Implication of histidine at the active site of exo-beta-(1-3)-D-glucanase from Basidiomycete sp. QM 806

Nutrition Intervention in Cancer

Nutrition intervention supports the patient with malnutrition secondary to cancer and its treatment and has been used in the primary and secondary prevention of common forms of cancer. During the emotional stress of dealing with cancer at any stage, patients derive increased quality of life and a sense of control over their lives as the result of receiving supportive advice on diet and lifestyle. Therefore, the use of nutrition intervention in cancer patients is justified in the absence of absolute proof of efficacy as long as it is done safely and with the consent of the cancer patient.

Implication of tryptophan and histidine in the active site of endo-polygalacturonase from Aspergillus ustus: elucidation of the reaction mechanism

The mechanism of action for the hydrolysis of polygalacturonic acid by the enzyme endo-polygalacturonase (poly(1,4-alpha-D-galacturonide) glycanohydrolase, EC 3.2.1.15) was investigated. The enzyme from Aspergillus ustus was purified to homogeneity and used for the study. The endo-polygalacturonase had a molecular weight of 36,000 daltons, a pI of 8.3, specific activity of 785 units/mg, Km of 0.82 mg/ml, and Vmax of 976 micromoles of product min-1 mg-1. Amino acids involved in the catalysis were identified by chemical modification and the active site characterized. Inhibition by hydroxynitrobenzyl bromide and diethylpyrocarbonate, followed by substrate protection studies showed that tryptophan and histidine were involved at or near the active site. Kinetic constants of partially inhibited enzyme, suggest the involvement of tryptophan in substrate binding and histidine in catalysis. Quenching of tryptophan fluorescence of the enzyme in the presence of polygalacturonic acid substantiated the conclusion that tryptophan was involved in substrate binding. An isotope effect of 1.8 was observed with deuterated water on the Vmax of the endo-polygalacturonase, with the proton inventory giving a linear relationship. The proposed mechanism involves a single proton transfer from the histidine residue of the enzyme to the glycosidic oxygen and hydrolysis by the addition of a water molecule.

Molecular biology of yeast exoglucanases

Three exoglucanase (Exg) genes have been reported in Saccharomyces cerevisiae. Gene EXG1 encodes the major isoenzyme (ExgI). Differential glycosylation of the primary translation product throughout the secretory pathway results in the secretion of several glycoforms. The major glycoform (ExgIb) contains two short carboxypeptidase Y-like oligosaccharides attached to both potential glycosylation sites present in the molecule. A minor glycoform (ExgIa) arises from the former by elongation of the second oligosaccharide. The protein portion is processed in the secretory pathway by the Kex2 protease. Gene EXG2 encodes a 63 kDa polypeptide with 12 potential glycosylation sites. The predicted protein, ExgII, carries a signal peptide at the amino terminus and a glycosyl-phosphatidyl inositol anchoring motif at the carboxyl end. The latter appears responsible for the particulate nature of this isoenzyme, since its elimination results in the secretion of this activity into the culture medium. Gene SSG1 encodes a 52 kDa polypeptide which is specifically synthesized during sporulation of diploids. SSG1 expression is under control of both sexual (a1-alpha 2 element) and nutritional control. Although homozygous ssg1/ssg1 diploid strains are still able to complete sporulation, they exhibited a delay in the appearance of mature asci. Single or double disruption of EXG1 and EXG2 did not result in any relevant phenotype and the triple mutant behaved as ssg1/ssg1. A ExgI-related enzyme is secreted by Candida albicans. All these four enzymes share 8 highly conserved regions in the same relative positions, indicating that they derived from a common ancestor. However, no clear function has so far been demonstrated for them.

