Influence of pH on indole-dependent heterodimeric interactions between Anopheles gambiae odorant-binding proteins OBP1 and OBP4.

Insect Odorant Binding Proteins (OBPs) constitute important components of their olfactory apparatus, as they are essential for odor recognition. OBPs undergo conformational changes upon pH change, altering their interactions with odorants. Moreover, they can form heterodimers with novel binding characteristics. Anopheles gambiae OBP1 and OBP4 were found capable of forming heterodimers possibly involved in the specific perception of the attractant indole. In order to understand how these OBPs interact in the presence of indole and to investigate the likelihood of a pH-dependent heterodimerization mechanism, the crystal structures of OBP4 at pH 4.6 and 8.5 were determined. Structural comparison to each other and with the OBP4-indole complex (3Q8I, pH 6.85) revealed a flexible N-terminus and conformational changes in the α4-loop-α5 region at acidic pH. Fluorescence competition assays showed a weak binding of indole to OBP4 that becomes further impaired at acidic pH. Additional Molecular Dynamic and Differential Scanning Calorimetry studies displayed that the influence of pH on OBP4 stability is significant compared to the modest effect of indole. Furthermore, OBP1-OBP4 heterodimeric models were generated at pH 4.5, 6.5, and 8.5, and compared concerning their interface energy and cross-correlated motions in the absence and presence of indole. The results indicate that the increase in pH may induce the stabilization of OBP4 by increasing its helicity, thereby enabling indole binding at neutral pH that further stabilizes the protein and possibly promotes the creation of a binding site for OBP1. A decrease in interface stability and loss of correlated motions upon transition to acidic pH may provoke the heterodimeric dissociation allowing indole release. Finally, we propose a potential OBP1-OBP4 heterodimer formation/disruption mechanism induced by pH change and indole binding.

2D finger-printing and molecular docking studies identified potent mosquito repellents targeting odorant binding protein 1.

Personal protection measures against the mosquitoes like the use of repellents constitute valuable tools in the effort to prevent the transmission of vector-borne diseases. Therefore, the discovery of novel repellent molecules which will be effective at lower concentrations and provide a longer duration of protection remains an urgent need. Mosquito Odorant-Binding Proteins (OBPs) involved in the initial steps of the olfactory signal transduction cascade have been recognized not only as passive carriers of odors and pheromones but also as the first molecular filter to discriminate semiochemicals, hence serving as molecular targets for the design of novel pest control agents. Among the three-dimensional structures of mosquito OBPs solved in the last decades, the OBP1 complexes with known repellents have been widely used as reference structures in docking analysis and molecular dynamics simulation studies for the structure-based discovery of new molecules with repellent activity. Herein, ten compounds known to be active against mosquitoes and/or displaying a binding affinity for Anopheles gambiae AgamOBP1 were used as queries in an in silico screening of over 96 million chemical samples in order to detect molecules with structural similarity. Further filtering of the acquired hits on the basis of toxicity, vapor pressure, and commercial availability resulted in 120 unique molecules that were subjected to molecular docking studies against OBP1. For seventeen potential OBP1-binders, the free energy of binding (FEB) and mode of interaction with the protein were further estimated by molecular docking simulations leading to the selection of eight molecules exhibiting the highest similarity with their parental compounds and favorable energy values. The in vitro determination of their binding affinity to AgamOBP1 and the evaluation of their repellent activity against female Aedes albopictus mosquitoes revealed that our combined ligand similarity screening and OBP1 structure-based molecular docking successfully detected three molecules with enhanced repellent properties. A novel DEET-like repellent with lower volatility (8.55 × 10-4 mmHg) but a higher binding affinity for OBP1 than DEET (1.35 × 10-3 mmHg). A highly active repellent molecule that is predicted to bind to the secondary Icaridin (sIC)-binding site of OBP1 with higher affinity than to the DEET-site and, therefore, represents a new scaffold to be exploited for the discovery of binders targeting multiple OBP sites. Finally, a third potent repellent exhibiting a high degree of volatility was found to be a strong DEET-site binder of OBP1 that could be used in slow-release formulations.

