Identification of small molecule inhibitors of botulinum neurotoxin serotype E via footprint similarity.

Botulinum neurotoxins (BoNT) are among the most poisonous substances known, and of the 7 serotypes (A-G) identified thus far at least 4 can cause death in humans. The goal of this work was identification of inhibitors that specifically target the light chain catalytic site of the highly pathogenic but lesser-studied E serotype (BoNT/E). Large-scale computational screening, employing the program DOCK, was used to perform atomic-level docking of 1.4 million small molecules to prioritize those making favorable interactions with the BoNT/E site. In particular, 'footprint similarity' (FPS) scoring was used to identify compounds that could potentially mimic features on the known substrate tetrapeptide RIME. Among 92 compounds purchased and experimentally tested, compound C562-1101 emerged as the most promising hit with an apparent IC50 value three-fold more potent than that of the first reported BoNT/E small molecule inhibitor NSC-77053. Additional analysis showed the predicted binding pose of C562-1101 was geometrically and energetically stable over an ensemble of structures generated by molecular dynamic simulations and that many of the intended interactions seen with RIME were maintained. Several analogs were also computationally designed and predicted to have further molecular mimicry thereby demonstrating the potential utility of footprint-based scoring protocols to help guide hit refinement.

Small molecule non-peptide inhibitors of botulinum neurotoxin serotype E: Structure-activity relationship and a pharmacophore model.

Botulinum neurotoxins (BoNTs) are the most poisonous biological substance known to humans. They cause flaccid paralysis by blocking the release of acetylcholine at the neuromuscular junction. Here, we report a number of small molecule non-peptide inhibitors of BoNT serotype E. The structure-activity relationship and a pharmacophore model are presented. Although non-peptidic in nature, these inhibitors mimic key features of the uncleavable substrate peptide Arg-Ile-Met-Glu (RIME) of the SNAP-25 protein. Among the compounds tested, most of the potent inhibitors bear a zinc-chelating moiety connected to a hydrophobic and aromatic moiety through a carboxyl or amide linker. All of them show low micromolar IC50 values.

ß-Hydroxyacyl-acyl Carrier Protein Dehydratase (FabZ) from Francisella tularensis and Yersinia pestis: Structure Determination, Enzymatic Characterization, and Cross-Inhibition Studies.

The bacterial system for fatty acid biosynthesis (FAS) contains several enzymes whose sequence and structure are highly conserved across a vast array of pathogens. This, coupled with their low homology and difference in organization compared to the equivalent system in humans, makes the FAS pathway an excellent target for antimicrobial drug development. To this end, we have cloned, expressed, and purified the ß-hydroxyacyl-acyl carrier protein dehydratase (FabZ) from both Francisella tularensis (FtFabZ) and Yersinia pestis (YpFabZ). We also solved the crystal structures and performed an enzymatic characterization of both enzymes and several mutant forms of YpFabZ. Additionally, we have discovered two novel inhibitors of FabZ, mangostin and stictic acid, which show similar potencies against both YpFabZ and FtFabZ. Lastly, we selected several compounds from the literature that have been shown to be active against single homologues of FabZ and tested them against both YpFabZ and FtFabZ. These results have revealed clues as to which scaffolds are likely to lead to broad-spectrum antimicrobials targeted against FabZ as well as modifications to existing FabZ inhibitors that may improve potency.

Molecular Assembly of Clostridium botulinum progenitor M complex of type E.

Clostridium botulinum neurotoxin (BoNT) is released as a progenitor complex, in association with a non-toxic-non-hemagglutinin protein (NTNH) and other associated proteins. We have determined the crystal structure of M type Progenitor complex of botulinum neurotoxin E [PTC-E(M)], a heterodimer of BoNT and NTNH. The crystal structure reveals that the complex exists as a tight, interlocked heterodimer of BoNT and NTNH. The crystal structure explains the mechanism of molecular assembly of the complex and reveals several acidic clusters at the interface responsible for association at low acidic pH and disassociation at basic/neutral pH. The similarity of the general architecture between the PTC-E(M) and the previously determined PTC-A(M) strongly suggests that the progenitor M complexes of all botulinum serotypes may have similar molecular arrangement, although the neurotoxins apparently can take very different conformation when they are released from the M complex.

