The catalytic amino-acid residues in the active site of cellobiohydrolase 1 are involved in chiral recognition

Chiral separation in the class of proton pump inhibitors by chromatographic and electromigration techniques: An overview

Proton pump inhibitors (PPIs) are benzimidazole-derivative chiral sulfoxides, frequently used in the treatment of gastric hyperacidity-related disorders. Due to their stereoselective metabolism, the eutomeric forms of PPIs can present a more advantageous pharmacokinetic profile by comparison with the distomers or racemates. Moreover, two representatives of the class are used in therapy both as racemates and as pure enantiomers (esomeprazole, dexlansoprazole). A relatively large number of enantioseparation methods employed for the stereoselective determination of PPIs from pharmaceutical, biological, and environmental matrices were published in the past three decades. The purpose of the current overview is to provide a systematic survey of the available chiral separation methods published since the introduction of PPIs in the therapy up to the present. Analytical and bioanalytical methods using different chromatographic and electromigration techniques reported for the enantioseparation of omeprazole, lansoprazole, pantoprazole, rabeprazole, ilaprazole, and tenatoprazole are included. The analytical conditions of the presented methods are summarized in three comprehensive tables, while a critical discussion of the applied techniques, possible mechanism of enantiorecognition, and future perspectives on the topic are also presented.

Enantioselective Binding of Propranolol and Analogues Thereof to Cellobiohydrolase Cel7A

At the catalytic site for the hydrolysis of cellulose the enzyme cellobiohydrolase Cel7A binds the enantiomers of the adrenergic beta-blocker propranolol with different selectivity. Methyl-to-hydroxymethyl group modifications of propranolol, which result in higher affinity and improved selectivity, were herein studied by ¹ H,¹ H and ¹ H,¹³ C scalar spin-spin coupling constants as well as utilizing the nuclear Overhauser effect (NOE) in conjunction with molecular dynamics simulations of the ligands per se, which showed the presence of all-antiperiplanar conformations, except for the one containing a vicinal oxygen-oxygen arrangement governed by the gauche effect. For the ligand-protein complexes investigated by NMR spectroscopy using, inter alia, transferred NOESY and saturation-transfer difference (STD) NMR experiments the S-isomers were shown to bind with a higher affinity and a conformation similar to that preferred in solution, in contrast to the R-isomer. The fact that the S-form of the propranolol enantiomer is pre-arranged for binding to the protein is also observed for a crystal structure of dihydroxy-(S)-propranolol and Cel7A presented herein. Whereas the binding of propranolol is entropy driven, the complexation with the dihydroxy analogue is anticipated to be favored also by an enthalpic term, such as for its enantiomer, that is, dihydroxy-(R)-propranolol, because hydrogen-bond donation replaces the corresponding bonding from hydroxyl groups in glucosyl residues of the natural substrate. In addition to a favorable entropy component, albeit lesser in magnitude, this represents an effect of enthalpy-to-entropy compensation in ligand-protein interactions.

Separation of enantiomers on chiral stationary phase based on cellulase: Effect of preparation method and silica particle diameters on chiral recognition ability

Cellulase (Cel) was immobilized onto aminopropyl-silica gels via its amino and carboxy groups, respectively, using N,N'-disuccinimidyl carbonate, and 1-ethyl-3-(3'-dimethylaminopropyl)carbodimide and N-hydroxysulfosuccinimide. They were termed N-Cel and C-Cel, respectively. Despite their smaller retention factors on a C-Cel column, the enantioseparation factors and resolution of β-blockers, propranolol, alprenolol, oxprenolol and pindolol, were similar with N- and C-Cel columns. In addition, C-Cel was prepared using aminopropyl-silica gels, whose nominal particle diameters were 5 and 3, and 2.1μm, respectively. A C-Cel column prepared with 2.1-μm aminopropyl-silica gels gave the highest enantioselectivity and column efficiency among three C-Cel columns. Furthermore, the influence of N,N-dimethyl-n-octylamine (DMOA) or cellobiose concentrations on the retentivity and enantioselectivity for β-blockers on a C-Cel column was investigated. The results indicate that single-site competition of β-blockers with DMOA or cellobiose on the catalytic binding site of Cel and the further bindings at the secondary site in a non-competitive fashion could occur. Furthermore, the enantioselective bindings of β-blockers could occur at the catalytic biding cite of Cel and at the secondary binding site.

