The pentein superfamily is a mechanistically diverse superfamily encompassing both noncatalytic proteins and enzymes that catalyze hydrolase, dihydrolase and amidinotransfer reactions on guanidine substrates. Despite generally low sequence identity, they possess a conserved structural fold and display common mechanistic themes in catalysis. The structurally characterized catalytic penteins possess a conserved core of residues that include a Cys, His and two polar, guanidine-binding residues. All known catalytic penteins use the core Cys to attack the substrate’s guanidine moiety to form a covalent thiouronium adduct and all cleave one or more of the guanidine C–N bonds. The mechanistic information compiled to date supports the hypothesis that this superfamily may have evolved divergently from a catalytically promiscuous ancestor.
Pentein; Guanidine; Arginine deiminase; Dimethylarginine dimethylaminohydrolase; Agmatine deiminase; Peptidylarginine deiminase; Arginine:glycine amidinotransferase; Arginine:inosamine phosphate; amidinotransferase; Nα-succinylarginine dihydrolase; Guanidino-modifying superfamily
Due to the global threat of antibiotic resistance mediated by New Delhi metallo-beta-lactamase-1 (NDM-1) and the lack of structurally diverse inhibitors reported for this enzyme, we developed screening and counter-screening assays for manual and automated formats. The manual assay is a trans-well absorbance-based endpoint assay in 96-well plates and has a Z’ factor of 0.8. The automated assay is an epi-absorbance endpoint assay in 384-well plates, has a Z’ factor of ≥ 0.8, good signal / baseline ratios (> 3.8), and is likely scalable for high-throughput screening (HTS). A TEM-1-based counter-screen is also presented to eliminate false positives due to assay interference or off-target activities. A pilot screen of a pharmacologically characterized compound library identified two thiol-modifying compounds as authentic NDM-1 inhibitors: p-hloromecuribenzoate (p-CMB) and nitroprusside. Recombinant NDM-1 has one Cys residue that serves as a conserved active-site primary zinc ligand and is selectively modified by p-CMB as confirmed by LC-MS/MS. However a C208D mutation results in an enzyme that maintains almost full lactamase activity, yet is completely resistant to the inhibitor. These results predict that covalent targeting of the conserved active-site Cys residue may have drawbacks as a drug design strategy.
AiiA is a metal-dependent N-acyl homoserine lactone hydrolase that displays broad substrate specificity, but shows preference for substrates with long N-acyl substitutions. Previously, crystal structures of AiiA in complex with the ring-opened product N-hexanoyl-l-homoserine revealed binding interactions near the metal center, but did not identify a binding pocket for the N-acyl chains of longer substrates. Here we report the crystal structure of an AiiA mutant, F107W, determined in the presence and absence of N-decanoyl-l-homoserine. F107 is located in a hydrophobic cavity adjacent to the previously identified ligand binding pocket, and F107W results in the formation of an unexpected interaction with the ring-opened product. Notably, the structure reveals a previously unidentified hydrophobic binding pocket for the substrate’s N-acyl chain. Two aromatic residues, F64 and F68 form a hydrophobic clamp, centered around the seventh carbon in the product-bound structure’s decanoyl chain, making an interaction that would also be available for longer substrates, but not for shorter substrates. Steady-state kinetics using substrates of various lengths with AiiA bearing mutations at the hydrophobic clamp, including insertion of a redox sensitive cysteine pair, confirms the importance of this hydrophobic feature for substrate preference. Identifying the specificity determinants of AiiA will aid the development of more selective quorum-quenching enzymes as tools and as potential therapeutics.