Interactions of a potent cyclic peptide inhibitor with the light chain of botulinum neurotoxin A: Insights from X-ray crystallography.

The seven antigenically distinct serotypes (A-G) of botulinum neurotoxin (BoNT) are responsible for the deadly disease botulism. BoNT serotype A (BoNT/A) exerts its lethal action by cleaving the SNARE protein SNAP-25, leading to inhibition of neurotransmitter release, flaccid paralysis and autonomic dysfunction. BoNTs are dichain proteins consisting of a ∼ 100 kDa heavy chain and a ∼ 50 kDa light chain; the former is responsible for neurospecific binding, internalization and translocation, and the latter for cleavage of neuronal SNARE proteins. Because of their extreme toxicity and history of weaponization, the BoNTs are regarded as potential biowarfare/bioterrorism agents. No post-symptomatic therapeutic interventions are available for BoNT intoxication other than intensive care; therefore it is imperative to develop specific antidotes against this neurotoxin. To this end, a cyclic peptide inhibitor (CPI-1) was evaluated in a FRET assay for its ability to inhibit BoNT/A light chain (Balc). CPI was found to be highly potent, exhibiting a Ki of 12.3 nM with full-length Balc448 and 39.2 nM using a truncated crystallizable form of the light chain (Balc424). Cocrystallization studies revealed that in the Balc424-CPI-1 complex, the inhibitor adopts a helical conformation, occupies a high percentage of the active site cavity and interacts in an amphipathic manner with critical active site residues. The data suggest that CPI-1 prevents SNAP-25 from accessing the Balc active site by blocking both the substrate binding path at the surface and the Zn(2+) binding region involved in catalysis. This differs from linear peptide inhibitors described to date which block only the latter.

Computer-aided identification, synthesis, and biological evaluation of novel inhibitors for botulinum neurotoxin serotype A.

Botulinum neurotoxins (BoNTs) are among the most potent biological toxin known to humans, and are classified as Category A bioterrorism agents by the Centers for Disease Control and prevention (CDC). There are seven known BoNT serotypes (A-G) which have been thus far identified in literature. BoNTs have been shown to block neurotransmitter release by cleaving proteins of the soluble NSF attachment protein receptor (SNARE) complex. Disruption of the SNARE complex precludes motor neuron failure which ultimately results in flaccid paralysis in humans and animals. Currently, there are no effective therapeutic treatments against the neurotoxin light chain (LC) after translocation into the cytosols of motor neurons. In this work, high-throughput in silico screening was employed to screen a library of commercially available compounds from ZINC database against BoNT/A-LC. Among the hit compounds from the in silico screening, two lead compounds were identified and found to have potent inhibitory activity against BoNT/A-LC in vitro, as well as in Neuro-2a cells. A few analogs of the lead compounds were synthesized and their potency examined. One of these analogs showed an enhanced activity than the lead compounds.

Recent developments with metalloprotease inhibitor class of drug candidates for botulinum neurotoxins.

Botulinum Neurotoxins are the most poisonous of all toxins with lethal dose in nanogram quantities. They are potential biological warfare and bioterrorism agents due to their high toxicity and ease of preparation. On the other hand BoNTs are also being increasingly used for therapeutic and cosmetic purposes, and with that the chances of accidental overdose are increasing. And despite the potential damage they could cause to human health, there are no post-intoxication drugs available so far. But progress is being made in this direction. The crystal structures in native form and bound with substrate peptides have been determined, and these are enabling structure-based drug discovery possible. High throughput assays have also been designed to speed up the screening progress. Substrate-based and small molecule inhibitors have been identified. But turning high affinity inhibitors into clinically viable drug candidates has remained a challenge. We discuss here the latest developments and the future challenges in drug discovery for Botulinum neurotoxins.

Functional annotation and structural characterization of a novel lactonase hydrolyzing D-xylono-1,4-lactone-5-phosphate and L-arabino-1,4-lactone-5-phosphate.

