Veerabahu Shanmugasundaram

Molecular Similarity Measures

Molecular similarity is a pervasive concept in chemistry. It is essential to many aspects of chemical reasoning and analysis and is perhaps the fundamental assumption underlying medicinal chemistry. Dissimilarity, the complement of similarity, also plays a major role in a growing number of applications of molecular diversity in combinatorial chemistry, high-throughput screening, and related fields. How molecular information is represented, called the representation problem, is important to the type of molecular similarity analysis (MSA) that can be carried out in any given situation. In this work, four types of mathematical structure are used to represent molecular information: sets, graphs, vectors, and functions. Molecular similarity is a pairwise relationship that induces structure into sets of molecules, giving rise to the concept of chemical space. Although all three concepts - molecular similarity, molecular representation, and chemical space - are treated in this chapter, the emphasis is on molecular similarity measures. Similarity measures, also called similarity coefficients or indices, are functions that map pairs of compatible molecular representations that are of the same mathematical form into real numbers usually, but not always, lying on the unit interval. This chapter presents a somewhat pedagogical discussion of many types of molecular similarity measures, their strengths and limitations, and their relationship to one another. An expanded account of the material on chemical spaces presented in the first edition of this book is also provided. It includes a discussion of the topography of activity landscapes and the role that activity cliffs in these landscapes play in structure-activity studies.

Thermodynamic analysis of water molecules at the surface of proteins and applications to binding site prediction and characterization

Water plays an essential role in determining the structure and function of all biological systems. Recent methodological advances allow for an accurate and efficient estimation of the thermodynamic properties of water molecules at the surface of proteins. In this work, we characterize these thermodynamic properties and relate them to various structural and functional characteristics of the protein. We find that high‐energy hydration sites often exist near protein motifs typically characterized as hydrophilic, such as backbone amide groups. We also find that waters around alpha helices and beta sheets tend to be less stable than waters around loops. Furthermore, we find no significant correlation between the hydration site‐free energy and the solvent accessible surface area of the site. In addition, we find that the distribution of high‐energy hydration sites on the protein surface can be used to identify the location of binding sites and that binding sites of druggable targets tend to have a greater density of thermodynamically unstable hydration sites. Using this information, we characterize the FKBP12 protein and show good agreement between fragment screening hit rates from NMR spectroscopy and hydration site energetics. Finally, we show that water molecules observed in crystal structures are less stable on average than bulk water as a consequence of the high degree of spatial localization, thereby resulting in a significant loss in entropy. These findings should help to better understand the characteristics of waters at the surface of proteins and are expected to lead to insights that can guide structure‐based drug design efforts. Proteins 2011.

Chemical and computational methods for the characterization of covalent reactive groups for the prospective design of irreversible inhibitors.

Interest in drugs that covalently modify their target is driven by the desire for enhanced efficacy that can result from the silencing of enzymatic activity until protein resynthesis can occur, along with the potential for increased selectivity by targeting uniquely positioned nucleophilic residues in the protein. However, covalent approaches carry additional risk for toxicities or hypersensitivity reactions that can result from covalent modification of unintended targets. Here we describe methods for measuring the reactivity of covalent reactive groups (CRGs) with a biologically relevant nucleophile, glutathione (GSH), along with kinetic data for a broad array of electrophiles. We also describe a computational method for predicting electrophilic reactivity, which taken together can be applied to the prospective design of thiol-reactive covalent inhibitors.

Structural basis for effectiveness of siderophore-conjugated monocarbams against clinically relevant strains of Pseudomonas aeruginosa

Pseudomonas aeruginosa is an opportunistic Gram-negative pathogen that causes nosocomial infections for which there are limited treatment options. Penicillin-binding protein PBP3, a key therapeutic target, is an essential enzyme responsible for the final steps of peptidoglycan synthesis and is covalently inactivated by β-lactam antibiotics. Here we disclose the first high resolution cocrystal structures of the P. aeruginosa PBP3 with both novel and marketed β-lactams. These structures reveal a conformational rearrangement of Tyr532 and Phe533 and a ligand-induced conformational change of Tyr409 and Arg489. The well-known affinity of the monobactam aztreonam for P. aeruginosa PBP3 is due to a distinct hydrophobic aromatic wall composed of Tyr503, Tyr532, and Phe533 interacting with the gem-dimethyl group. The structure of MC-1, a new siderophore-conjugated monocarbam complexed with PBP3 provides molecular insights for lead optimization. Importantly, we have identified a novel conformation that is distinct to the high-molecular-weight class B PBP subfamily, which is identifiable by common features such as a hydrophobic aromatic wall formed by Tyr503, Tyr532, and Phe533 and the structural flexibility of Tyr409 flanked by two glycine residues. This is also the first example of a siderophore-conjugated triazolone-linked monocarbam complexed with any PBP. Energetic analysis of tightly and loosely held computed hydration sites indicates protein desolvation effects contribute significantly to PBP3 binding, and analysis of hydration site energies allows rank ordering of the second-order acylation rate constants. Taken together, these structural, biochemical, and computational studies provide a molecular basis for recognition of P. aeruginosa PBP3 and open avenues for future design of inhibitors of this class of PBPs.