The structure of AgamOBP5 in complex with the natural insect repellents Carvacrol and Thymol: Crystallographic, fluorescence and thermodynamic binding studies.

Among several proteins participating in the olfactory perception process of insects, Odorant Binding Proteins (OBPs) are today considered valid targets for the discovery of compounds that interfere with their host-detection behavior. The 3D structures of Anopheles gambiae mosquito AgamOBP1 in complex with the known synthetic repellents DEET and Icaridin have provided valuable information on the structural characteristics that govern their selective binding. However, no structure of a plant-derived repellent bound to an OBP has been available until now. Herein, we present the novel three-dimensional crystal structures of AgamOBP5 in complex with two natural phenolic monoterpenoid repellents, Carvacrol and Thymol, and the MPD molecule. Structural analysis revealed that both monoterpenoids occupy a binding site (Site-1) by adopting two alternative conformations. An additional Carvacrol was also bound to a secondary site (Site-2) near the central cavity entrance. A protein-ligand hydrogen-bond network supplemented by van der Waals interactions spans the entire binding cavity, bridging α4, α6, and α3 helices and stabilizing the overall structure. Fluorescence competition and Differential Scanning Calorimetry experiments verified the presence of two binding sites and the stabilization effect on AgamOBP5. While Carvacrol and Thymol bind to Site-1 with equal affinity in the submicromolar range, they exhibit a significantly lower and distinct binding capacity for Site-2 with Kd's of ~7 µΜ and ~18 µΜ, respectively. Finally, a comparison of AgamOBP5 complexes with the AgamOBP4-Indole structure revealed that variations of ligand-interacting aminoacids such as A109T, I72M, A112L, and A105T cause two structurally similar and homologous proteins to display different binding specificities.

A Combined Computational Methodology for the Discovery of Hit Compounds with Putative Insect Repellency Properties.

Mosquitoes and other hematophagous arthropods, the primary vectors of multiple parasites and viruses, are responsible for the transmission of serious diseases to humans. Nowadays, the interest is focused on the development of novel repellents to the existing ones with advanced properties. The present study attempts the discovery of novel hit compounds which may evolve as insect repellents using a combined computational methodology targeting the Odorant Binding Protein 1 (OBP1). The in silico results indicated two compounds, namely coniferyl alcohol and 1,2-diphenyl-2-propanol, which were further evaluated (a) in vitro for their binding affinity to AgamOBP1 and (b) in vivo using dose-dependent repellence tests against the aggressive-day biting Aedes albopictus. The combination of in vitro and in vivo results pointed that coniferyl alcohol and 1,2-diphenyl-2-propanol exhibited high binding affinity over OBP1 with 69.4 and 84.7 nM, respectively as well as efficient repellent activity. Compounds were also tested for their dose-dependent repellency activity in vivo against Aedes albopictus. Overall, the selected compounds can serve as scaffolds for the development of novel repellents.

Behavioural response of Culex pipiens biotype molestus to oviposition pheromone.

Insect behaviour relies on an olfactory sensory system that controls a range of activities, from food choice and mating to oviposition, where pheromones play a central role. In Culex mosquitoes, egg-laying is accompanied by the release of mosquito oviposition pheromone (MOP), which has been shown to affect the oviposition behaviour of conspecifics. Here, we investigated for the first time the effect of MOP on the oviposition rate of Culex pipiens biotype molestus, examining separately males and females, before and after mating and oviposition. Our results demonstrate that MOP is more likely to act as an oviposition stimulant rather than an attractant, since more gravid females laid eggs in its presence, while the number of male or female mosquitoes (virgin or mated) captured on pheromone-treated pots was similar to those treated with control water.

Glycogen phosphorylase revisited: extending the resolution of the R- and T-state structures of the free enzyme and in complex with allosteric activators.