A novel lactonase from Mycoplasma synoviae 53 (MS53_0025) and Mycoplasma agalactiae PG2 (MAG_6390) was characterized by protein structure determination, molecular docking, gene context analysis, and library screening. The crystal structure of MS53_0025 was determined to a resolution of 2.06 Å. This protein adopts a typical amidohydrolase (ß/α)8-fold and contains a binuclear zinc center located at the C-terminal end of the ß-barrel. A phosphate molecule was bound in the active site and hydrogen bonds to Lys217, Lys244, Tyr245, Arg275, and Tyr278. Both docking and gene context analysis were used to narrow the theoretical substrate profile of the enzyme, thus directing empirical screening to identify that MS53_0025 and MAG_6390 catalyze the hydrolysis of d-xylono-1,4-lactone-5-phosphate (2) with kcat/Km values of 4.7 × 10(4) and 5.7 × 10(4) M(-1) s(-1) and l-arabino-1,4-lactone-5-phosphate (7) with kcat/Km values of 1.3 × 10(4) and 2.2 × 10(4) M(-1) s(-1), respectively. The identification of the substrate profile of these two phospho-furanose lactonases emerged only when all methods were integrated and therefore provides a blueprint for future substrate identification of highly related amidohydrolase superfamily members.

Discovery of a novel L-lyxonate degradation pathway in Pseudomonas aeruginosa PAO1.

The l-lyxonate dehydratase (LyxD) in vitro enzymatic activity and in vivo metabolic function were assigned to members of an isofunctional family within the mandelate racemase (MR) subgroup of the enolase superfamily. This study combined in vitro and in vivo data to confirm that the dehydration of l-lyxonate is the biological role of the members of this family. In vitro kinetic experiments revealed catalytic efficiencies of ∼10(4) M(-1) s(-1) as previously observed for members of other families in the MR subgroup. Growth studies revealed that l-lyxonate is a carbon source for Pseudomonas aeruginosa PAO1; transcriptomics using qRT-PCR established that the gene encoding LyxD as well as several other conserved proximal genes were upregulated in cells grown on l-lyxonate. The proximal genes were shown to be involved in a pathway for the degradation of l-lyxonate, in which the first step is dehydration by LyxD followed by dehydration of the 2-keto-3-deoxy-l-lyxonate product by 2-keto-3-deoxy-l-lyxonate dehydratase to yield α-ketoglutarate semialdehyde. In the final step, α-ketoglutarate semialdehyde is oxidized by a dehydrogenase to α-ketoglutarate, an intermediate in the citric acid cycle. An X-ray structure for the LyxD from Labrenzia aggregata IAM 12614 with Mg(2+) in the active site was determined that confirmed the expectation based on sequence alignments that LyxDs possess a conserved catalytic His-Asp dyad at the end of seventh and sixth ß-strands of the (ß/α)7ß-barrel domain as well as a conserved KxR motif at the end of second ß-strand; substitutions for His 316 or Arg 179 inactivated the enzyme. This is the first example of both the LyxD function in the enolase superfamily and a pathway for the catabolism of l-lyxonate.

Loss of quaternary structure is associated with rapid sequence divergence in the OSBS family.

The rate of protein evolution is determined by a combination of selective pressure on protein function and biophysical constraints on protein folding and structure. Determining the relative contributions of these properties is an unsolved problem in molecular evolution with broad implications for protein engineering and function prediction. As a case study, we examined the structural divergence of the rapidly evolving o-succinylbenzoate synthase (OSBS) family, which catalyzes a step in menaquinone synthesis in diverse microorganisms and plants. On average, the OSBS family is much more divergent than other protein families from the same set of species, with the most divergent family members sharing <15% sequence identity. Comparing 11 representative structures revealed that loss of quaternary structure and large deletions or insertions are associated with the family's rapid evolution. Neither of these properties has been investigated in previous studies to identify factors that affect the rate of protein evolution. Intriguingly, one subfamily retained a multimeric quaternary structure and has small insertions and deletions compared with related enzymes that catalyze diverse reactions. Many proteins in this subfamily catalyze both OSBS and N-succinylamino acid racemization (NSAR). Retention of ancestral structural characteristics in the NSAR/OSBS subfamily suggests that the rate of protein evolution is not proportional to the capacity to evolve new protein functions. Instead, structural features that are conserved among proteins with diverse functions might contribute to the evolution of new functions.

