Geeta Nanda

Induction of xylanase in aspergillus ochraceus

Aspergillus ochraceus produced xylanase inductively in washed glucose-grown mycelia incubated with methyl β-d-xyloside. The production of xylan endo-l,4-β-xylosidase (xylanase) was 4.2 times greater than that obtained in xylan medium. The inducer was not metabolized and was most active at a concentration of 0.3 g/L with an incubation period of 16 h. The uptake of the inducer by the mycelia seemed to be energy-dependent. Methyl α-d-xyloside repressed xylanase synthesis. Glucose, cyeloheximide, actinomycin D and eAMP were found to inhibit xylanase induction by methyl β-D-xyloside.

Biochemical approaches of increasing thermostability of β‐amylase from Bacillus megaterium B6

Studies on the irreversible thermoinactivation of β‐amylase from Bacillus megaterium B6 exposed to 60°C revealed that the deactivation mechanism probably results from the oxidation of thiols present at the active site of the enzyme. Several attempts were made to increase its thermostability, which indicated that Mn2+ played a key role in determining thermostability and partially reactivating the inactivated enzyme. Immobilization of β‐amylase through gel‐entrapment and covalent crosslinking brought about a remarkable increase in thermotolerance with about a 14‐fold increase in catalytic half‐life.

Thermostability of β-xylosidase from Aspergillus sydowii MG49

Heating of Aspergillus β‐xylosidase at 85°C ± 1°C and pH 5.5–6.0 (optimum for activity), causes irreversible, covalent thermoinactivation of the enzyme, involving oxidation of the thiol groups that are required for catalysis. Exogenous addition of cysteine, DTT, GSH and mercaptoethanol stabilizes the enzyme by extending its half‐life. A similar effect is also exhibited by bivalent cations like Mg2+, Mn2+, Co2+, Ca2+and Zn2+ while, on the other hand Cu2+ accelerates thermoinactivation. Chemical modification of crude β‐xylosidase with cross‐linking agents like glutaraldehyde or covalent immobilization to a nonspecific protein like gelatin and BSA also enhances enzyme thermostability. These results suggest that addition of thiols and bivalent metal ions to a crude β‐xylosidase preparation or immobilization/chemical modification enhances its thermal stability, thus preventing loss of catalytic activity at elevated temperatures.

Production of xylanolytic enzymes during growth on pulverized grass by Aspergillus ochraceus-42

Xylanase and β‐xylosidase with activity of 6.46 U mg‐1 and 0.500 U mg‐1, respectively, were produced extracellularly by Aspergillus ochraceus during growth on pulverized grass in liquid state fermentation, compared to 9.3 U mg‐1 and 0.74 U mg‐1 when pure xylan was used. The culture filtrate was devoid of any cellulase activity. Xylanolytic enzymes were produced optimally in 144 h of incubation on 1% pulverized grass, pH 6.5. About 8.43% (w/w) sugars were liberated from alkali‐treated grass in 6 h by the synergistic effect of xylanolytic enzymes. The half‐lives for xylanase and β‐xylosidase at 50°C were 210 min and 300 min, respectively, and half‐life increased with the increase in protein concentration. Both mono‐ and divalent cations, especially K+ and Zn2+, exhibited a profound effect on the rate of enzyme saccharification.

β-Amylase fromBacillus megaterium

Bacillus megaterium strain B6 producing extracellular β-amylase was isolated and grown in a medium supplemented with waste potato starch. It showed highest enzyme synthesis in the early stationary phase. The partially purified β-amylase had a temperature optimum at 60°C and a pH optimum at 6.9 and was not affected by Schardinger dextrins. These properties would allow its application in sugar industry.

Variation of antimetabolite sensitivity with different carbon sources inBacillus subtilis

The susceptibility ofBacillus subtilis to amino acid analogues was found to be markedly influenced by the carbon source used in the test media. Thialysine inhibited the bacterium with a greater number of carbon sources than the other two analogues tested. 5-Hydroxylysine was inhibitory with glycerol, lactose,D-xylose,L-arabinose and soluble starch while ethionine showed toxicity with lactose,D-xylose andL-arabinose. None of these analogues were toxic at the levels tested whenD-galactose was used as carbon source. The bacterium was not susceptible to thialysine with glycerol, to 5-hydroxylysine withL-arabinose and to ethionine with lactose.

Adsorption of β‐amylase from Bacillus carotarum B6 onto different native starches

β‐Amylase from Bacillus carotarum B6, a soil isolate, was readily adsorbed onto native raw starch from various sources. The absorbability was highest toward arrowroot starch and was complete within 30 min of exposure. Adsorption was remarkably affected in the presence of surfactants like Tween 80.

High activity xylanase from Aspergillus sydowii MG49 during growth on jute stalk lignocellulose

Aspergillus sydowil MG49 produced 33.0 U mg‐1 of extracellular xylanase activity when grown in liquid state fermeniation (LSF) with 1% ground jute stalk as the sole carbon source compared to 56.0 U mg‐1 when pure xylan was used. Optimum time‐course and pH for maximum enzyme production were 144 h and 4.0 respectively. The culture filtrate was devoid of any cellulase and β‐xylosidase activity. The xylanase exhibited optimum activity at 60°C and pH 5.5. Partially‐fermented jute stalk could be recycled at least twice for xylanase production, exhibiting 25.8 and 17.4 U mg‐1 activity in two later consecutive cycles respectively.