The crystal structures of free T-state and R-state glycogen phosphorylase (GP) and of R-state GP in complex with the allosteric activators IMP and AMP are reported at improved resolution. GP is a validated pharmaceutical target for the development of antihyperglycaemic agents, and the reported structures may have a significant impact on structure-based drug-design efforts. Comparisons with previously reported structures at lower resolution reveal the detailed conformation of important structural features in the allosteric transition of GP from the T-state to the R-state. The conformation of the N-terminal segment (residues 7-17), the position of which was not located in previous T-state structures, was revealed to form an α-helix (now termed α0). The conformation of this segment (which contains Ser14, phosphorylation of which leads to the activation of GP) is significantly different between the T-state and the R-state, pointing in opposite directions. In the T-state it is packed between helices α4 and α16 (residues 104-115 and 497-508, respectively), while in the R-state it is packed against helix α1 (residues 22'-38') and towards the loop connecting helices α4' and α5' of the neighbouring subunit. The allosteric binding site where AMP and IMP bind is formed by the ordering of a loop (residues 313-326) which is disordered in the free structure, and adopts a conformation dictated mainly by the type of nucleotide that binds at this site.

The architecture of hydrogen and sulfur σ-hole interactions explain differences in the inhibitory potency of C-ß-d-glucopyranosyl thiazoles, imidazoles and an N-ß-d glucopyranosyl tetrazole for human liver glycogen phosphorylase and offer new insights to structure-based design.

C-Glucopyranosyl imidazoles, thiazoles, and an N-glucopyranosyl tetrazole were assessed in vitro and ex vivo for their inhibitory efficiency against isoforms of glycogen phosphorylase (GP; a validated pharmacological target for the development of anti-hyperglycaemic agents). Imidazoles proved to be more potent inhibitors than the corresponding thiazoles or the tetrazole. The most potent derivative has a 2-naphthyl substituent, a Ki value of 3.2 µM for hepatic glycogen phosphorylase, displaying also 60% inhibition of GP activity in HepG2 cells, compared to control vehicle treated cells, at 100 µM. X-Ray crystallography studies of the protein - inhibitor complexes revealed the importance of the architecture of inhibitor associated hydrogen bonds or sulfur σ-hole bond interactions to Asn284 OD1, offering new insights to structure-based design efforts. Moreover, while the 2-glucopyranosyl-tetrazole seems to bind differently from the corresponding 1,2,3-triazole compound, the two inhibitors are equipotent.

Glucopyranosylidene-spiro-imidazolinones, a New Ring System: Synthesis and Evaluation as Glycogen Phosphorylase Inhibitors by Enzyme Kinetics and X-ray Crystallography.

Epimeric series of aryl-substituted glucopyranosylidene-spiro-imidazolinones, an unprecedented new ring system, were synthesized from the corresponding Schiff bases of O-perbenzoylated (gluculopyranosylamine)onamides by intramolecular ring closure of the aldimine moieties with the carboxamide group elicited by N-bromosuccinimide in pyridine. Test compounds were obtained by Zemplén O-debenzoylation. Stereochemistry and ring tautomers of the new compounds were investigated by NMR, time-dependent density functional theory (TDDFT)-electronic circular dichroism, and DFT-NMR methods. Kinetic studies with rabbit muscle and human liver glycogen phosphorylases showed that the (R)-imidazolinones were 14-216 times more potent than the (S) epimers. The 2-naphthyl-substituted (R)-imidazolinone was the best inhibitor of the human enzyme (Ki 1.7 µM) and also acted on HepG2 cells (IC50 177 µM). X-ray crystallography revealed that only the (R) epimers bound in the crystal. Their inhibitory efficacy is based on the hydrogen-bonding interactions of the carbonyl oxygen and the NH of the imidazolinone ring.

High Consistency of Structure-Based Design and X-Ray Crystallography: Design, Synthesis, Kinetic Evaluation and Crystallographic Binding Mode Determination of Biphenyl-N-acyl-ß-d-Glucopyranosylamines as Glycogen Phosphorylase Inhibitors.

Structure-based design and synthesis of two biphenyl-N-acyl-ß-d-glucopyranosylamine derivatives as well as their assessment as inhibitors of human liver glycogen phosphorylase (hlGPa, a pharmaceutical target for type 2 diabetes) is presented. X-ray crystallography revealed the importance of structural water molecules and that the inhibitory efficacy correlates with the degree of disturbance caused by the inhibitor binding to a loop crucial for the catalytic mechanism. The in silico-derived models of the binding mode generated during the design process corresponded very well with the crystallographic data.

