Binding of p-substituted-phenyl glycosides to the lectins from the pea and the lentil

The effect of p-substitution on the behaviour of phenyl α-D-mannopyranosides as specific ligands for the lectins from Pisum sativum (pea) and Lens culinaris (lentil) is reported. The binding affinities are related to the electronic properties of the substituents, expressed as the Hammett substituent constant (p = ?0.4). Inductive and mesomeric contributions are best considered separately. Neither steric hindrance nor hydrophobic binding of the substituents is observed. For the binding of the corresponding α-D-glucopyranosides to the Pisum sativum lectin, the same effect is maintained (p = ?0.4). In a series of p-substituted-phenyl β-D-glucopyranosides, the nature of the substituent has almost no effect on the binding to the Pisum sativum lectin.     

The methanolysis of some derivatives of 2,3,4-tri-O-benzyl-α-D-glucopyranosyl bromide in the presence and absence of silver salts

In contrast to reactions with high concentration, reactions of several derivatives of 2,3,4-tri-0-benzyl-α-D-glucopyranosyl bromide with low concentrations of methanol gave mainly the α-D-glucosides regardless of the structure of the C-6 substituent. Methanolysis of the same α-D-glucosyl bromides or the corresponding chlorides in the presence of silver tetrafluoroborate or hexafluorophosphate at —78° gave mainly the β-D-glucosides. The use of these silver salts led to side reactions, particularly when the glucosyl halide had an acyl blocking group at C-6. The side reactions were minimized when silver trifluoromethanesulfonate (triflate) was used. The relative amounts of α- and β-D-glucosides produced in the presence of silver triflate depended on the structure of the C-6 substituent and the solvent polarity. A rapid methanolysis of 2,3,4-tri-0-benzyl-6-0-(N-phenylcarbamoyl)-α-D-glucopyranosyl bromide with silver triflate in ether at —78° gave a high proportion of the methyl α-D-glucoside.The results of direct methanolysis seem to be due to competitive methanolysis of the anomeric bromides and a p?ush-pull$\text{\$}\text{?}$mechanism is postulated in the presence of silver tetrafluoroborate or hexafluorophosphate. Glucosyl triflate intermediates are proposed for the silver triflate-assisted methanolyses.     

Alcohol Induced Reversal of Enantioselectivity in a Lipase Catalyzed Resolution of 2-Chloropropionic Acid

Alcohol induced reversal of enantioselectivity in the esterification of 2-chloropropionic acid using lipase from Candida cylindracea has been investigated. It was found that an alcohol having substituents both at the α- and the β-carbon preferentially esterified the S-acid, while a straight chain alcohol preferentially esterified the R-acid. Intermediate enantioselectivities were obtained with alcohols having substituents either at the α- or the β-carbon, but still in favor of the R-acid. An acyl binding domain composed of three subsites is proposed for this lipase; one for the hydrocarbon chain, a second for a methyl substituent and a third for an electronegative substituent.     

Mutual Protein-Ligand Conformational Selection Drives cGMP vs. cAMP Selectivity in Protein Kinase G

Protein kinase G (PKG) is a major receptor of cGMP, and controls signaling pathways distinct from those regulated by cAMP. However, the contributions of the two substituents that differentiate cGMP from cAMP (i.e. 6-oxo and 2-NH₂) to the cGMP-versus-cAMP selectivity of PKG remain unclear. Here, using NMR to map how binding affinity and dynamics of the protein and ligand vary along a ligand double-substitution cycle, we show that the contributions of the two substituents to binding affinity are surprisingly non-additive. Such non-additivity stems primarily from mutual protein–ligand conformational selection, whereby not only does the ligand select for a preferred protein conformation upon binding, but also, the protein selects for a preferred ligand conformation. The 6-oxo substituent mainly controls the conformational equilibrium of the bound protein, while the 2-NH₂ substituent primarily controls the conformational equilibrium of the unbound ligand (i.e. syn versus anti). Therefore, understanding the conformational dynamics of both the protein and ligand is essential to explain the cGMP-versus-cAMP selectivity of PKG.     