AiiA; lactonase; dizinc hydrolase; substrate specificity; quorum quenching; N-acyl homoserine lactone
Potent and selective inhibitors of the enzyme dimethylarginine dimethylaminohydrolase (DDAH) are useful as molecular probes to better understand cellular regulation of nitric oxide. Inhibitors are also potential therapeutic agents for treatment of pathological states associated with the inappropriate overproduction of nitric oxide, such as septic shock, selected types of cancer, and other conditions. Inhibitors with structures dissimilar to substrate may overcome limitations inherent to substrate analogs. Therefore, to identify structurally-diverse inhibitor scaffolds, high-throughput screening (HTS) of a 4000-member library of fragment-sized molecules was completed using the Pseudomonas aeruginosa DDAH and human DDAH-1 isoforms. Use of a substrate concentration equal to its KM value during the primary screen allowed for the detection of inhibitors with different modes of inhibition. A series of validation tests were designed and implemented in the identification of four inhibitors of human DDAH-1 that were unknown prior to the screen. Two inhibitors share a 4-halopyridine scaffold and act as quiescent affinity labels that selectively and covalently modify the active-site Cys residue. Two inhibitors are benzimidazole-like compounds that reversibly and competitively inhibit human DDAH-1 with Ligand Efficiency values ≥ 0.3 kcal / mol / heavy (non-hydrogen) atom, indicating their suitability for further development. Both inhibitor scaffolds have available sites to derivatize for further optimization. Therefore, use of this fragment-based HTS approach is demonstrated to successfully identify two novel scaffolds for development of DDAH-1 inhibitors.
Fragment library; High-throughput screen; Inhibitor discovery; Dimethylarginine dimethylaminohydrolase; Nitric oxide
N-Acyl-l-homoserine lactones (AHLs) are a major class of quorum sensing signals used by Gram-negative bacteria to regulate gene expression in a population-dependent manner, thereby enabling group behavior. Enzymes capable of generating and catabolizing AHL signals are of significant interest for the study of microbial ecology and quorum-sensing pathways, for understanding the systems that bacteria have evolved to interact with small molecule signals, and for their possible use in therapeutic and industrial applications. The recent structural and functional studies reviewed here provide detailed insight into the chemistry and enzymology of bacterial communication.
Inhibitors of human dimethylarginine dimethylaminohydrolase-1 (DDAH-1) are of therapeutic interest for controlling pathological nitric oxide production. Only a limited number of biologically useful inhibitors have been identified, so structurally diverse lead compounds are desired. In contrast with previous assays that do not possesses adequate sensitivity for optimal screening, herein is reported a high-throughput assay that uses an alternative thiol-releasing substrate, S-methyl-L-thiocitrulline, and a thiol-reactive fluorophore, 7-diethylamino-3-(4′-maleimidylphenyl)-4-methylcoumarin, to enable continuous detection of product formation by DDAH-1. The assay is applied to query two commercial libraries totaling 4,446 compounds and two representative hits are described, including a known DDAH-1 inhibitor. This is the most sensitive DDAH-1 assay reported to date, and enables screening of compound libraries using [S] =KM conditions, while displaying Z′ factors from 0.6 – 0.8. Therefore, this strategy now makes possible high-throughput screening for human DDAH-1 inhibitors in pursuit of molecular probes and drugs to control excessive nitric oxide production.
Dimethylarginine dimethylaminohydrolase; high-throughput screening; nitric oxide; CPM
Dimethylarginine dimethylaminohydrolase (DDAH) is an endogenous regulator of nitric oxide production and represents a potential therapeutic target. However, only a small number of biologically useful inhibitors have been reported, and many of these are substrate analogues. To seek more diverse scaffolds, we developed a high-throughput screening (HTS) assay and queried two small libraries totaling 2446 compounds. The HTS assay proved to be robust, reproducible, and scalable, with Z′ factors ≥ 0.78. One inhibitor, ebselen, is structurally divergent from substrate and was characterized in detail. This selenazole covalently inactivates DDAH in vitro and in cultured cells. The rate constant for inactivation of DDAH (44000 ± 2400 M–1 s–1) is greater than those reported for any other target, suggesting that this pathway is an important aspect of ebselen's total pharmacological effects.