Initial recognition of a cellodextrin chain in the cellulose-binding tunnel may affect cellobiohydrolase directional specificity

Cellobiohydrolases processively hydrolyze glycosidic linkages in individual polymer chains of cellulose microfibrils, and typically exhibit specificity for either the reducing or nonreducing end of cellulose. Here, we conduct molecular dynamics simulations and free energy calculations to examine the initial binding of a cellulose chain into the catalytic tunnel of the reducing-end-specific Family 7 cellobiohydrolase (Cel7A) from Hypocrea jecorina. In unrestrained simulations, the cellulose diffuses into the tunnel from the -7 to the -5 positions, and the associated free energy profiles exhibit no barriers for initial processivity. The comparison of the free energy profiles for different cellulose chain orientations show a thermodynamic preference for the reducing end, suggesting that the preferential initial binding may affect the directional specificity of the enzyme by impeding nonproductive (nonreducing end) binding. Finally, the Trp-40 at the tunnel entrance is shown with free energy calculations to have a significant effect on initial chain complexation in Cel7A.

Recent developments on cellulases and carbohydrate-binding modules with cellulose affinity

This review concerns basic research on cellulases and cellulose-specific carbohydrate-binding modules (CBMs). As a background, glycosyl hydrolases are also briefly reviewed. The nomenclature of cellulases and CBMs is discussed. The main cellulase-producing organisms and their cellulases are described. Synergy, enantioseparation, cellulases in plants, cellulosomes, cellulases and CBMs as analytical tools and cellulase-like enzymes are also briefly reviewed.

Separation of drug enantiomers by liquid chromatography and capillary electrophoresis, using immobilized proteins as chiral selectors

Proteins display interesting chiral discrimination properties owing to multiple possibilities of intermolecular interactions with chiral compounds. This review deals with proteins which have been used as immobilized chiral selectors for the enantioseparation of drugs in liquid chromatography and capillary electrophoresis. The main procedures allowing the immobilization of proteins onto matrices, such as silica and zirconia particles, membranes and capillaries are first presented. Then the factors affecting the enantioseparation of drugs in liquid chromatography, using various protein-based chiral stationary phases (CSPs), are reviewed and discussed. Last, chiral separations already achieved using immobilized protein selectors in affinity capillary electrochromatography (ACEC) are presented and compared in terms of efficiency, stability and reproducibility.

Protein-based chiral stationary phases for high-performance liquid chromatography enantioseparations

The enantioseparations of various compounds using proteins as the chiral selectors in high-performance liquid chromatography (HPLC) are considered in this review. The proteins used include albumins such as bovine serum albumin and human serum albumin, glycoproteins such as alpha1-acid glycoprotein, ovomucoid, ovoglycoprotein, avidin and riboflavin binding protein, enzymes such as trypsin, alpha-chymotrypsin, cellobiohydrolase I, lysozyme, pepsin and amyloglucosidase, and other proteins such as ovotransferrin and beta-lactoglobulin. This review deals with the properties of HPLC chiral stationary phases based on proteins, and the enantioselective properties and chiral recognition mechanisms of these stationary phases.

