Butyrylcholinesterase interactions with amylin may protect pancreatic cells in metabolic syndrome

The metabolic syndrome (MetS) is a risk factor for type 2 diabetes mellitus (T2DM). However, the mechanisms underlying the transition from MetS to T2DM are unknown. Our goal was to study the potential contribution of butyrylcholinesterase (BChE) to this process. We first determined the hydrolytic activity of BChE in serum from MetS, T2DM and healthy individuals. The ‘Kalow’ variant of BChE (BChE-K), which has been proposed to be a risk factor for T2DM, was genotyped in the last two groups. Our results show that in MetS patients serum BChE activity is elevated compared to T2DM patients and healthy controls (P < 0.001). The BChE-K genotype showed similar prevalence in T2DM and healthy individuals, excluding this genotype as a risk factor for T2DM. However, the activity differences remained unexplained. Previous results from our laboratory have shown BChE to attenuate the formation of β-amyloid fibrils, and protect cultured neurons from their cytotoxicity. Therefore, we next studied the in vitro interactions between recombinant human butyrylcholinesterase and amylin by surface plasmon resonance, Thioflavine T fluorescence assay and cross-linking, and used cultured pancreatic β cells to test protection by BChE from amylin cytotoxicity. We demonstrate that BChE interacts with amylin through its core domain and efficiently attenuates both amylin fibril and oligomer formation. Furthermore, application of BChE to cultured β cells protects them from amylin cytotoxicity. Taken together, our results suggest that MetS-associated BChE increases could protect pancreatic β-cells in vivo by decreasing the formation of toxic amylin oligomers.

Therapeutic antibodies: Discovery and development using the ProteOn XPR36 biosensor interaction array system

Therapeutic monoclonal antibodies are becoming a significant and rapidly growing class of therapeutic pharmaceuticals. Their discovery and development requires fast and high-throughput methodologies for screening and selecting appropriate candidate antibodies having high affinity for the target as well as high specificity and low cross-reactivity. This study demonstrates the use of the ProteOn XPR36 protein interaction array system and its novel approach, termed One-Shot Kinetics, for the rapid screening and selection of high-affinity antibodies. This approach allows multiple quantitative protein binding analyses in parallel, providing association, dissociation, and affinity constants for several antibodies or supernatants simultaneously in one experiment. We show that the ProteOn XPR36 system is a valuable tool for use across multiple stages of the therapeutic antibody discovery and development process, enabling efficient and rapid screening after panning, affinity maturation, assay validation, and clone selection.

Methyl jasmonate binds to and detaches mitochondria-bound hexokinase

Cellular bio-energetic metabolism and mitochondria are recognized as potential targets for anticancer agents, due to the numerous relevant peculiarities cancer cells exhibit. Jasmonates are anticancer agents that interact directly with mitochondria. The aim of this study was to identify mitochondrial molecular targets of jasmonates. We report that jasmonates bind to hexokinase and detach it from the mitochondria and its mitochondrial anchor-the voltage-dependent anion channel (VDAC), as judged by hexokinase immunochemical and activity determinations, surface plasmon resonance analysis and planar lipid bilayer VDAC-activity analysis. Furthermore, the susceptibility of cancer cells and mitochondria to jasmonates is dependent on the expression of hexokinase, evaluated using hexokinase-overexpressing transfectants and its mitochondrial association. Many types of cancer cells exhibit overexpression of the key glycolytic enzyme, hexokinase, and its excessive binding to mitochondria. These characteristics are considered to play a pivotal role in cancer cell growth rate and survival. Thus, our findings provide an explanation for the selective effects of jasmonates on cancer cells. Most importantly, this is the first demonstration of a cytotoxic mechanism based on direct interaction between an anticancer agent and hexokinase. The proposed mechanism can serve to guide development of a new selective approach for cancer therapy.