Identification of novel bioinspired synthetic mosquito repellents by combined ligand-based screening and OBP-structure-based molecular docking.

In this work we report a fast and efficient virtual screening protocol for discovery of novel bioinspired synthetic mosquito repellents with lower volatility and, in all likelihood, increased protection time as compared with their plant-derived parental compounds. Our screening protocol comprises two filtering steps. The first filter is based on the shape and chemical similarity to known plant-derived repellents, whereas the second filter is based on the predicted similarity of the ligand's binding mode to the Anopheles gambiae odorant binding protein (AgamOBP1) relative to that of DEET and Icaridin to the same OBP. Using this protocol, a chemical library containing 42,755 synthetic molecules was screened in silico and sixteen selected compounds were tested for their affinity to AgamOBP1 in vitro and repellence against A. gambiae female mosquitoes using a warm-body repellent assay. One of them showed DEET-like repellence (91%) but with significantly lower volatility (2.84 × 10-6 mmHg) than either DEET (1.35 × 10-3 mmHg) or its parental cuminic acid (3.08 × 10-3 mmHg), and four other compounds were found to exhibit repellent indices between 69 and 79%. Overall, a correlation was not evident between repellence and OBP-binding strength. In contrast, a correlation between binding mode and repellence was found.

Nanomolar Inhibitors of Glycogen Phosphorylase Based on ß-d-Glucosaminyl Heterocycles: A Combined Synthetic, Enzyme Kinetic, and Protein Crystallography Study.

Aryl substituted 1-(ß-d-glucosaminyl)-1,2,3-triazoles as well as C-ß-d-glucosaminyl 1,2,4-triazoles and imidazoles were synthesized and tested as inhibitors against muscle and liver isoforms of glycogen phosphorylase (GP). While the N-ß-d-glucosaminyl 1,2,3-triazoles showed weak or no inhibition, the C-ß-d-glucosaminyl derivatives had potent activity, and the best inhibitor was the 2-(ß-d-glucosaminyl)-4(5)-(2-naphthyl)-imidazole with a Ki value of 143 nM against human liver GPa. An X-ray crystallography study of the rabbit muscle GPb inhibitor complexes revealed structural features of the strong binding and offered an explanation for the differences in inhibitory potency between glucosyl and glucosaminyl derivatives and also for the differences between imidazole and 1,2,4-triazole analogues.

The crystal structure of the AgamOBP1•Icaridin complex reveals alternative binding modes and stereo-selective repellent recognition.

Anopheles gambiae Odorant Binding Protein 1 in complex with the most widely used insect repellent DEET, was the first reported crystal structure of an olfactory macromolecule with a repellent, and paved the way for OBP1-structure-based approaches for discovery of new host-seeking disruptors. In this work, we performed STD-NMR experiments to directly monitor and verify the formation of a complex between AgamOBP1 and Icaridin, an efficient DEET alternative. Furthermore, Isothermal Titration Calorimetry experiments provided evidence for two Icaridin-binding sites with different affinities (Kd = 0.034 and 0.714 mM) and thermodynamic profiles of ligand binding. To elucidate the binding mode of Icaridin, the crystal structure of AgamOBP1•Icaridin complex was determined at 1.75 Å resolution. We found that Icaridin binds to the DEET-binding site in two distinct orientations and also to a novel binding site located at the C-terminal region. Importantly, only the most active 1R,2S-isomer of Icaridin's equimolar diastereoisomeric mixture binds to the AgamOBP1 crystal, providing structural evidence for the possible contribution of OBP1 to the stereoselectivity of Icaridin perception in mosquitoes. Structural analysis revealed two ensembles of conformations differing mainly in spatial arrangement of their sec-butyl moieties. Moreover, structural comparison with DEET indicates a common recognition mechanism for these structurally related repellents. Ligand interactions with both sites and binding modes were further confirmed by 2D 1H-15N HSQC NMR spectroscopy. The identification of a novel repellent-binding site in AgamOBP1 and the observed structural conservation and stereoselectivity of its DEET/Icaridin-binding sites open new perspectives for the OBP1-structure-based discovery of next-generation insect repellents.

