been used to redesign glycoproteins and for the a priori design and construction of covalently constrained novel protein scaffolds not found in nature. Versatile and precise labeling of protein molecules by chemical synthesis facilitates effective application of advanced physical methods including multidimensional nuclear magnetic resonance and time‐resolved FTIR for the elucidation of protein structure–activity relationships. The chemistries used for total synthesis of proteins have been adapted to making artificial molecular devices and protein‐inspired nanomolecular constructs. Research to develop mirror image life in the laboratory is in its very earliest stages, based on the total chemical synthesis of d ‐protein forms of polymerase enzymes.     

An Automated High-Throughput Cell-Based Multiplexed Flow Cytometry Assay to Identify Novel Compounds to Target Candida albicans Virulence-Related Proteins

Although three major classes of systemic antifungal agents are clinically available, each is characterized by important limitations. Thus, there has been considerable ongoing effort to develop novel and repurposed agents for the therapy of invasive fungal infections. In an effort to address these needs, we developed a novel high-throughput, multiplexed screening method that utilizes small molecules to probe candidate drug targets in the opportunistic fungal pathogen Candida albicans. This method is amenable to high-throughput automated screening and is based upon detection of changes in GFP levels of individually tagged target proteins. We first selected four GFP-tagged membrane-bound proteins associated with virulence or antifungal drug resistance in C. albicans. We demonstrated proof-of-principle that modulation of fluorescence intensity can be used to assay the expression of specific GFP-tagged target proteins to inhibitors (and inducers), and this change is measurable within the HyperCyt automated flow cytometry sampling system. Next, we generated a multiplex of differentially color-coded C. albicans strains bearing C-terminal GFP-tags of each gene encoding candidate drug targets incubated in the presence of small molecules from the Prestwick Chemical Library in 384-well microtiter plate format. Following incubation, cells were sampled through the HyperCyt system and modulation of protein levels, as indicated by changes in GFP-levels of each strain, was used to identify compounds of interest. The hit rate for both inducers and inhibitors identified in the primary screen did not exceed 1% of the total number of compounds in the small-molecule library that was probed, as would be expected from a robust target-specific, high-throughput screening campaign. Secondary assays for virulence characteristics based on null mutant strains were then used to further validate specificity. In all, this study presents a method for the identification and verification of new antifungal drugs targeted to fungal virulence proteins using C. albicans as a model fungal pathogen.     

New approaches towards the identification of antibiotic and vaccine targets in Streptococcus pneumoniae

Streptococcus pneumoniae causes more than one million deaths every year, mostly of young children in developing countries, due to pneumonia, bacteremia and meningitis. The emergence and dissemination of drug-resistant pneumococcal strains, coupled to changing patterns of virulence and the inadequacy of available vaccines, calls for an aggressive search for novel targets for antibiotic and vaccine development. Microbial genomics techniques allow genetic and biochemical tools to be employed to tackle discovery, design and development of new anti-infective agents based on the identification of hundreds of new targets. In this review, novel approaches employed to identify potential antibiotic and vaccine targets in S. pneumoniae are highlighted. Recently identified virulence factors, as well as molecules essential for bacterial viability, cell wall integrity and infectivity, are discussed.     

Solvent-induced proteome profiling for proteomic quantitation and target discovery of small molecular drugs

Target identification by modification-free proteomic approaches can potentially reveal the pharmacological mechanism of small molecular compounds. By combining the recent solvent-induced protein precipitation (SIP) method with TMT-labeling quantitative proteomics, we propose solvent-induced proteome profiling (SIPP) approach to identify small molecule–protein interactions. The SIPP approach enables to depict denaturation curves of the target protein by varying concentrations of organic solvents to induce unfolding and precipitation of the cellular proteome. By using this approach, we have successfully identified the known targets of market drugs and natural products and extended the proteome information of SIP for target identification.     