Glycosynthase activity of Geobacillus stearothermophilus GH52 beta-xylosidase: efficient synthesis of xylooligosaccharides from alpha-D-xylopyranosyl fluoride through a conjugated reaction

Glycosynthases are mutant glycosidases in which the acidic nucleophile is replaced by a small inert residue. In the presence of glycosyl fluorides of the opposite anomeric configuration (to that of their natural substrates), these enzymes can catalyze glycosidic bond formation with various acceptors. In this study we demonstrate that XynB2E335G, a nucleophile-deficient mutant of a glycoside hydrolase family 52 beta-xylosidase from G. stearothermophilus, can function as an efficient glycosynthase, using alpha-D-xylopyranosyl fluoride as a donor and various aryl sugars as acceptors. The mutant enzyme can also catalyze the self-condensation reaction of alpha-D-xylopyranosyl fluoride, providing mainly alpha-D-xylobiosyl fluoride. The self-condensation kinetics exhibited apparent classical Michaelis-Menten behavior, with kinetic constants of 1.3 s(-1) and 2.2 mM for k(cat) and K(M(acceptor)), respectively, and a k(cat)/K(M(acceptor)) value of 0.59 s(-1) mM(-1). When the beta-xylosidase E335G mutant was combined with a glycoside hydrolase family 10 glycosynthase, high-molecular-weight xylooligomers were readily obtained from the affordable alpha-D-xylopyranosyl fluoride as the sole substrate.

Exploring “one-shot” kinetics and small molecule analysis using the ProteOn XPR36 array biosensor

A ProteOn XPR36 parallel array biosensor was used to characterize the binding kinetics of a set of small molecule/enzyme interactions. Using one injection with the ProteOn's crisscrossing flow path system, we collected response data for six different concentrations of each analyte over six different target protein surfaces. This "one-shot" approach to kinetic analysis significantly improves throughput while generating high-quality data even for low-molecular-mass analytes. We found that the affinities determined for nine sulfonamide-based inhibitors of the enzyme carbonic anhydrase II were highly correlated with the values determined using isothermal titration calorimetry. We also measured the temperature dependence (from 15 to 35 degrees C) of the kinetics for four of the inhibitor/enzyme interactions. Our results illustrate the potential of this new parallel-processing biosensor to increase the speed of kinetic analysis in drug discovery and expand the applications of real-time protein interaction arrays.

Enzyme–Substrate Complex Structures of a GH39 β-Xylosidase from Geobacillus stearothermophilus

Beta-D-Xylosidases are glycoside hydrolases that catalyse the release of xylose units from short xylooligosaccharides and are engaged in the final breakdown of plant cell-wall hemicelluloses. beta-D-Xylosidases are found in glycoside hydrolase families 3, 39, 43, 52 and 54. The first crystal structure of a GH39 beta-xylosidase revealed a multi-domain organization with the catalytic domain having the canonical (beta/alpha)8 barrel fold. Here, we report the crystal structure of the GH39 Geobacillus stearothermophilus beta-D-xylosidase, inactivated by a point mutation of the general acid-base residue E160A, in complex with the chromogenic substrate molecule 2,5-dinitrophenyl-beta-D-xyloside. Surprisingly, six of the eight active sites present in the crystallographic asymmetric unit contain the trapped covalent glycosyl-enzyme intermediate, while two of them still contain the uncleaved substrate. The structural characterization of these two critical species along the reaction coordinate of this enzyme identifies the residues forming its xyloside-binding pocket as well as those essential for its aglycone recognition.

Biochemical Characterization and Identification of the Catalytic Residues of a Family 43 β-d-Xylosidase from Geobacillus stearothermophilus T-6