Structural characterization and function determination of a nonspecific carboxylate esterase from the amidohydrolase superfamily with a promiscuous ability to hydrolyze methylphosphonate esters.

The uncharacterized protein Rsp3690 from Rhodobacter sphaeroides is a member of the amidohydrolase superfamily of enzymes. In this investigation the gene for Rsp3690 was expressed in Escherichia coli and purified to homogeneity, and the three-dimensional structure was determined to a resolution of 1.8 Å. The protein folds as a distorted (ß/α)8-barrel, and the subunits associate as a homotetramer. The active site is localized to the C-terminal end of the ß-barrel and is highlighted by the formation of a binuclear metal center with two manganese ions that are bridged by Glu-175 and hydroxide. The remaining ligands to the metal center include His-32, His-34, His-207, His-236, and Asp-302. Rsp3690 was shown to catalyze the hydrolysis of a wide variety of carboxylate esters, in addition to organophosphate and organophosphonate esters. The best carboxylate ester substrates identified for Rsp3690 included 2-naphthyl acetate (kcat/Km = 1.0 × 10(5) M(-1) s(-1)), 2-naphthyl propionate (kcat/Km = 1.5 × 10(5) M(-1) s(-1)), 1-naphthyl acetate (kcat/Km = 7.5 × 10(3) M(-1) s(-1)), 4-methylumbelliferyl acetate (kcat/Km = 2.7 × 10(3) M(-1) s(-1)), 4-nitrophenyl acetate (kcat/Km = 2.3 × 10(5) M(-1) s(-1)), and 4-nitrophenyl butyrate (kcat/Km = 8.8 × 10(5) M(-1) s(-1)). The best organophosphonate ester substrates included ethyl 4-nitrophenyl methylphosphonate (kcat/Km = 3.8 × 10(5) M(-1) s(-1)) and isobutyl 4-nitrophenyl methylphosphonate (kcat/Km = 1.1 × 10(4) M(-1) s(-1)). The (SP)-enantiomer of isobutyl 4-nitrophenyl methylphosphonate was hydrolyzed 10 times faster than the less toxic (RP)-enantiomer. The high inherent catalytic activity of Rsp3690 for the hydrolysis of the toxic enantiomer of methylphosphonate esters make this enzyme an attractive target for directed evolution investigations.

Divergent evolution of ligand binding in the o-succinylbenzoate synthase family.

Thermobifida fusca o-succinylbenzoate synthase (OSBS), a member of the enolase superfamily that catalyzes a step in menaquinone biosynthesis, has an amino acid sequence that is 22 and 28% identical with those of two previously characterized OSBS enzymes from Escherichia coli and Amycolatopsis sp. T-1-60, respectively. These values are considerably lower than typical levels of sequence identity among homologous proteins that have the same function. To determine how such divergent enzymes catalyze the same reaction, we determined the structure of T. fusca OSBS and identified amino acids that are important for ligand binding. We discovered significant differences in structure and conformational flexibility between T. fusca OSBS and other members of the enolase superfamily. In particular, the 20s loop, a flexible loop in the active site that permits ligand binding and release in most enolase superfamily proteins, has a four-amino acid deletion and is well-ordered in T. fusca OSBS. Instead, the flexibility of a different region allows the substrate to enter from the other side of the active site. T. fusca OSBS was more tolerant of mutations at residues that were critical for activity in E. coli OSBS. Also, replacing active site amino acids found in one protein with the amino acids that occur at the same place in the other protein reduces the catalytic efficiency. Thus, the extraordinary divergence between these proteins does not appear to reflect a higher tolerance of mutations. Instead, large deletions outside the active site were accompanied by alteration of active site size and electrostatic interactions, resulting in small but significant differences in ligand binding.

Deamination of 6-aminodeoxyfutalosine in menaquinone biosynthesis by distantly related enzymes.