Small molecule microarrays: recent advances and applications

Directed or exploratory drug development programs constantly seek robust screening platforms for the high fidelity identification and validation of potential targets. Small-molecule microarrays (SMMs) have risen to this call by elegantly forging the capability of combinatorial chemistry in producing myriad compounds with the powerful throughput afforded by microarrays. This synergism offers scientists a versatile tool for rapid compound analysis and discovery. Microarrays of small molecules have already been successfully applied in important areas ranging from protein profiling to the discovery of therapeutic leads. Recent interesting developments towards improved immobilization strategies and library creation methods, together with novel advances herein described, have set the stage for SMMs to take on wider and more routine applications in academia and industry. As a rapidly maturing technology, SMMs pave the way forward in high-throughput exploration, both in the identification of biologically significant natural and synthetic small molecules and in harnessing their vast potential towards medicinal and diagnostic applications.     

Target discovery in small-molecule cell-based screens by in situ proteome reactivity profiling

Chemical genomics aims to discover small molecules that affect biological processes through the perturbation of protein function. However, determining the protein targets of bioactive compounds remains a formidable challenge. We address this problem here through the creation of a natural product-inspired small-molecule library bearing protein-reactive elements. Cell-based screening identified a compound, MJE3, that inhibits breast cancer cell proliferation. In situ proteome reactivity profiling revealed that MJE3, but not other library members, covalently labeled the glycolytic enzyme phosphoglycerate mutase 1 (PGAM1), resulting in enzyme inhibition. Interestingly, MJE3 labeling and inhibition of PGAM1 were observed exclusively in intact cells. These results support the hypothesis that cancer cells depend on glycolysis for viability and promote PGAM1 as a potential therapeutic target. More generally, the incorporation of protein-reactive compounds into chemical genomics screens offers a means to discover targets of bioactive small molecules in living systems, thereby enabling downstream mechanistic investigations.     

Binding-Site Assessment by Virtual Fragment Screening

The accurate prediction of protein druggability (propensity to bind high-affinity drug-like small molecules) would greatly benefit the fields of chemical genomics and drug discovery. We have developed a novel approach to quantitatively assess protein druggability by computationally screening a fragment-like compound library. In analogy to NMR-based fragment screening, we dock ∼11000 fragments against a given binding site and compute a computational hit rate based on the fraction of molecules that exceed an empirically chosen score cutoff. We perform a large-scale evaluation of the approach on four datasets, totaling 152 binding sites. We demonstrate that computed hit rates correlate with hit rates measured experimentally in a previously published NMR-based screening method. Secondly, we show that the in silico fragment screening method can be used to distinguish known druggable and non-druggable targets, including both enzymes and protein-protein interaction sites. Finally, we explore the sensitivity of the results to different receptor conformations, including flexible protein-protein interaction sites. Besides its original aim to assess druggability of different protein targets, this method could be used to identifying druggable conformations of flexible binding site for lead discovery, and suggesting strategies for growing or joining initial fragment hits to obtain more potent inhibitors.     

Small and lethal: searching for new antibacterial compounds with novel modes of action.

The discovery of drugs used to combat infectious diseases is in the process of constant change to address the ever-worsening problem of antibiotic resistance in pathogens and a lack of recent success in discovering new antibacterial drugs. In the past 2 decades, research in both academia and industry has made use of molecular biology, genetics, and comparative genomics, which has led to the development of key technologies for the discovery of novel antibacterial agents. Genome-scale efforts have led to the identification of numerous molecular targets. Chemical diversity from synthetic combinatorial libraries and natural products is being used to screen for new molecules. A wide variety of approaches are being used in the search for novel antibiotics, and these can be categorized as being either biochemically focused or cell based. The over-riding goal of all methods in use today is to discover new chemical matter with novel mechanisms of action against drug-resistant pathogens.     