A Phanerochaete chrysosporium beta-D-glucosidase/beta-D-xylosidase with specificity for (1-->3)-beta-D-glucan linkages

Phanerochaete chrysosporium is the best studied organism with respect to lignin degradation, but its degradation of the xylan component of lignocellulose is only now being studied. When grown on oat spelt xylan (mainly arabinoxylan), it produces an enzyme with beta-D-xylosidase and beta-D-glucosidase activity. This enzyme was purified by ultrafiltration followed by ammonium sulphate precipitation, anion-exchange chromatography using DEAE Biogel and Mono Q, and gel filtration using Superose 12. It is extracellular, with an apparent M(r) value of 44,500 as determined by SDS-PAGE; the pI is 4.67 and activity is maximal at pH 5 and 60 degrees C. The enzyme is of particular interest because its principal activity is against laminaribiose (3-O-beta-D-glucopyranosyl-D-glucopyranose and laminarin [(1-->3)-beta-D-glucan with ca. 3% of beta-(1-->6) branches] rather than cellobiose and xylobiose. It was competitively inhibited by D-glucono-1,5-lactone and deoxynojirimycin; with p-nitrophenyl beta-D-xylopyranoside as substrate, the Ki values were 32 and 87.5 microM, respectively, and with p-nitrophenyl beta-D-glucopyranoside, they were 35 and 68.7 microM, respectively. The Km values with p-nitrophenyl beta-D-xylopyranoside and p-nitrophenyl beta-D-glucopyranoside as substrates were 3.51 and 5.30 mM, respectively.

Hydrolysis of glycosylpyridinium ions by anomeric-configuration-inverting glycosidases

The hydrolyses of five beta-D-xylopyranosylpyridinium ions by the beta-D-xylosidase of Bacillus pumilus proceed with kcat values 10(8)-10(9)-fold larger than the rates of spontaneous hydrolysis of the same compounds. Log(kcat) values correlate well with aglycon pK(a) [B1g(V) = -0.52, r = 0.99], whereas the correlation of log(kcat/Km) is poor [r = 0.77; beta 1g(V/K) = approximately -0.6]. The (1-->3)-beta-D-glucanase of Sporotrichum dimorphosporum hydrolyses 4-bromo-2-(beta-D-glucopyranosyl)isoquinolinium ion with a rate enhancement of 10(8). The amyloglucosidase II of Aspergillus niger hydrolyses three alpha-D-glucopyranosylpyridinium ions with rate enhancements of 10(5)-10(8). The efficient hydrolysis of glycosylpyridinium ions by these three inverting glycosidases, the catalytic mechanism of which is unlikely to involve a nucleophile from the enzyme, makes it improbable that the hydrolysis of glycosylpyridinium ions by retaining glycosidases, discovered some years ago, is initiated by addition of a catalytic nucleophilic carboxylate group of the enzyme to the pyridinium ring.

Formation, separation and characterization of three beta-1,3-glucanases from Sclerotium glucanicum

The appearance of beta-1,3-glucanases in supernatants of Sclerotium glucanicum cultures was followed by SDS-PAGE and shown to be dependent on cultivation time. Three beta-1,3-glucanases were isolated and purified. Glucanase I and III appeared homogeneous on SDS-PAGE with molecular masses of 85 and 33.5 kDa, respectively. Enzyme I was an endo-splitting beta-1,3-glucanase. In hydrolyzing laminarin it released glucose, laminaritriose and laminaribiose as major endproducts and smaller amounts of higher oligosaccharides. Enzyme III was an exo-beta-1,3-glucanase removing glucose from laminarin and gentiobiose and glucose from scleroglucan. For laminarin as substrate the Km of enzyme I and III was 2.5 and 3.33 mg/ml, respectively. Enzyme II was only partially purified and found to be also an exo-beta-1,3-glucanase, releasing glucose as the only hydrolysis product from laminarin. It did not attack scleroglucan. Its molecular weight was determined to be 78 kDa. Optimum pH and temperature of the three enzymes were determined. The three activities were significantly inhibited by 1 mM Hg2+.

Microbial beta-glucanases different from cellulases

The beta-glucans different from cellulose are the most abundant class of polysaccharides. They are found in microorganisms and higher plants as structural entities of cell wall, as cytoplasmic and vacuolar reserve materials, and as extracellular substances. Enzyme systems capable to hydrolyze beta-glucans are produced by different microorganisms. The occurrence and nature of beta-glucanases and their substrates are reviewed. The regulation of biosynthesis of these enzymes, their properties, substrate and product specificities, mode of action and molecular cloning are described. The participation of beta-glucanases in the morphogenetic events of yeast cell is presented. The role and synergism of different types of 1,3-beta-glucanases in microbial cell wall lysis and the potential application for isolation of intracellular materials like proteins, carbohydrates, enzymes and as an analytical tool are discussed in the light of current knowledge.