Affinity purification of beta-amylases originating from plant using cyclomaltohexaose-immobilized Sepharose 6B in the presence of ammonium sulfate

A new interpretation of sulfate activation of rabbit muscle glycogen phosphorylase

It is widely known that sulfate ion at high concentration serves like an allosteric activator of glycogen phosphorylase (GP). Based on the crystallographic studies on GP, it has been assumed that the sulfate ion is bound close to the phosphorylatable Ser¹⁴ site of nonactivated GP, causing a conformational change to catalytically-active GP. However, there are also reports that sulfate ion inhibits allosterically-activated GP by preventing the phosphate substrate from attaching to the catalytic site. In the present study, using a high concentration of sulfate ion, significant enhancement of GP activity was observed when macromolecular glycogen was used as substrate but not when smaller maltohexaose was used. In glycogen solution, nonreducing-end glucose residues are localized on the surface of glycogen and are not distributed homogenously in the solution. Using cyclodextrin-immobilized column chromatography, we found that sulfate at high concentration promoted GP-dextrin binding through the dextrin-binding site (DBS) located away from the catalytic site. This result is consistent with the properties of the DBSs found in glycogen-debranching enzyme and β-amylase. Therefore, we propose a new interpretation of the sulfate activation of GP, wherein sulfate ions at high concentration promote glycogen-binding to the DBS directly, and glycogen-binding to the catalytic site indirectly. Our findings were successfully applied to the affinity purification of porcine brain GP.

Purification and characterization of a soluble recombinant human ST6Gal I functionally expressed in Escherichia coli

A soluble and active form of recombinant human ST6Gal I was expressed in Escherichia coli. The gene encoding the soluble form of ST6Gal I lacking the membrane and cytosolic regions was introduced into a bacterial expression vector, pMAL-p2X, fused in frame with a maltose-binding protein (MBP) tag. Low-temperature cultivation at 13 degrees C during IPTG-induction significantly improved both solubility and MBP-tagging of the recombinant enzyme expressed in bacteria. The supernatant prepared by disruption of the cells demonstrated sialic acid transfer activity to both an oligosaccharide and a glycoprotein, asialofetuin, indicating that the enzyme expressed in bacteria is soluble and active. The MBP-tagged enzyme was efficiently purified by a combination of cation-exchange column and amylase-conjugated agarose column chromatography. The purified recombinant enzyme exerted enzymatic activity even in the absence of detergents in the reaction mixture. Acceptor substrate specificity of the enzyme was marginally different from that of rat liver ST6Gal I. These observations suggest that membrane and cytosolic regions of ST6Gal I may affect the properties of the enzyme. The purified recombinant enzyme was applied to convert desialylated fetuin to resialylated fetuin. Lectin blotting demonstrated that resialylated fetuin possesses a single Neu5Ac alpha 2-6 residue. The resialylated fetuin efficiently blocked hemagglutination induced by influenza virus strain A/Memphis/1/71 (H3N2), indicating that resialylated carbohydrate chains on the protein are so active as to competitively inhibit virus-receptor interaction. In conclusion, soluble recombinant ST6Gal I obtained using our bacterial expression system is a valuable tool to investigate the molecular mechanisms of biological and pathological interactions mediated via carbohydrates.

High expression of glycogen-debranching enzyme in Escherichia coli and its competent purification method

Glycogen-debranching enzyme (GDE) gene from Saccharomyces cerevisiae was cloned and expressed into Escherichia coli. A 99.3% homology was found between the nucleotide sequences of GDE gene harbored in the recombinant E. coli plasmid (pTrc99A) and the open reading frame (902039-906646 position) of the 4608-bp fragment of S. cerevisiae chromosome XVI. We investigated the best conditions for GDE expression. When the cultivation temperature of recombinant E. coli strains was lowered to 25 degrees C and the isopropyl-beta-d-thiogalactopyranoside (IPTG) concentration used for induction was decreased to as low as 0.02 mM, a total of about 33 mg of recombinant GDE can be isolated from a liter culture as estimated by amylo-1,6-glucosidase activity. Consecutively, we developed a new method for purifying GDE. The method requires only a single-step purification via beta-cyclodextrin-immobilized Sepharose 6B (beta-CD Sepharose 6B) affinity chromatography and renders a 90% recovery of the enzyme. Moreover, the purified recombinant GDE is a homogeneous protein and possesses the same characteristics as those of S. cerevisiae. With the highly expressed GDE in recombinant E. coli and a rapid and effective purification method, we successfully resolved the hurdle always faced for obtaining an ample amount of purified GDE. The availability of GDE, hence, may allow advancement on GDE studies and provide new prospects for GDE on biotechnological application.

Chromatographic methods for amylases

This review surveys recent developments in chromatographic methods for the separation of amylases from complex extracts, including the separation of isozymes. It contains two tables with the properties and molecular characteristics of alpha-and beta-amylases from different sources as well as an updated review of methods for the determination of amylase activity. The main subject of this review is a detailed evaluation of the application of newly developed chromatographic methods for the purification of amylases.

Functional analysis of Glu380 and Leu383 of soybean beta-amylase. A proposed action mechanism

Soybean beta-amylase, comprising a (beta/alpha)8-barrel core with a mobile loop, similar to that of triose phosphate isomerase, was mutated by site-directed mutagenesis at residues Glu380 and Leu383. X-ray crystallographic findings suggest that Glu380 is the counterpart of the catalytic site (Glu186) and that Leu383, located near the active-site cavity, forms an inclusion complex with cyclomaltohexaose. Separate substitutions of Glu380 by Gln and Asp completely eliminated the activity without inducing any significant changes in the circular dichroic spectra nor in the binding affinity for cyclomaltohexaose. Glu380, in cooperation with Glu186, therefore, is clearly indispensable for the liberation of beta-maltose from starch. Substitutions of Leu383 by Ile and Gln, in contrast, led to remarkable increases in the Km values of both mutants when compared to that of the non-mutant enzyme. The mutants also showed marked reductions in their binding affinities to cyclomaltohexaose. Overall, it would appear that the kcat/Km of soybean beta-amylase increases in proportion to the length of the substrate molecule, and depends also on the characteristics of the side chain of the residue at position 383. Leu383, therefore, may be important for both substrate penetration and subsequent retention at the active site. Based on the foregoing, we propose an action mechanism of soybean beta-amylase involving the interactions of three essential amino acid residues (Asp101, Glu186 and Glu380) in concert with Leu383, and assumed an indispensable role for Asp101.

Residues essential for catalytic activity of soybean beta-amylase

To determine which amino acid residues are essential for the catalytic activity of soybean beta-amylase, deoxyoligonucleotide site-directed mutagenesis was employed against aspartyl, glutamyl, and cysteinyl residues located in highly conserved regions found in beta-amylase family to date. Both substitution of aspartic acid at position 101 and that of glutamic acid at position 186 of the enzyme by neutral and acidic amino acids, respectively, led to the complete elimination of activity, but did not induce any significant changes in circular dichroic spectra or the binding affinity for cyclomaltohexaose, a substrate analogue. Taking account of the results obtained here, the above two amino acid residues are involved in the catalytic site of soybean beta-amylase. The replacement of glutamic acid at position 345 decreased activity to below 6% of the non-mutant level, implying that this residue may also play a crucial role in beta-amylase activity, although it may not be involved at the catalytic site itself. In contrast, substitution of cysteinyl residue at position 95 by a serinyl residue led to a drastic reducing of the optimal temperature (from 50 degrees C to 30 degrees C), suggesting that this cysteinyl residue is responsible for the thermal stability of the enzyme.