Beta-D-xylosidases are hemilcellulases that hydrolyze short xylooligosaccharides into xylose units. Here, we describe the characterization and kinetic analysis of a family 43 beta-xylosidase from Geobacillus stearothermophilus T-6 (XynB3). Enzymes in this family use an inverting single-displacement mechanism with two conserved carboxylic acids, a general acid, and a general base. XynB3 was most active at 65 degrees C and pH 6.5, with clear preference to xylose-based substrates. Products analysis indicated that XynB3 is an exoglycosidase that cleaves single xylose units from the nonreducing end of xylooligomers. On the basis of sequence homology, amino acids Asp15 and Glu187 were suggested to act as the general-base and general-acid catalytic residues, respectively. Kinetic analysis with substrates bearing different leaving groups showed that, for the wild-type enzyme, the k(cat) and k(cat)/K(m) values were only marginally affected by the leaving-group reactivity, whereas for the E187G mutant, both values exhibited significantly greater dependency on the pK(a) of the leaving group. The pH-dependence activity profile of the putative general-acid mutant (E187G) revealed that the protonated catalytic residue was removed. Addition of the exogenous nucleophile azide did not affect the activities of the wild type or the E187G mutant but rescued the activity of the D15G mutant. On the basis of thin-layer chromatography and (1)H NMR analyses, xylose and not xylose azide was the only product of the accelerated reaction, suggesting that the azide ion does not attack the anomeric carbon directly but presumably activates a water molecule. Together, these results confirm the suggested catalytic role of Glu187 and Asp15 in XynB3 and provide the first unequivocal evidence regarding the exact roles of the catalytic residues in an inverting GH43 glycosidase.

Crystallization and preliminary crystallographic analysis of a thermostable family 52 β-D-xylosidase fromGeobacillus stearothermophilusT-6

Beta-D-xylosidases (EC 3.2.1.37) are hemicellulases that hydrolyze short xylooligosaccharides into single xylose units. In this study, the first crystallization and preliminary X-ray analysis of a family 52 glycoside hydrolase, the beta-D-xylosidase (XynB2) from Geobacillus stearothermophilus T-6, is described. XynB2 is a dimeric protein consisting of two identical subunits of 705 amino acids with a calculated molecular weight of 79 894 Da. XynB2 was crystallized by the hanging-drop vapour-diffusion method and the crystals were found to belong to space group P1, with unit-cell parameters a = 80.6, b = 97.5, c = 107.2 A, alpha = 107.4, beta = 98.2, gamma = 106.6 degrees. The native crystals diffracted X-rays to a resolution of 2.0 A.

Crystallization and preliminary X-ray analysis of family 39 β-D-xylosidase fromGeobacillus stearothermophilusT-6

beta-D-Xylosidases (EC 3.2.1.37) are hemicellulases that hydrolyze short xylooligosaccharides into single xylose units. In this study, the crystallization and preliminary X-ray analysis of the beta-D-xylosidase (XynB1) from Geobacillus stearothermophilus T-6, a family 39 glycoside hydrolase, are described. XynB1 is a tetrameric protein consisting of four identical subunits of 503 amino acids and with a calculated molecular weight of 58 001 Da. Both the native and the selenomethionine-containing XynB1 were crystallized by the hanging-drop vapour-diffusion method and the crystals were found to belong to space group P2(1)2(1)2(1), with unit-cell parameters a = 92.7, b = 165.7, c = 311.0 A. The native crystals diffracted X-rays to a resolution of 2.1 A.

Detailed kinetic analysis of a family 52 glycoside hydrolase: a beta-xylosidase from Geobacillus stearothermophilus

Geobacillus stearothermophilus T-6 encodes for a beta-xylosidase (XynB2) from family 52 of glycoside hydrolases that was previously shown to hydrolyze its substrate with net retention of the anomeric configuration. XynB2 significantly prefers substrates with xylose as the glycone moiety and exhibits a typical bell-shaped pH dependence curve. Binding properties of xylobiose and xylotriose to the active site were measured using isothermal titration calorimetry (ITC). Binding reactions were enthalpy driven with xylobiose binding more tightly than xylotriose to the active site. The kinetic constants of XynB2 were measured for the hydrolysis of a variety of aryl beta-D-xylopyranoside substrates bearing different leaving groups. The Brønsted plot of log k(cat) versus the pK(a) value of the aglycon leaving group reveals a biphasic relationship, consistent with a double-displacement mechanism as expected for retaining glycoside hydrolases. Hydrolysis rates for substrates with poor leaving groups (pK(a) > 8) vary widely with the aglycon reactivity, indicating that, for these substrates, the bond cleavage is rate limiting. However, no such dependence is observed for more reactive substrates (pK(a) < 8), indicating that in this case hydrolysis of the xylosyl-enzyme intermediate is rate limiting. Secondary kinetic isotope effects suggest that the intermediate breakdown proceeds with modest oxocarbenium ion character at the transition state, and bond cleavage proceeds with even lower oxocarbenium ion character. Inhibition studies with several gluco analogue inhibitors could be measured since XynB2 has low, yet sufficient, activity toward 4-nitrophenyl beta-D-glucopyranose. As expected, inhibitors mimicking the proposed transition state structure, such as 1-deoxynojirimycin, bind with much higher affinity to XynB2 than ground state inhibitors.