Dimethylarginine dimethylaminohydrolase; high-throughput screening; ebselen; covalent inhibitors; nitric oxide
Small molecules capable of selective covalent protein modification are of significant interest for the development of biological probes and therapeutics. We recently reported that 2-methyl-4-bromopyridine is a quiescent affinity label for the nitric oxide controlling enzyme dimethylarginine dimethylaminohydrolase (DDAH) [Johnson, C.M., Linsky, T.W., Yoon, D.W., Person, M.D. & Fast, W. (2011) J. Am. Chem. Soc. 133, 1553-1562]. Discovery of this novel protein modifier raised the possibility that the 4-halopyridine motif may be suitable for wider application. Therefore, the inactivation mechanism of the related compound 2-hydroxymethyl-4-chloropyridine is probed here in more detail. Solution studies support an inactivation mechanism in which the active-site Asp66 residue stabilizes the pyridinium form of the inactivator, which has enhanced reactivity toward the active site Cys, resulting in covalent bond formation, loss of the halide, and irreversible inactivation. A 2.18 Å resolution X-ray crystal structure of the inactivated complex elucidates the orientation of the inactivator and its covalent attachment to the active-site Cys, but the structural model does not show an interaction between the inactivator and Asp66. Molecular modeling is used to investigate inactivator binding, reaction, and also a final pyridinium deprotonation step that accounts for the apparent differences between the solution-based and structural studies with respect to the role of Asp66. This work integrates multiple approaches to elucidate the inactivation mechanism of a novel 4-halopyridine “warhead,” emphasizing the strategy of using pyridinium formation as a “switch” to enhance reactivity when bound to the target protein.
Peptidylarginine deiminase (PAD) catalyzes the posttranslational citrullination of selected proteins in a calcium dependent manner. The PAD4 isoform has been implicated in multiple sclerosis, Rheumatoid arthritis, some types of cancer, and plays a role in gene regulation. However, the substrate selectivity of PAD4 is not well defined, nor is the impact of citrullination on many other pathways. Here, a high-density protein array is used as a primary screen to identify 40 previously unreported PAD4 substrates, 10 of which are selected and verified in a cell lysate-based secondary assay. One of the most prominent hits, human 40S ribosomal protein S2 (RPS2), is characterized in detail. PAD4 citrullinates the Arg-Gly repeat region of RPS2, which is also an established site for Arg methylation by protein arginine methyltransferase 3 (PRMT3). As in other systems, crosstalk is observed; citrullination and methylation modifications are found to be antagonistic to each other, suggesting a conserved posttranslational regulatory strategy. Both PAD4 and PRMT3 are found to co-sediment with the free 40S ribosomal subunit fraction from cell extracts. These findings are consistent with participation of citrullination in the regulation of RPS2 and ribosome assembly. This application of protein arrays to reveal new PAD4 substrates suggests a role for citrullination in a number of different cellular pathways.
In an effort to develop novel covalent modifiers of dimethylarginine dimethylaminohydrolase (DDAH) that are useful for biological applications, a set of “fragment”-sized inhibitors that were identified using a high-throughput screen are tested for time-dependent inhibition. One structural class of inactivators, 4-halopyridines, show time- and concentration-dependent inactivation of DDAH and the inactivation mechanism of one example, 4-bromo-2-methylpyridine (1), is characterized in detail. The neutral form of halopyridines is not very reactive with excess glutathione. However, 1 readily reacts, with loss of its halide, in a selective, covalent and irreversible manner with the active-site Cys249 of DDAH. This active-site Cys is not particularly reactive (pKa ca. 8.8) and 1 does not inactivate papain (Cys pKa ca. ≤ 4), suggesting that, unlike many reagents, Cys nucleophilicity is not a predominating factor in selectivity. Rather, binding and stabilization of the more reactive pyridinium form of the inactivator by a second moiety, Asp66, is required for facile reaction. This constraint imparts a unique selectivity profile to these inactivators. To our knowledge, halopyridines have not previously been reported as protein modifiers, and therefore represent a first-in-class example of a novel type of quiescent affinity label.
C-alkyl amidine analogs of asymmetric Nω,Nω -dimethyl-L-arginine are dual-targeted inhibitors of both human DDAH-1 and nitric oxide (NO) synthase, and provide a promising scaffold for developing therapeutics to control NO overproduction in a variety of pathologies including septic shock and some cancers. Using a two-part clickchemistry-mediated activity probe, a homologated series of C-alkyl amidines are ranked for their ability to inhibit DDAH-1 within cultured HEK 293T cells. N5-(1-iminopentyl)-L-ornithine was determined to be the most potent compound in vitro (Kd = 7 μM) as well as in cultured cells, and the binding conformation and covalent reversible mode of inhibition was investigated by comparison of interactions made with DDAH-1 and a catalytically inactive C274S variant as gauged by Xray crystallography and isothermal titration calorimetry. By interrupting the ability of the inhibitor to form a covalent bond, the contribution of this interaction can be estimated. These results suggest further stabilization of the covalent adduct as a promising strategy for lead optimization in the design of effective reagents to block NO synthesis.