Structural basis for enantiomer binding and separation of a common beta-blocker: crystal structure of cellobiohydrolase Cel7A with bound (S)-propranolol at 1.9 A resolution

Cellobiohydrolase Cel7A (previously called CBH 1), the major cellulase produced by the mould fungus Trichoderma reesei, has been successfully exploited as a chiral selector for separation of stereo-isomers of some important pharmaceutical compounds, e.g. adrenergic beta-blockers. Previous investigations, including experiments with catalytically deficient mutants of Cel7A, point unanimously to the active site as being responsible for discrimination of enantiomers. In this work the structural basis for enantioselectivity of basic drugs by Cel7A has been studied by X-ray crystallography. The catalytic domain of Cel7A was co-crystallised with the (S)-enantiomer of a common beta-blocker, propranolol, at pH 7, and the structure of the complex was determined and refined at 1. 9 A resolution. Indeed, (S)-propranolol binds at the active site, in glucosyl-binding subsites -1/+1. The catalytic residues Glu212 and Glu217 make tight salt links with the secondary amino group of (S)-propranolol. The oxygen atom attached to the chiral centre of (S)-propranolol forms hydrogen bonds to the nucleophile Glu212 O(epsilon1) and to Gln175 N(epsilon2), whereas the aromatic naphthyl moiety stacks with the indole ring of Trp376 in site +1. The bidentate charge interaction with the catalytic glutamate residues is apparently crucial, since no enantioselectivity has been obtained with the catalytically deficient mutants E212Q and E217Q. Activity inhibition experiments with wild-type Cel7A were performed in conditions close to those used for crystallisation. Competitive inhibition constants for (R)- and (S)-propranolol were determined at 220 microM and 44 microM, respectively, corresponding to binding free energies of 20 kJ/mol and 24 kJ/mol, respectively. The K(i) value for (R)-propranolol was 57-fold lower than the highest concentration, 12.5 mM, used in co-crystallisation experiments. Still several attempts to obtain a complex with the (R)-enantiomer have failed. By using cellobiose as a selective competing ligand, the retention of the enantiomers of propranolol on the chiral stationary phase (CSP) based on Cel7A mutant D214N were resolved into enantioselective and non- selective binding. The enantioselective binding was weaker for both enantiomers on D214N-CSP than on wild-type-CSP.

Influence of the solute hydrophobicity on the enantioselective adsorption of beta-blockers on a cellulase protein used as the chiral selector

Adsorption isotherm data were acquired at different eluent pH values for the enantiomers of several beta-blockers on cellobiohydrolase I on silica gel. They fit well to the biLangmuir model, allowing the determination of the equilibrium constants and the monolayer capacities for chiral and nonselective adsorption. The adsorption of the S-enantiomers (eluted second) is exothermic at low pH, endothermic at high pH, and athermal in a narrow pH range depending on the beta-blocker. This transition pH range is lower for S-alprenolol than for the more hydrophobic S-propranolol, although their endothermic adsorption originates from hydrophobic interactions. This surprising observation is explained by the relative values of the isotherm coefficients. S-Alprenolol seems to have a more pronounced endothermic behavior than S-propranolol because the nonselective interactions of both compounds with the stationary phase are exothermic but their contribution to retention, relative to that of the endothermic chiral interactions, is less important for alprenolol. The order of increasing energy of the chiral interactions is the same as that of hydrophobicity, propranolol>alprenolol>metoprolol.

Cellobiohydrolase 58 (P.c. Cel 7D) is complementary to the homologous CBH I (T.r. Cel 7A) in enantioseparations

Cellobiohydrolase 58 (EC 3.2.1.91, P.c. Cel 7D) from Phanerochaete chrysosporium was immobilized on silica and the resulting material, CBH 58-silica, was then used as a chiral stationary phase (CSP) in liquid chromatographic separations of enantiomers. The enantioselectivities obtained on CBH 58-silica were compared with those on CBH I-silica (a phase based on a corresponding cellulase from Trichoderma reesei). CBH 58-silica displayed higher selectivity than CBH I-silica for the more hydrophilic compounds, such as atenolol and metoprolol, although great similarities in chiral separation of beta-adrenergic antagonists were found between the two phases. None of the acidic compounds tested could be resolved on the CBH 58 phase. Moreover, the solutes were retained more on the CBH 58 phase in general, indicating an improved application potential in bioanalysis. Addition of cellobiose or lactose, both of which are inhibitors of cellulases, to the mobile phase impaired the enantioselectivity, indicating an overlap of the enantioselective and catalytic sites. The chiral analytes also functioned as competitive inhibitors and their inhibition constants were determined.