Proteins of unknown function belonging to cog1816 and cog0402 were characterized. Sav2595 from Steptomyces avermitilis MA-4680, Acel0264 from Acidothermus cellulolyticus 11B, Nis0429 from Nitratiruptor sp. SB155-2 and Dr0824 from Deinococcus radiodurans R1 were cloned, purified, and their substrate profiles determined. These enzymes were previously incorrectly annotated as adenosine deaminases or chlorohydrolases. It was shown here that these enzymes actually deaminate 6-aminodeoxyfutalosine. The deamination of 6-aminodeoxyfutalosine is part of an alternative menaquinone biosynthetic pathway that involves the formation of futalosine. 6-Aminodeoxyfutalosine is deaminated by these enzymes with catalytic efficiencies greater than 10(5) M(-1) s(-1), Km values of 0.9-6.0 µM, and kcat values of 1.2-8.6 s(-1). Adenosine, 2'-deoxyadenosine, thiomethyladenosine, and S-adenosylhomocysteine are deaminated at least an order of magnitude slower than 6-aminodeoxyfutalosine. The crystal structure of Nis0429 was determined and the substrate, 6-aminodeoxyfutalosine, was positioned in the active site on the basis of the presence of adventitiously bound benzoic acid. In this model, Ser-145 interacts with the carboxylate moiety of the substrate. The structure of Dr0824 was also determined, but a collapsed active site pocket prevented docking of substrates. A computational model of Sav2595 was built on the basis of the crystal structure of adenosine deaminase and substrates were docked. The model predicted a conserved arginine after ß-strand 1 to be partially responsible for the substrate specificity of Sav2595.

The C terminus of the catalytic domain of type A botulinum neurotoxin may facilitate product release from the active site.

Botulinum neurotoxins are the most toxic of all compounds. The toxicity is related to a poor zinc endopeptidase activity located in a 50-kDa domain known as light chain (Lc) of the toxin. The C-terminal tail of Lc is not visible in any of the currently available x-ray structures, and it has no known function but undergoes autocatalytic truncations during purification and storage. By synthesizing C-terminal peptides of various lengths, in this study, we have shown that these peptides competitively inhibit the normal catalytic activity of Lc of serotype A (LcA) and have defined the length of the mature LcA to consist of the first 444 residues. Two catalytically inactive mutants also inhibited LcA activity. Our results suggested that the C terminus of LcA might interact at or near its own active site. By using synthetic C-terminal peptides from LcB, LcC1, LcD, LcE, and LcF and their respective substrate peptides, we have shown that the inhibition of activity is specific only for LcA. Although a potent inhibitor with a Ki of 4.5 µm, the largest of our LcA C-terminal peptides stimulated LcA activity when added at near-stoichiometric concentration to three versions of LcA differing in their C-terminal lengths. The result suggested a product removal role of the LcA C terminus. This suggestion is supported by a weak but specific interaction determined by isothermal titration calorimetry between an LcA C-terminal peptide and N-terminal product from a peptide substrate of LcA. Our results also underscore the importance of using a mature LcA as an inhibitor screening target.

Structure of alanine racemase from Oenococcus oeni with bound pyridoxal 5'-phosphate.

The crystal structure of alanine racemase from Oenococcus oeni has been determined at 1.7 Šresolution using the single-wavelength anomalous dispersion (SAD) method and selenium-labelled protein. The protein exists as a symmetric dimer in the crystal, with both protomers contributing to the two active sites. Pyridoxal 5'-phosphate, a cofactor, is bound to each monomer and forms a Schiff base with Lys39. Structural comparison of alanine racemase from O. oeni (Alr) with homologous family members revealed similar domain organization and cofactor binding.

Structure-based drug discovery for botulinum neurotoxins.

Clostridium botulinum neurotoxin is the most poisonous substance known to humans. It is a potential biowarfare threat and a public health hazard. The only therapeutics available is antibody treatment which will not be effective for post-exposure therapy. There are no drugs available for post-intoxication treatment. Accordingly, it is imperative to develop effective drugs to counter botulism. Available structural information on botulinum neurotoxins both alone and in complex with their substrates offers an efficient method for designing structure-based drugs to treat botulism.