DDAH; nitric oxide; inhibitors; covalentreversible; activity probe
Dimethylarginine dimethylaminohydrolase (DDAH) is an endogenous regulator of nitric oxide production and represents a potential therapeutic target. However, only a small number of biologically useful inhibitors have been reported, and many of these are substrate analogs. To seek more diverse scaffolds, we developed a high-throughput screening (HTS) assay and queried two small libraries totaling 2446 compounds. The HTS assay proved to be robust, reproducible and scalable, with Z' factors ≥ 0.78. One inhibitor, ebselen, is structurally divergent from substrate and was characterized in detail. This selenazole covalently inactivates DDAH in vitro and in cultured cells. The rate constant for inactivation of DDAH (44,000 ± 2,400 M−1s−1) is greater than those reported for any other target, suggesting this pathway is an important aspect of ebselen's total pharmacological effects.
Dimethylarginine dimethylaminohydrolase; high-throughput screening; ebselen; covalent inhibitors; nitric oxide
Molecules that block nitric oxide's (NO) biosynthesis are of significant interest. For example, nitric oxide synthase (NOS) inhibitors have been suggested as anti-tumor therapeutics, as have inhibitors of dimethylarginine dimethylaminohydrolase (DDAH), an enzyme that catabolizes endogenous NOS inhibitors. Dual-targeted inhibitors hold promise as more effective reagents to block NO biosynthesis than single-targeted compounds. In this study, a small set of known NOS inhibitors are surveyed as inhibitors of recombinant human DDAH-1. From these, an alkylamidine scaffold is selected for homologation. Stepwise lengthening of one substituent converts an NOS-selective inhibitor into a dual-targeted NOS/DDAH-1 inhibitor and then into a DDAH-1 selective inhibitor, as seen in the inhibition constants of N5-(1-iminoethyl)-, N5-(1-iminopropyl)-, N5-(1-iminopentyl)- and N5-(1-iminohexyl)-l-ornithine for neuronal NOS (1.7, 3, 20, >1,900 μM, respectively) and DDAH-1 (990, 52, 7.5, 110 μM, respectively). A 1.9Å X-ray crystal structure of the N5-(1-iminopropyl)-l-ornithine : DDAH-1 complex indicates covalent bond formation between the inhibitor's amidino carbon and the active-site Cys274, and solution studies show reversible competitive inhibition, consistent with a reversible covalent mode of DDAH inhibition by alkylamidine inhibitors. These represent a versatile scaffold for the development of a targeted polypharmacological approach to control NO biosynthesis.
In an effort to determine the physiological role of A. thaliana Glx2-1, we attempted to uncover a substrate for the enzyme. Glx2-1 did not effectively process 192 diverse substrates found in a commercial screen used for microorganism identification, or exhibit arylsulfatase, lactonase or phosphotriesterase activities. However, Glx2-1 does exhibit β-lactamase activity with kcat/KM values from 103 to 105 M−1 s−1. Glx2-1 can hydrolyze cephalosporins and carbapenems, albeit with rate constants slower than most metallo-β-lactamases. The potential role of a β-lactamase in the mitochondria of plant cells is briefly discussed.
Many enzymes in the pentein superfamily use a transient covalent intermediate in their catalytic mechanisms. Here, we use a mutant (H162G) dimethylarginine dimethylaminohydrolase from Pseudomonas aeruginosa and an alternative substrate, S-methyl-L-thiocitrulline, to trap, crystallize and determine the 2.8 Å resolution structure of a stable covalent adduct which mimics this reaction intermediate. Observed interactions between the trapped adduct and active site residues along with comparison to a previously known product-bound structure provide insight into the normal catalytic mechanism. The plane of the trapped thiouronium intermediate is angled away from that seen in the product and substrate complexes, allowing for an altered angle of attack between the nucleophiles of the first and second half reactions. The stable covalent adduct is also capable of further reaction. Addition of exogenous imidazole can rescue the original hydrolytic activity. Notably, addition of other exogenous amines can instead yield substituted arginine products. These alternative products arise from partitioning of the trapped intermediate into the evolutionarily related amidinotransferase reaction pathway. The enzyme scaffold provides both selectivity and catalysis for the amidinotransferase reaction, underscoring commonalities between different reaction pathways found in this mechanistically diverse enzyme superfamily. The promiscuous partitioning of this covalent intermediate may also help to illuminate the evolutionary history of these enzymes.