Stereoselectivity and reactivity of α-chymotrypsin modified at methionine-192

Native α-chymotrypsin (N-Chtr) and two modified forms, methionine-192 sulfoxide (O-Chtr) and methionine-192-S-(N-2-carboxyisopropyl)carbamylmethyl (Al-Chtr) α-chy-motrypsin have been compared in hydrolysis of methyl β-phenylpropionate and its d-and l-α-substituted derivatives, in which substituents are Cl, OH, OCOCH₃, NHCHO, and NHCOCH₃. In general, both modifications lower the reactivity of the enzymes in terms of k_cat and K_m(app); the decrease is greater toward l than toward d-enantiomers, and stereoselectivity is decreased. It is proposed that the modifications change the methionine-192 side chain from hydrophobic to hydrophilic, bring it into solution, and enlarge the Met-192-Ser-214 passage in which α-substituents fit. This leads to weaker binding, allows more freedom of motion of substrates and decreases reactivity, while allowing easier access of d-α-substituents. The large modifying substituent of Al-Chtr counters this effect by steric interaction with the large d-α-acetoxy and d-α-acylamido substituents, while the small polar modification in O-Chtr favors hydrolysis of these d-substrates.     

α-D-Mannosidase-catalyzed hydrolysis of substituted phenyl α-D-mannopyranosides

The influence substituents on the hydrolysis of substituted phenyl α-D-mannopyranosides by α-D-mannosidase from Medicago sativa L. has been investigated. As indicated by structure-activity relations, the electronic effect of the substituent has an influence on the rate of formation of the intermediate mannosyl-enzyme complex. This effect depends not only on the nature of the substituent, but also on its position (meta or para) and on the temperature of the experiment. Hammett-type linear free energy relationships show that the reaction constant p changes its sign at ～27°. Substrates with strong electron-withdrawing groups show values of log V that are linearly related to 1/T, whereas the Arrhenius plots for other substrates are severely curved. This complex behaviour is tentatively explained by assuming that some meta-substituents have an unusual, temperature- and substituent-dependent influence on the formation of the Michaelis—Menten complex.     

Immunochemical studies on the combining site of the d-galactose/N-acetyl-d-galactosamine specific lectin from Erythrina cristagalli seeds

The combining site of the Erythrina cristagalli lectin was studied by quantitative precipitin and precipitin inhibition assays. The lectin precipitated best with two fractions of a precursor human ovarian cyst blood group substance with I and i activities. A₁, A₂, B, H, Le^a, and Le^b blood group substances precipitated poorly to moderately and substances of the same blood group activity precipitated to varying extents. These differences are attributable to heterogeneity resulting from incomplete biosynthesis of carbohydrate chains. Specific precipitates with the poorly reactive blood group substances were found to be more soluble than those reacting strongly. Precipitation was minimally affected by EDTA or divalent cations. Among the monosaccharides and glycosides tested for inhibition of precipitation, p-nitrophenyl βdGal was most active and was 10 times more active than methyl βdGal, indicating involvement of hydrophobic contacts in site specificity. Methyl αdGalNAc, p-nitrophenyl αdGalNAc, methyl αdGal, N-acetyl-d-galactosamine, p-nitrophenyl αdGal, methyl βdGal, and p-nitrophenyl βdGalNAc were progressively less active than p-nitrophenyl βdGal. The best disaccharide inhibitor dGalβ1 → 4dGlcNAc was 7.5 times more potent than p-nitrophenyl βdGal. A tetraantennary and triantennary oligosaccharide containing four and three dGalβ1 → 4dGlcNAcβ1 → branches, respectively, were, because of cooperative binding effects, 1.6 and 2.5 times more active than the bi- and monoantennary oligosaccharides, respectively. dGalβ1 → 4dGlcNAcβ1 → 6dGal and dGalβ1 → 4dGlcNAcβ1 → 2dMan had the same activity, being 1.5 times more active than dGalβ1 → 4dGlcNAc, which was 2.6 and 8.5 times more active than dGalβ1 → 3dGlcNAc and dGalβ1 → 3dGlc, respectively. Substitutions by N-acetyl-d-galactos-amine or l-fucose on the d-galactose of inhibitory compounds blocked activity. These results suggest that a hydrophobic interaction with the subterminal sugar is important in the binding and that the specificity of the lectin combining site involves a terminal dGalβ1 → 4dGlcNAc and the β linkage of a third sugar.     