Structure based inhibitor design targeting glycogen phosphorylase B. Virtual screening, synthesis, biochemical and biological assessment of novel N-acyl-ß-d-glucopyranosylamines.

Glycogen phosphorylase (GP) is a validated target for the development of new type 2 diabetes treatments. Exploiting the Zinc docking database, we report the in silico screening of 1888 N-acyl-ß-d-glucopyranosylamines putative GP inhibitors differing only in their R groups. CombiGlide and GOLD docking programs with different scoring functions were employed with the best performing methods combined in a 'consensus scoring' approach to ranking of ligand binding affinities for the active site. Six selected candidates from the screening were then synthesized and their inhibitory potency was assessed both in vitro and ex vivo. Their inhibition constants' values, in vitro, ranged from 5 to 377µM while two of them were effective at causing inactivation of GP in rat hepatocytes at low µM concentrations. The crystal structures of GP in complex with the inhibitors were defined and provided the structural basis for their inhibitory potency and data for further structure based design of more potent inhibitors.

Glucopyranosylidene-spiro-iminothiazolidinone, a new bicyclic ring system: synthesis, derivatization, and evaluation for inhibition of glycogen phosphorylase by enzyme kinetic and crystallographic methods.

The reaction of thiourea with O-perbenzoylated C-(1-bromo-1-deoxy-ß-D-glucopyranosyl)formamide gave the new anomeric spirocycle 1R-1,5-anhydro-D-glucitol-spiro-[1,5]-2-imino-1,3-thiazolidin-4-one. Acylation and sulfonylation with the corresponding acyl chlorides (RCOCl or RSO2Cl where R=tBu, Ph, 4-Me-C6H4, 1- and 2-naphthyl) produced the corresponding 2-acylimino- and 2-sulfonylimino-thiazolidinones, respectively. Alkylation by MeI, allyl-bromide and BnBr produced mixtures of the respective N-alkylimino- and N,N'-dialkyl-imino-thiazolidinones, while reactions with 1,2-dibromoethane and 1,3-dibromopropane furnished spirocyclic 5,6-dihydro-imidazo[2,1-b]thiazolidin-3-one and 6,7-dihydro-5H-thiazolidino[3,2-a]pyrimidin-3-one, respectively. Removal of the O-benzoyl protecting groups by the Zemplén protocol led to test compounds most of which proved micromolar inhibitors of rabbit muscle glycogen phosphorylase b (RMGPb). Best inhibitors were the 2-benzoylimino- (Ki=9µM) and the 2-naphthoylimino-thiazolidinones (Ki=10 µM). Crystallographic studies of the unsubstituted spiro-thiazolidinone and the above most efficient inhibitors in complex with RMGPb confirmed the preference and inhibitory effect that aromatic (and especially 2-naphthyl) derivatives show for the catalytic site promoting the inactive conformation of the enzyme.

Crystal and solution studies of the "Plus-C" odorant-binding protein 48 from Anopheles gambiae: control of binding specificity through three-dimensional domain swapping.

Much physiological and behavioral evidence has been provided suggesting that insect odorant-binding proteins (OBPs) are indispensable for odorant recognition and thus are appealing targets for structure-based discovery and design of novel host-seeking disruptors. Despite the fact that more than 60 putative OBP-encoding genes have been identified in the malaria vector Anopheles gambiae, the crystal structures of only six of them are known. It is therefore clear that OBP structure determination constitutes the bottleneck for structure-based approaches to mosquito repellent/attractant discovery. Here, we describe the three-dimensional structure of an A. gambiae "Plus-C" group OBP (AgamOBP48), which exhibits the second highest expression levels in female antennae. This structure represents the first example of a three-dimensional domain-swapped dimer in dipteran species. A combined binding site is formed at the dimer interface by equal contribution of each monomer. Structural comparisons with the monomeric AgamOBP47 revealed that the major structural difference between the two Plus-C proteins localizes in their N- and C-terminal regions, and their concerted conformational change may account for monomer-swapped dimer conversion and furthermore the formation of novel binding pockets. Using a combination of gel filtration chromatography, differential scanning calorimetry, and analytical ultracentrifugation, we demonstrate the AgamOBP48 dimerization in solution. Eventually, molecular modeling calculations were used to predict the binding mode of the most potent synthetic ligand of AgamOBP48 known so far, discovered by ligand- and structure-based virtual screening. The structure-aided identification of multiple OBP binders represents a powerful tool to be employed in the effort to control transmission of the vector-borne diseases.