Potent Inhibitors of LpxC for the Treatment of Gram-Negative Infections

In this paper, we present the synthesis and SAR as well as selectivity, pharmacokinetic, and infection model data for representative analogues of a novel series of potent antibacterial LpxC inhibitors represented by hydroxamic acid.

Estimation of aqueous solubility of organic compounds with QSPR approach.

AbstractPurpose. To derive a QSPR model for estimation of aqueous solubility of organic compounds. Methods. Solubility data for 930 diverse compounds was investigated with principal component regression analysis. This set of compounds consists of pharmaceuticals, pollutants, nutrients, herbicides, and pesticides. The diversity of this collection was analyzed using MACCS fingerprint and BCUT chemistry space. Results. The training set of the solubility data is as diverse as the Available Chemicals Directory, and more diverse than the MDL Drug Data Report. Forty-six molecular descriptors were screened using a genetic algorithm. A QSPR model with a squared correlation coefficient (r2) of 0.92, a root mean square error of 0.53 log molar solubility (log Sw), an average absolute estimation error of 0.36 log Sw, and a cross-validated q2 of 0.91 was derived. The QSPR model was validated with a test set of 249 compounds not included in the training set. The absolute estimation error for the test set of compounds was 0.39 log Sw. Conclusions. A highly predictive QSPR model for estimating aqueous solubility was derived and validated. This model can be used to estimate aqueous solubility for virtual screening and combinatorial library design.

Siderophore Receptor-Mediated Uptake of Lactivicin Analogues in Gram-Negative Bacteria

Multidrug-resistant Gram-negative pathogens are an emerging threat to human health, and addressing this challenge will require development of new antibacterial agents. This can be achieved through an improved molecular understanding of drug-target interactions combined with enhanced delivery of these agents to the site of action. Herein we describe the first application of siderophore receptor-mediated drug uptake of lactivicin analogues as a strategy that enables the development of novel antibacterial agents against clinically relevant Gram-negative bacteria. We report the first crystal structures of several sideromimic conjugated compounds bound to penicillin binding proteins PBP3 and PBP1a from Pseudomonas aeruginosa and characterize the reactivity of lactivicin and β-lactam core structures. Results from drug sensitivity studies with β-lactamase enzymes are presented, as well as a structure-based hypothesis to reduce susceptibility to this enzyme class. Finally, mechanistic studies demonstrating that sideromimic modification alters the drug uptake process are discussed.

Distinctive Attributes of β-Lactam Target Proteins in Acinetobacter baumannii Relevant to Development of New Antibiotics

Multi-drug-resistant forms of the Gram-negative pathogen Acinetobacter baumannii are an emerging threat to human health and further complicate the general problem of treating serious bacterial infections. Meeting this challenge requires an improved understanding of the relationships between the structures of major therapeutic targets in this organism and the activity levels exhibited against it by different antibiotics. Here we report the first crystal structures of A. baumannii penicillin-binding proteins (PBPs) covalently inactivated by four β-lactam antibiotics. We also relate the results to kinetic, biophysical, and computational data. The structure of the class A protein PBP1a was solved in apo form and for its covalent conjugates with benzyl penicillin, imipenem, aztreonam, and the siderophore-conjugated monocarbam MC-1. It included a novel domain genetically spliced into a surface loop of the transpeptidase domain that contains three conserved loops. Also reported here is the first high-resolution structure of the A. baumannii class B enzyme PBP3 in apo form. Comparison of this structure with that of MC-1-derivatized PBP3 of Pseudomonas aeruginosa identified differences between these orthologous proteins in A. baumannii and P. aeruginosa. Thermodynamic analyses indicated that desolvation effects in the PBP3 ligand-binding sites contributed significantly to the thermal stability of the enzyme-antibiotic covalent complexes. Across a significant range of values, they correlated well with results from studies of inactivation kinetics and the protein structures. The structural, biophysical, and computational data help rationalize differences in the functional performance of antibiotics against different protein targets and can be used to guide the design of future agents.

Pyridone-Conjugated Monobactam Antibiotics with Gram-Negative Activity

Herein we describe the structure-aided design and synthesis of a series of pyridone-conjugated monobactam analogues with in vitro antibacterial activity against clinically relevant Gram-negative species including Pseudomonas aeruginosa , Klebsiella pneumoniae , and Escherichia coli . Rat pharmacokinetic studies with compound 17 demonstrate low clearance and low plasma protein binding. In addition, evidence is provided for a number of analogues suggesting that the siderophore receptors PiuA and PirA play a role in drug uptake in P. aeruginosa strain PAO1.

Heterocyclic methylsulfone hydroxamic acid LpxC inhibitors as Gram-negative antibacterial agents

The synthesis and antibacterial activity of heterocyclic methylsulfone hydroxamates is presented. Compounds in this series are potent inhibitors of the LpxC enzyme, a key enzyme involved in the production of lipopolysaccharide (LPS) found in the outer membrane of Gram-negative bacteria. SAR evaluation of compounds in this series revealed analogs with potent antibacterial activity against challenging Gram-negative species such as Pseudomonas aeruginosa and Klebsiella pneumoniae.