Reductive Biotransformation of Benzaldehyde Derivatives by Baker's Yeast in Non-Conventional Media: Effect of Substrate Hydrophobicity on the Biocatalytic Reaction

The reduction of six derivatives of benzaldehyde, α,α,α-trifluorotolualdehyde, nitrobenzaldehyde, anisaldehyde, fluorobenzaldehyde, tolualdehyde, chlorobenzaldehyde, each of which was substituted in the ortho-, meta- and para-positions, was investigated in whole cell yeast biotransformations conducted in non-conventional media. Correlation between hydrophobic (π) and electronic (σ) parameters of the substituent of the substrate and biocatalytic activity in organic solvent media (hexane containing 2% v/v water) was evaluated. While catalytic activity in general decreased as the substituent hydrophobicity (π) increased, the trend was more pronounced for ortho- substituents compared to meta- and para-. When the electronic parameters (σ) of the meta- and para- substituents were correlated with the catalytic activity, the opposite was observed, namely the catalytic activity increased as the electronic parameter of the substituent increased. This observation was similar for meta- and para- substituents.

Preliminary studies on the relationships between solvent and substrate polarity on the one hand and catalytic activity on the other are also presented and discussed.     

Immunochemical studies on the reactivities and combining sites of the d-galactopyranose- and 2-acetamido-2-deoxy-d-galactopyranose-specific lectin purified from Sophora japonica seeds

Sophora japonica lectin agglutinates human B erythrocytes strongly and A₁ erythrocytes weakly. Bivalent metal ions such as Ca²⁺, Mn²⁺, or Mg²⁺ were shown to be essential for hemagglutinating and precipitating activities. At optimal concentrations of bivalent metal ions, hemagglutinating activity was highest between pH 8.5 and 9.0 and decreased sharply below pH 8.5, whereas precipitating capacity was maximal between pH 6.7 and 9.5. The combining site of the S. japonica lectin was explored by quantitative precipitin and precipitin inhibition assays. This lectin showed substantial differences in precipitation with several blood group B substances ascribable to heterogeneity resulting from incomplete biosynthesis of their carbohydrate side chains. The lectin precipitated moderately well with A₁ substance and precursor blood group I fractions (OG). It precipitated weakly or not at all with A₂, H, or Le^a substances. In inhibition assays, glycosides of dGalNAc were about five to six times better than those of dGal; dGalNAc itself was about six times better than dGal. Nitrophenyl glycosides were all substantially better than the methyl glycosides, indicating a hydrophobic contribution to the site subterminal to the nonreducing moiety. Although nitrophenyl β-glycosides were much better than the corresponding α-glycosides, the methyl α-and βDGalNAcp were equal in activity as were methyl α- and βDGalp. Among the oligosaccharides tested, the β-linked N-tosyl-l-serine glycoside of dGalβ1 → 3dGalNAc was best and was as active as p-nitrophenyl βDGalNAcp, whereas dGalβ1 → 3dGalNAc α-N-tosyl serine and the nitrophenyl and phenyl α-glycosides of dGalβ1 → 3dGalNAc were much less active, suggesting that the hydrophobic moiety and/or a subterminal dGalNAc β-linked and substituted on carbon 3 play an important role in binding and that a β-linked glycoside of dGalβ1 → 3dGalNAc may be an essential requirement for binding. The results of inhibition studies with other oligosaccharides indicate that a subterminal dGlcNAc substituted on carbon 3 or 4 by dGalβ may contribute somewhat to binding and that whether the dGlcNAc is linked β1 → 3 or β1 → 6 to a third sugar does not contribute to or interfere with binding. The β1 → 3 linkage of the terminal dGal to the subterminal amino sugar is significant since dGalβ1 → 4dGlcNAc was one-half as active as the corresponding β1 → 3-linked compound and the subterminal sugar must be unsubstituted for optimal binding. N-Acetyllactosamine was 50% more active than lactose, indicating that the subterminal N-acetamido group was also contributing significantly to binding. A variety of other sugars, glycosides, and oligosaccharides showed little or not activity. From the oligosaccharides available, the combining size of this lectin would appear to be least as large a β-linked disaccharide and most complementary to dGalβ1 → 3dGalNAc β-linked to tosyl-l-serine the most active compound tested.     