Sourcing the affinity of flavonoids for the glycogen phosphorylase inhibitor site via crystallography, kinetics and QM/MM-PBSA binding studies: comparison of chrysin and flavopiridol.

Flavonoids have been discovered as novel inhibitors of glycogen phosphorylase (GP), a target to control hyperglycemia in type 2 diabetes. To elucidate the mechanism of inhibition, we have determined the crystal structure of the GPb-chrysin complex at 1.9 Å resolution. Chrysin is accommodated at the inhibitor site intercalating between the aromatic side chains of Phe285 and Tyr613 through π-stacking interactions. Chrysin binds to GPb approximately 15 times weaker (Ki=19.01 µM) than flavopiridol (Ki=1.24 µM), exclusively at the inhibitor site, and both inhibitors display similar behavior with respect to AMP. To identify the source of flavopiridols' stronger affinity, molecular docking with Glide and postdocking binding free energy calculations using QM/MM-PBSA have been performed and compared. Whereas docking failed to correctly rank inhibitor binding conformations, the QM/MM-PBSA method employing M06-2X/6-31+G to model the π-stacking interactions correctly reproduced the experimental results. Flavopiridols' greater binding affinity is sourced to favorable interactions of the cationic 4-hydroxypiperidin-1-yl substituent with GPb, with desolvation effects limited by the substituent conformation adopted in the crystallographic complex. Further successful predictions using QM/MM-PBSA for the flavonoid quercetagetin (which binds at the allosteric site) leads us to propose the methodology as a useful and inexpensive tool to predict flavonoid binding.

N-(4-Substituted-benzoyl)-N'-(ß-d-glucopyranosyl)ureas as inhibitors of glycogen phosphorylase: Synthesis and evaluation by kinetic, crystallographic, and molecular modelling methods.

N-(4-Substituted-benzoyl)-N'-(ß-d-glucopyranosyl) ureas (substituents: Me, Ph, Cl, OH, OMe, NO(2), NH(2), COOH, and COOMe) were synthesised by ZnCl(2) catalysed acylation of O-peracetylated ß-d-glucopyranosyl urea as well as in reactions of O-peracetylated or O-unprotected glucopyranosylamines and acyl-isocyanates. O-deprotections were carried out by base or acid catalysed transesterifications where necessary. Kinetic studies revealed that most of these compounds were low micromolar inhibitors of rabbit muscle glycogen phosphorylase b (RMGPb). The best inhibitor was the 4-methylbenzoyl compound (K(i)=2.3µM). Crystallographic analyses of complexes of several of the compounds with RMGPb showed that the analogues exploited, together with water molecules, the available space at the ß-pocket subsite and induced a more extended shift of the 280s loop compared to RMGPb in complex with the unsubstituted benzoyl urea. The results suggest the key role of the water molecules in ligand binding and structure-based ligand design. Molecular docking study of selected inhibitors was done to show the ability of the binding affinity prediction. The binding affinity of the highest scored docked poses was calculated and correlated with experimentally measured K(i) values. Results show that correlation is high with the R-squared (R(2)) coefficient over 0.9.

The σ-hole phenomenon of halogen atoms forms the structural basis of the strong inhibitory potency of C5 halogen substituted glucopyranosyl nucleosides towards glycogen phosphorylase b.