The N-acyl-l-homoserine lactone hydrolases (AHL lactonases) have attracted considerable attention because of their ability to quench AHL-mediated quorum-sensing pathways in Gram-negative bacteria and because of their relation to other enzymes in the metallo-β-lactamase superfamily. To elucidate the detailed catalytic mechanism of AHL lactonase, mutations are made on residues that presumably contribute to substrate binding and catalysis. Steady-state kinetic studies are carried out on both the wild-type and mutant enzymes using a spectrum of substrates. Two mutations, Y194F and D108N, present significant effects on the overall catalysis. On the basis of a high-resolution structural model of the enzyme−product complex, a hybrid quantum mechanical/molecular mechanical method is used to model the substrate binding orientation and to probe the effect of the Y194F mutation. Combining all experimental and computational results, we propose a detailed mechanism for the ring-opening hydrolysis of AHL substrates as catalyzed by the AHL lactonase from Bacillus thuringiensis. Several features of the mechanism that are also found in related enzymes are discussed and may help to define an evolutionary thread that connects the hydrolytic enzymes of this mechanistically diverse superfamily.
Enzymes capable of hydrolyzing N-acyl-l-homoserine lactones (AHLs) used in some bacterial quorum-sensing pathways are of considerable interest for their ability to block undesirable phenotypes. Most known AHL hydrolases that catalyze ring opening (AHL lactonases) are members of the metallo-β-lactamase enzyme superfamily and rely on a dinuclear zinc site for catalysis and stability. Here we report the three-dimensional structures of three product complexes formed with the AHL lactonase from Bacillus thuringiensis. Structures of the lactonase bound with two different concentrations of the ring-opened product of N-hexanoyl-l-homoserine lactone are determined at 0.95 and 1.4 Å resolution and exhibit different product configurations. A structure of the ring-opened product of the non-natural N-hexanoyl-l-homocysteine thiolactone at 1.3 Å resolution is also determined. On the basis of these product-bound structures, a substrate-binding model is presented that differs from previous proposals. Additionally, the proximity of the product to active-site residues and observed changes in protein conformation and metal coordination provide insight into the catalytic mechanism of this quorum-quenching metalloenzyme.
Lactone-hydrolyzing enzymes derived from some Bacillus species are capable of disrupting quorumsensing in bacteria that use N-acyl-l-homoserine lactones (AHLs) as intercellular signaling molecules. Despite the promise of these quorum-quenching enzymes as therapeutic and anti-biofouling agents, the ring opening mechanism and the role of metal ions in catalysis have not been elucidated. Labeling studies using 18O, 2H, and the AHL lactonase from Bacillus thuringiensis implicate an addition-elimination pathway for ring opening in which a solvent-derived oxygen is incorporated into the product carboxylate, identifying the alcohol as the leaving group. 1H-NMR is used to show that metal binding is required to maintain proper folding. A thio effect is measured for hydrolysis of N-hexanoyl-l-homoserine lactone and the corresponding thiolactone by AHL lactonase disubstituted with alternative metal ions including Mn2+, Co2+, Zn2+, and Cd2+. The magnitude of the thio effect on kcat values and the thiophilicity of the metal ion substitutions vary in parallel and are consistent with a kinetically significant interaction between the leaving group and the active-site metal center during turnover. X-ray absorption spectroscopy confirms that dicobalt substitution does not result in large structural perturbations at the active-site. Finally, substitution of the dinuclear metal site with Cd2+ results in a greatly enhanced catalyst that can hydrolyze AHLs 1600 to 24000-fold faster than other reported quorum-quenching enzymes.