A new series of N2-substituted-5-(p-toluenesulfonylamino)phthalimide analogues as α-glucosidase inhibitors

Several members of a new family of non-sugar-type α-glycosidase inhibitors, bearing a 5-(p-toluenesulfonylamino)phthalimide moiety and various substituent at the N2 position, were synthesized and their activities were investigated. The newly synthesized compounds displayed different inhibition profile towards yeast α-glycosidase and rat intestinal α-glycosidase. Almost all the compounds had strong inhibitory activities against yeast α-glycosidase. Regarding rat intestinal α-glycosidase, only analogs with N2-aromatic substituents displayed varying degrees of inhibitory activities on rat intestinal maltase and lactase and nearly all compounds showed no inhibition against rat intestinal α-amylase. Structure–activity relationship studies indicated that 5-(p-toluenesulfonylamino)phthalimide moiety is a favorable scaffold to exert the α-glucosidase inhibitory activity and substituents at the N2 position have considerable influence on the efficacy of the inhibition activities.     

Comparative molecular field analysis of fenoterol derivatives interacting with an agonist-stabilized form of the β2-adrenergic receptor

The β₂-adrenergic receptor (β₂-AR) agonist [³H]-(R,R′)-methoxyfenoterol was employed as the marker ligand in displacement studies measuring the binding affinities (K_i values) of the stereoisomers of a series of 4′-methoxyfenoterol analogs in which the length of the alkyl substituent at α′ position was varied from 0 to 3 carbon atoms. The binding affinities of the compounds were additionally determined using the inverse agonist [³H]-CGP-12177 as the marker ligand and the ability of the compounds to stimulate cAMP accumulation, measured as EC₅₀ values, were determined in HEK293 cells expressing the β₂-AR. The data indicate that the highest binding affinities and functional activities were produced by methyl and ethyl substituents at the α′ position. The results also indicate that the K_i values obtained using [³H]-(R,R′)-methoxyfenoterol as the marker ligand modeled the EC₅₀ values obtained from cAMP stimulation better than the data obtained using [³H]-CGP-12177 as the marker ligand. The data from this study was combined with data from previous studies and processed using the Comparative Molecular Field Analysis approach to produce a CoMFA model reflecting the binding to the β₂-AR conformation probed by [³H]-(R,R′)-4′-methoxyfenoterol. The CoMFA model of the agonist-stabilized β₂-AR suggests that the binding of the fenoterol analogs to an agonist-stabilized conformation of the β₂-AR is governed to a greater extend by steric effects than binding to the [³H]-CGP-12177-stabilized conformation(s) in which electrostatic interactions play a more predominate role.     

Some structural features of a new sulphated heteropolysaccharide from Padina pavonia

An acid-extractable, water-soluble, polysaccharide sulphate, isolated from Padina pavonia, comprised variable proportions of glucuronic acid, galactose, glucose, mannose, xylose, and fucose in addition to a protein moiety. Partial acid hydrolysis and autohydrolysis of the free acid polysaccharide yielded several oligosaccharides. Evidence from periodate oxidation studies indicated that the inner polysaccharide portion is composed of (1 → 4)-linked β-D-glucuronic acid, (1 → 4)-linked β-D-mannose and (1 → 4)-linked β-D-glucose residues. The heteropolymeric partially sulphated exterior portion is attached to the inner part and comprises various ratios of (1 → 4)-linked β-D-galactose, β-D-galactose-3-sulphate residues, (1 → 4)-linked β-D-glucose residues, (1 → 2)-linked α-L-fucose 4-sulphate residues and (1 → 3)-linked β-D-xylose residues.     

Quantitative structure-activity relationship of cytokinin-active adenine and urea derivatives

The substituent effect of N⁶-alkyl and -aralkyl adenines on the promotion of the growth of tobacco callus was analysed quantitatively using physico-chemical substituent parameters and regression analysis. The results indicated an optimum steric condition for activity in terms of the maximum width of the N⁶-substituents from the bond-axis connecting the N⁶-atom with its α carbon atom. The electron withdrawing effect of the N⁶-substituent enhances the activity. The substituent effect on the cytokinin activity of phenyl- and diphenyl- urea derivatives determined by Bruce and Zwar using the tobacco pith-block assay was also analysed. The results suggest that position-specific steric and hydrophobic effects of aromatic substituents participate in the variation in activity rationalizing the general trend of the activity; meta >para >ortho derivatives, for both series of compounds. The electronic effect is significant for the activity of diphenylureas but not for that of phenylureas which show somewhat different modes of interaction between the two series at the site of action. Based on inferences made from the correlations, hypothetical maps for the mode of interaction of these three sets of compounds at the site of action have been proposed.     