Functional annotation and three-dimensional structure of an incorrectly annotated dihydroorotase from cog3964 in the amidohydrolase superfamily.

The substrate specificities of two incorrectly annotated enzymes belonging to cog3964 from the amidohydrolase superfamily were determined. This group of enzymes are currently misannotated as either dihydroorotases or adenine deaminases. Atu3266 from Agrobacterium tumefaciens C58 and Oant2987 from Ochrobactrum anthropi ATCC 49188 were found to catalyze the hydrolysis of acetyl-(R)-mandelate and similar esters with values of k(cat)/K(m) that exceed 10(5) M(-1) s(-1). These enzymes do not catalyze the deamination of adenine or the hydrolysis of dihydroorotate. Atu3266 was crystallized and the structure determined to a resolution of 2.62 Å. The protein folds as a distorted (ß/α)(8) barrel and binds two zincs in the active site. The substrate profile was determined via a combination of computational docking to the three-dimensional structure of Atu3266 and screening of a highly focused library of potential substrates. The initial weak hit was the hydrolysis of N-acetyl-D-serine (k(cat)/K(m) = 4 M(-1) s(-1)). This was followed by the progressive identification of acetyl-(R)-glycerate (k(cat)/K(m) = 4 × 10(2) M(-1) s(-1)), acetyl glycolate (k(cat)/K(m) = 1.3 × 10(4) M(-1) s(-1)), and ultimately acetyl-(R)-mandelate (k(cat)/K(m) = 2.8 × 10(5) M(-1) s(-1)).

Structure of a periplasmic glucose-binding protein from Thermotoga maritima.

ABC transport systems have been characterized in organisms ranging from bacteria to humans. In most bacterial systems, the periplasmic component is the primary determinant of specificity of the transport complex as a whole. Here, the X-ray crystal structure of a periplasmic glucose-binding protein (GBP) from Thermotoga maritima determined at 2.4 Šresolution is reported. The molecule consists of two similar α/ß domains connected by a three-stranded hinge region. In the current structure, a ligand (ß-D-glucose) is buried between the two domains, which have adopted a closed conformation. Details of the substrate-binding sites revealed features that determine substrate specificity. In toto, ten residues from both domains form eight hydrogen bonds to the bound sugar and four aromatic residues (two from each domain) stabilize the substrate through stacking interactions.

Peptide inhibitors of botulinum neurotoxin serotype A: design, inhibition, cocrystal structures, structure-activity relationship and pharmacophore modeling.

Clostridium botulinum neurotoxins are classified as Category A bioterrorism agents by the Centers for Disease Control and Prevention (CDC). The seven serotypes (A-G) of the botulinum neurotoxin, the causative agent of the disease botulism, block neurotransmitter release by specifically cleaving one of the three SNARE (soluble N-ethylmaleimide-sensitive factor attachment protein receptor) proteins and induce flaccid paralysis. Using a structure-based drug-design approach, a number of peptide inhibitors were designed and their inhibitory activity against botulinum serotype A (BoNT/A) protease was determined. The most potent peptide, RRGF, inhibited BoNT/A protease with an IC(50) of 0.9 µM and a K(i) of 358 nM. High-resolution crystal structures of various peptide inhibitors in complex with the BoNT/A protease domain were also determined. Based on the inhibitory activities and the atomic interactions deduced from the cocrystal structures, the structure-activity relationship was analyzed and a pharmacophore model was developed. Unlike the currently available models, this pharmacophore model is based on a number of enzyme-inhibitor peptide cocrystal structures and improved the existing models significantly, incorporating new features.

Discovery of a fluorene class of compounds as inhibitors of botulinum neurotoxin serotype E by virtual screening.

Botulinum neurotoxins are one of the most poisonous biological substances known to humans and present a potential bioterrorism threat. There are no therapeutic interventions developed so far. Here, we report the first small molecule non-peptide inhibitor for botulinum neurotoxin serotype E discovered by structure-based virtual screening and propose a mechanism for its inhibitory activity.