C5 halogen substituted glucopyranosyl nucleosides (1-(ß-D-glucopyranosyl)-5-X-uracil; X=Cl, Br, I) have been discovered as some of the most potent active site inhibitors of glycogen phosphorylase (GP), with respective K(i) values of 1.02, 3.27, and 1.94 µM. The ability of the halogen atom to form intermolecular electrostatic interactions through the σ-hole phenomenon rather than through steric effects alone forms the structural basis of their improved inhibitory potential relative to the unsubstituted 1-(ß-D-glucopyranosyl)uracil (K(i) =12.39 µM), as revealed by X-ray crystallography and modeling calculations exploiting quantum mechanics methods. Good agreement was obtained between kinetics results and relative binding affinities calculated by QM/MM-PBSA methodology for various substitutions at C5. Ex vivo experiments demonstrated that the most potent derivative (X=Cl) toward purified GP has no cytotoxicity and moderate inhibitory potency at the cellular level. In accordance, ADMET property predictions were performed, and suggest decreased polar surface areas as a potential means of improving activity in the cell.

hCINAP is an atypical mammalian nuclear adenylate kinase with an ATPase motif: structural and functional studies.

Human coilin interacting nuclear ATPase protein (hCINAP) directly interacts with coilin, a marker protein of Cajal Bodies (CBs), nuclear organelles involved in the maturation of small nuclear ribonucleoproteins UsnRNPs and snoRNPs. hCINAP has previously been designated as an adenylate kinase (AK6), but is very atypical as it exhibits unusually broad substrate specificity, structural features characteristic of ATPase/GTPase proteins (Walker motifs A and B) and also intrinsic ATPase activity. Despite its intriguing structure, unique properties and cellular localization, the enzymatic mechanism and biological function of hCINAP have remained poorly characterized. Here, we offer the first high-resolution structure of hCINAP in complex with the substrate ADP (and dADP), the structure of hCINAP with a sulfate ion bound at the AMP binding site, and the structure of the ternary complex hCINAP-Mg(2+) ADP-Pi. Induced fit docking calculations are used to predict the structure of the hCINAP-Mg(2+) ATP-AMP ternary complex. Structural analysis suggested a functional role for His79 in the Walker B motif. Kinetic analysis of mutant hCINAP-H79G indicates that His79 affects both AK and ATPase catalytic efficiency and induces homodimer formation. Finally, we show that in vivo expression of hCINAP-H79G in human cells is toxic and drastically deregulates the number and appearance of CBs in the cell nucleus. Our findings suggest that hCINAP may not simply regulate nucleotide homeostasis, but may have broader functionality, including control of CB assembly and disassembly in the nucleus of human cells.

Halogen-substituted (C-ß-D-glucopyranosyl)-hydroquinone regioisomers: synthesis, enzymatic evaluation and their binding to glycogen phosphorylase.

Electrophilic halogenation of C-(2,3,4,6-tetra-O-acetyl-ß-D-glucopyranosyl) 1,4-dimethoxybenzene (1) afforded regioselectively products halogenated at the para position to the D-glucosyl moiety (8, 9) that were deacetylated to 3 (chloride) and 16 (bromide). For preparing meta regioisomers, 1 was efficiently oxidized with CAN to afford C-(2,3,4,6-tetra-O-acetyl-ß-D-glucopyranosyl) 1,4-benzoquinone 2 which, in either MeOH or H(2)O-THF containing few equivalents of AcCl, added hydrochloric acid to produce predominantly meta (with respect to the sugar moiety) chlorinated hydroquinone derivatives 5 and 18, this latter being deacetylated to 4. The deacetylated meta (4, 5) or para (3, 16) halohydroquinones were evaluated as inhibitors of glycogen phosphorylase (GP, a molecular target for inhibition of hepatic glycogenolysis under high glucose concentrations) by kinetics and X-ray crystallography. These compounds are competitive inhibitors of GPb with respect to α-D-glucose-1-phosphate. The measured IC(50) values (µM) [169.9±10.0 (3), 95 (4), 39.8±0.3 (5) 136.4±4.9 (16)] showed that the meta halogenated inhibitors (4, 5) are more potent than their para analogs (3, 16). The crystal structures of GPb in complex with these compounds at high resolution (1.97-2.05 Å) revealed that the inhibitors are accommodated at the catalytic site and stabilize the T conformation of the enzyme. The differences in their inhibitory potency can be interpreted in terms of variations in the interactions with protein residues of the different substituents on the aromatic part of the inhibitors.