Étude complémentaire de la structure de trois galactoxyloglucanes (amyloïdes) de graines

D-Galacto-D-xylo-D-glucans (amyloids) from Balsamina, Tropaeolum, and Tamarindus seeds behave in a similar manner in the presence of various glycosidase preparations: slow depolymerization by enzymes from several germinated or non-germinated seeds, and hydrolysis into monosaccharides and oligosaccharides by commercial cellulase and hemicellulase preparations from fungi. A purified cellulase from Penicillium notatum gave a dialyzable fraction almost exclusively composed of α-D-xylopyranosyl-(1→6)-D-glucose residues and a nondialyzable fraction composed of chains of β-D-(1→4)[withsome (1→3)]-glucopyranosyl residues; β-D-galacto-pyranosyl-(1→2)-α-D-xylosyl groups are linked to some of the β-D-glucosyl residues at 0-6. The presence of (1→3)-linkages in the D-glucan chain of the Balsamina was verified by methylation and sequential periodate oxidation-borohydride reduction; the distribution of the substituents on the D-glucan chain is not regular. The main D-glucan backbone, where the β-D-glucosyl residues are partly linked at 0-6 to β-D-galactosyl-(1→2)-D-xylosyl groups, is linked to D-glucan chains where almost all the D-glucose units are linked at 0-6 by one α-D-xylosyl group. The presence of 3,6-di-O-methyl-D-glucose after permethylation and hydrolysis suggests that the xyloglucan chains are linked to 0-2 of the D-glucosyl units of the galactoxyloglucan backbone.     

Conformational characterization of human eukaryotic initiation factor 2α: A single tryptophan protein

The α-subunit of the human eukaryotic initiation factor 2 (heIF2α), a GTP binding protein, plays a major role in the initiation of protein synthesis. During various cytoplasmic stresses, eIF2α gets phosphorylated by eIF2α-specific kinases resulting in inhibition of protein synthesis. The cloned and over expressed heIF2α, a protein with a single tryptophan (trp) residue was examined for its conformational characteristics using steady-state and time-resolved tryptophan fluorescence, circular dichroism (CD) and hydrophobic dye binding. The steady-state fluorescence spectrum, fluorescence lifetimes (τ₁ = 1.13 ns and τ₂ = 4.74 ns) and solute quenching studies revealed the presence of trp conformers in hydrophobic and differential polar environment at any given time. Estimation of the α-helix and β-sheet content showed: (i) more compact structure at pH 2.0, (ii) distorted α-helix and rearranged β-sheet in presence of 4 M guanidine hydrochloride and (iii) retention of more than 50% ordered structure at 95 °C. Hydrophobic dye binding to the protein with loosened tertiary structure was observed at pH 2.0 indicating the existence of a molten globule-like structure. These observations indicate the inherent structural stability of the protein under various denaturing conditions.     

Studies on aroyl- and aryl-hydrazide derivatives from d-glycero-d-gulo-heptono-1,4-lactone

A series of aroyl- and aryl-hydrazide derivatives was prepared from d-glycero-d-gulo-heptono-1,4-lactone (1). The reactivity of the NH proton in these hydrazides, in terms of their dissociation constants (pK_a), was determined from their electronic spectra, and correlated to the Hammett σ values of the substituents. Comparable reactivities of the NH protons for the compounds, and the effect of the substituent, were studied by n.m.r. spectroscopy. Decomposition of the aroylhydrazides with copper(II) sulfate or nitrous acid resulted in the regeneration of 1.     