Retention mechanism of beta-blockers on an immobilized cellulase. Relative importance of the hydrophobic and ionic contributions to their enantioselective and nonselective interactions

The adsorption isotherms of the enantiomers of three beta-blockers, metoprolol, alprenolol, and propranolol, were measured on cellobiohydrolase I (CBH I) immobilized on silicagel, in the concentration range between 0.25 microM and 1.7 mM, at pH = 5.0, 5.5, and 6.0. In agreement with previous results, these data are accounted for by a two-sites physical model and fit closely to a Bilangmuir equation. The saturation capacities and the binding constants were determined for each enantiomer on the chiral and the nonchiral sites. The chiral sites are shown to be strongly ionic, in contrast to the nonchiral ones, which are mainly hydrophobic. However, the chiral binding of (S)-propranolol is endothermic, with a high adsorption entropy, in contrast to the chiral interactions of (R)-propranolol and to the nonchiral interactions, which are all exothermic. This indicates that hydrophobic interactions also play a role in the chiral binding. The dependence of the adsorption parameters on the hydrophobicity of the solute is discussed and interpreted in terms of the retention mechanism. The results are compared with the structure of the protein, recently elucidated by X-ray crystallography.

Chromatographic evaluation of structure selective and enantioselective retention of amines and acids on cellobiohydrolase I wild type and its mutant D214N

The mechanisms of structure selective and enantioselective retentions of amines and acids on two chiral stationary phases based on wild type cellobiohydrolase I (CBH I) and its mutant D214N have been investigated. All the amino alcohols tested had an enantioselective site that overlaps with the catalytically active site of CBH I, whereas the enantioselectivity of prilocaine was not affected by the mutation. The hydroxyl group of the amino alcohols did not seem to be an important contributor to the total binding strength whereas a bromo substituent in the aromatic ring promotes a high enantioselectivity (alpha=7.05). Interestingly, the chiral recognition site of the acid warfarin overlaps with the binding site of the amino alcohols. Di-p-toluoyltartaric acid and dibenzoyltartaric acid were strongly retained probably due to electrostatic attraction, but no enantioselectivity was observed. The difference in retention characteristics for the amino alcohols on the two stationary phases was strongly pH-dependent. A change in elution order of different amino alcohols occurred when changing the pH from 5.0 to 7.0. The difference between the two phases was lower at low pH. The retention times could also be affected by ionic strength and by use of cellobiose as a mobile phase additive but no indication of ion-pair retention of the amines was observed, when adding hexanesulphonate as counter ion to the mobile phase. The temperature dependence of the retention of the enantiomers of propranolol at pH 7.0 on the mutant D214N was similar to what was earlier observed on the wild type CBH I at lower pH.

Discrimination between enantioselective and non-selective binding sites on cellobiohydrolase-based stationary phases by site specific competing ligands

A systematic study was performed to investigate the influence of cellobiose or lactose on the enantioselective retention behaviour of some beta-blockers in liquid chromatography using Cellobiohydrolase (CHB) I from Trichoderma reesei or Cellobiohydrolase 58 from Phanerochaete chrysosporium immobilized on silica as stationary phases. The results revealed that the retention could be described by the function [equation; see text] where the observed capacity factor corresponds to the sum of an enantioselective mode being influenced by a site specific competing ligand (competitor) and a non-selective mode unaffected by the competitor. A non-constrained non-linear least-square regression gave in all cases virtually identical nondisplacable capacity factors (k'ns) for both enantiomers of the same drug. The experimental capacity factors (k'(x,C)) of the enantiomers all show a close fit to the adapted function. The Kd values calculated for the competitor were also virtually identical for each pair of enantiomers and were in accordance with Ki data determined for the competitors in classical enzyme kinetics experiments, demonstrating that one unique site; namely, the catalytic site, was responsible for the enantioselective binding. Similar results were obtained with the resolution of rac-alprenolol and rac-metoprolol on CBH I phase.