Sweating the small stuff: microRNA discovery in plants

The class of small RNAs known as microRNAs (miRNAs) has a demonstrated role in the negative regulation of gene expression in both plants and animals. These small molecules have been shown to play a critical role in a wide range of developmental and physiological pathways. Although hundreds of different miRNAs have now been identified using cloning and computational approaches, characterization of their targets and biological roles has been more limited. New sequencing technologies promise to accelerate the sequencing of small RNAs and additional genetic and genomic strategies are being applied to assess their regulatory function on RNA targets. These technologies will enable the identification of large numbers of small RNAs from diverse species, and comparative genomics approaches based on these data are likely to identify additional miRNAs. Combined with bioinformatics and experimental approaches to separate miRNAs from short-interfering RNAs (siRNAs), the pace of miRNA discovery is likely to accelerate, leading to an improved understanding of miRNA function and biological significance.     

Functional genomics: identifying drug targets for parasitic diseases

The genomic sequences of parasitic diseases are rapidly becoming available and, recently, the full sequence of Plasmodium falciparum has been published. Much has been promised from this genomic revolution including the identification of new drug targets and novel chemotherapeutic treatments for the control of parasitic diseases. The challenge to use this information efficiently will require functional genomics tools such as bioinformatics, microarrays, proteomics and chemical genomics to identify potential drug targets, and to allow the development of optimized lead compounds. The information generated from these tools will provide a crucial link from genomic analysis to drug discovery.     

Natural products against hematological malignancies and identification of their targets

Naturally occurring molecules derived from higher plants, animals, microorganisms and minerals play an important role in the discovery and development of novel therapeutic agents. The identification of molecular targets is of interest to elucidate the mode of action of these compounds, and it may be employed to set up target-based assays and allow structure-activity relationship studies to guide medicinal chemistry efforts toward lead optimization. In recent years, plant-derived natural compounds possessing potential anti-tumor activities have been garnering much interest and efforts are underway to identify their molecular targets. Here, we attempt to summarize the discoveries of several natural compounds with activities against hematological malignancies, such as adenanthin, oridonin, gambogic acid and wogonoside, the identification of their targets, and their modes of actions.     

Identification and validation of bioactive small molecule target through phenotypic screening

For effective bioactive small molecule discovery and development into new therapeutic drug, a systematic screening and target protein identification is required. Different from the conventional screening system, herein phenotypic screening in combination with multi-omics-based target identification and validation (MOTIV) is introduced. First, phenotypic screening provides visual effect of bioactive small molecules in the cell or organism level. It is important to know the effect on the cell or organism level since small molecules affect not only a single target but the entire cellular mechanism within a cell or organism. Secondly, MOTIV provides systemic approach to discover the target protein of bioactive small molecule. With the chemical genomics and proteomics approach of target identification methods, various target protein candidates are identified. Then network analysis and validations of these candidates result in identifying the biologically relevant target protein and cellular mechanism. Overall, the combination of phenotypic screening and MOTIV will provide an effective approach to discover new bioactive small molecules and their target protein and mechanism identification.     

Drugs against superbugs: private lessons from bacteriophages

Bacterial genomics has provided a plethora of potential targets for antibacterial drug discovery, however, success in the hunt for new antibiotics will hinge on selecting targets with the highest potential. A recent paper by Liu and coworkers describes a new approach to target selection that uncovers strategies used by bacteriophage to disable bacteria. The method uses key phage proteins to identify and validate vulnerable targets and exploits them further in the identification of new antibacterial leads.     

Computational analysis of ligand relationships within target families

Computational tools for the large-scale analysis and prediction of ligand-target interactions and the identification of small molecules having different selectivity profiles within target protein families complement research in chemical genetics and chemogenomics. For computational analysis and design, such tasks require a departure from the traditional focus on single targets, hit identification, and lead optimization. Recently, studies have been reported that profile compounds in silico against arrays of targets or systematically map ligand-target space. In order to identify small molecular probes that are suitable for chemical genetics applications, molecular diversity needs to be viewed in a way that partly differs from principles guiding conventional library design.