Recognition Mechanism of a Novel Gabapentinoid Drug,Mirogabalin, for Recombinant Human α2δ1, a Voltage-Gated Calcium Channel Subunit

Mirogabalin is a novel gabapentinoid drug with a hydrophobic bicyclo substituent on the γ-aminobutyric acid moiety that targets the voltage-gated calcium channel subunit α₂δ1. Here, to reveal the mirogabalin recognition mechanisms of α₂δ1, we present structures of recombinant human α₂δ1 with and without mirogabalin analyzed by cryo-electron microscopy. These structures show the binding of mirogabalin to the previously reported gabapentinoid binding site, which is the extracellular dCache_1 domain containing a conserved amino acid binding motif. A slight conformational change occurs around the residues positioned close to the hydrophobic group of mirogabalin. Mutagenesis binding assays identified that residues in the hydrophobic interaction region, in addition to several amino acid binding motif residues around the amino and carboxyl groups of mirogabalin, are critical for mirogabalin binding. The A215L mutation introduced to decrease the hydrophobic pocket volume predictably suppressed mirogabalin binding and promoted the binding of another ligand, L-Leu, with a smaller hydrophobic substituent than mirogabalin. Alterations of residues in the hydrophobic interaction region of α₂δ1 to those of the α₂δ2, α₂δ3, and α₂δ4 isoforms, of which α₂δ3 and α₂δ4 are gabapentin-insensitive, suppressed the binding of mirogabalin. These results support the importance of hydrophobic interactions in α₂δ1 ligand recognition.     

Aromatic phosphonates inhibit the lysophospholipase D activity of autotaxin

Autotaxin (ATX) is an attractive target for the anticancer therapeutics that inhibits angiogenesis, invasion and migration. ATX is an extracellular lysophospholipase D that hydrolyzes lysophosphatidylcholine to form the bioactive lipid lysophosphatidic acid. The aromatic phosphonate S32826 was the first described nanomolar inhibitor of ATX. However, the tridecylamide substituent on aromatic ring contributed to its poor solubility and bioavailability, severely limiting its utility in vivo. c Log P calculations revealed that the lipophilicity of S32826 could be lowered by shortening its hydrophobic chain and by introducing substituents alpha to the phosphonate. Herein, we describe the synthesis of a small set of α-substituted phosphonate analogs of S32826, and we show that shortening the chain and adding α-halo or α-hydroxy substituents increased solubility; however, ATX inhibition was reduced by most substitutions. An optimal compound was identified for examination of biological effects of ATX inhibition in vivo.     

Non-additivity of Functional Group Contributions in Protein-Ligand Binding: A Comprehensive Study by Crystallography and Isothermal Titration Calorimetry

Additivity of functional group contributions to protein-ligand binding is a very popular concept in medicinal chemistry as the basis of rational design and optimized lead structures. Most of the currently applied scoring functions for docking build on such additivity models. Even though the limitation of this concept is well known, case studies examining in detail why additivity fails at the molecular level are still very scarce. The present study shows, by use of crystal structure analysis and isothermal titration calorimetry for a congeneric series of thrombin inhibitors, that extensive cooperative effects between hydrophobic contacts and hydrogen bond formation are intimately coupled via dynamic properties of the formed complexes. The formation of optimal lipophilic contacts with the surface of the thrombin S3 pocket and the full desolvation of this pocket can conflict with the formation of an optimal hydrogen bond between ligand and protein. The mutual contributions of the competing interactions depend on the size of the ligand hydrophobic substituent and influence the residual mobility of ligand portions at the binding site. Analysis of the individual crystal structures and factorizing the free energy into enthalpy and entropy demonstrates that binding affinity of the ligands results from a mixture of enthalpic contributions from hydrogen bonding and hydrophobic contacts, and entropic considerations involving an increasing loss of residual mobility of the bound ligands. This complex picture of mutually competing and partially compensating enthalpic and entropic effects determines the non-additivity of free energy contributions to ligand binding at the molecular level.     

Structure-distribution studies on some 99mTc-o-hydroxybenzyliminodiacetic acid complexes

Biodistributions of a series of thirteen ^99mTc-o-hydroxy-benzyliminodiacetic acid complexes were carried out in rats and their hepatobiliary and urinary outputs correlated with lipophilicity, molecular weight, influence of substituent and plasma protein binding. Hepatobiliary output was moderate for those ligands with large alkyl substituents [t-butyl (36%), and iso-octyl (42%)] but compared to HIDA compounds was relatively low, indicating that they would not be suitable for clinical use. Halogen substituents had only a small effect on increasing hepatobiliary output but a large effect on reducing the urinary clearance.