Identification of the catalytic residues in family 52 glycoside hydrolase, a beta-xylosidase from Geobacillus stearothermophilus T-6

beta-d-Xylosidases (EC 3.2.1.37) are exo-type glycoside hydrolases that hydrolyze short xylooligosaccharides to xylose units. The enzymatic hydrolysis of the glycosidic bond involves two carboxylic acid residues, and their identification, together with the stereochemistry of the reaction, provides crucial information on the catalytic mechanism. Two catalytic mutants of a beta-xylosidase from Geobacillus stearothermophilus T-6 were subjected to detailed kinetic analysis to verify their role in catalysis. The activity of the E335G mutant decreased approximately 106-fold, and this activity was enhanced 103-fold in the presence of external nucleophiles such as formate and azide, resulting in a xylosyl-azide product with an opposite anomeric configuration. These results are consistent with Glu335 as the nucleophile in this retaining enzyme. The D495G mutant was subjected to detailed kinetic analysis using substrates bearing different leaving groups (pKa). The mutant exhibited 103-fold reduction in activity, and the Brønsted plot of log(kcat) versus pKa revealed that deglycosylation is the rate-limiting step, indicating that this step was reduced by 103-fold. The rates of the glycosylation step, as reflected by the specificity constant (kcat/Km), were similar to those of the wild type enzyme for hydrolysis of substrates requiring little protonic assistance (low pKa) but decreased 102-fold for those that require strong acid catalysis (high pKa). Furthermore, the pH dependence profile of the mutant enzyme revealed that acid catalysis is absent. Finally, the presence of azide significantly enhanced the mutant activity accompanied with the generation of a xylosyl-azide product with retained anomeric configuration. These results are consistent with Asp495 acting as the acid-base in XynB2.

Stereochemistry of family 52 glycosyl hydrolases: a beta-xylosidase from Bacillus stearothermophilus T-6 is a retaining enzyme

A beta-xylosidase from Bacillus stearothermophilus T-6 assigned to the uncharacterized glycosyl hydrolase family 52 was cloned, overexpressed in Escherichia coli and purified. The enzyme showed maximum activity at 65 degrees C and pH 5.6-6.3. The stereochemistry of the hydrolysis of p-nitrophenyl beta-D-xylopyranoside was followed by 1H-nuclear magnetic resonance. Time dependent spectrum analysis showed that the configuration of the anomeric carbon was retained, indicating that a retaining mechanism prevails in family 52 glycosyl hydrolases. Sequence alignment and site-directed mutagenesis enabled the identification of functionally important amino acid residues of which Glu337 and Glu413 are likely to be the two key catalytic residues involved in enzyme catalysis.

Glutamic acid 160 is the acid-base catalyst of beta-xylosidase from Bacillus stearothermophilus T-6: a family 39 glycoside hydrolase

A beta-xylosidase from Bacillus stearothermophilus T-6 was cloned, overexpressed in Escherichia coli and purified to homogeneity. Based on sequence alignment, the enzyme belongs to family 39 glycoside hydrolases, which itself forms part of the wider GH-A clan. The conserved Glu160 was proposed as the acid-base catalyst. An E160A mutant was constructed and subjected to steady state and pre-steady state kinetic analysis together with azide rescue and pH activity profiles. The observed results support the assignment of Glu160 as the acid-base catalytic residue.