In vitro mutagenesis of a xylanase from the extreme thermophile Caldocellum saccharolyticum

Extremophile - An Adaptive Strategy for Extreme Conditions and Applications

The concurrence of microorganisms in niches that are hostile like extremes of temperature, pH, salt concentration and high pressure depends upon novel molecular mechanisms to enhance the stability of their proteins, nucleic acids, lipids and cell membranes. The structural, physiological and genomic features of extremophiles that make them capable of withstanding extremely selective environmental conditions are particularly fascinating. Highly stable enzymes exhibiting several industrial and biotechnological properties are being isolated and purified from these extremophiles. Successful gene cloning of the purified extremozymes in the mesophilic hosts has already been done. Various extremozymes such as amylase, lipase, xylanase, cellulase and protease from thermophiles, halothermophiles and psychrophiles are of industrial interests due to their enhanced stability at forbidding conditions. In this review, we made an attempt to point out the unique features of extremophiles, particularly thermophiles and psychrophiles, at the structural, genomic and proteomic levels, which allow for functionality at harsh conditions focusing on the temperature tolerance by them.

Thermozymes

Enzymes synthesized by thermophiles (organisms with optimal growth temperatures > 60 degrees C) and hyperthermophiles (optimal growth temperatures > 80 degrees C) are typically thermostable (resistant to irreversible inactivation at high temperatures) and thermophilic (optimally active at high temperatures, i.e., > 60 degrees C). These enzymes, called thermozymes, share catalytic mechanisms with their mesophilic counterparts. When cloned and expressed in mesophilic hosts, thermozymes usually retain their thermal properties, suggesting that these properties are genetically encoded. Sequence alignments, amino acid content comparisons, and crystal structure comparisons indicate that thermozymes are, indeed, very similar to mesophilic enzymes. No obvious sequence or structural features account for enzyme thermostability and thermophilicity. Thermostability and thermophilicity molecular mechanisms are varied, differing from enzyme to enzyme. Thermostability and thermophilicity are usually caused by the accumulation of numerous subtle sequence differences. This review concentrates on the mechanisms involved in enzyme thermostability and thermophilicity. Their relationships with protein rigidity and flexibility and with protein folding and unfolding are discussed. Intrinsic stabilizing forces (e.g., salt bridges, hydrogen bonds, hydrophobic interactions) and extrinsic stabilizing factors are examined. Finally, thermozymes' potential as catalysts for industrial processes and specialty uses are discussed, and lines of development (through new applications, and protein engineering) are also proposed.

Xylanolytic enzyme production by an Aspergillus niger isolate

Production of xylanolytic enzymes by an Aspergillus niger CCMI 850 isolate was investigated in batch cultures. The effect of the composition of a fermentation medium that did not include chemical inducers, on beta-xylanase, beta-xylosidase, alpha-L-arabinofuranosidase, and total cellulase activity was studied. With 4% xylan as the carbon source, about 65 U/mL of beta-xylanase was obtained, whereas the total cellulase activity was undetectable, under the specified conditions. This beta-xylanase activity represents the highest reported for a wild-type strain of A. niger. The effect of pH and temperature on the activity of beta-xylanase was studied. Partial characterization of the beta-xylanase showed that with insoluble birchwood as substrate the Km and Vmax were 0.3 mM and 19 mumol/min, respectively. Aspects of using the crude beta-xylanase preparation for applications in the pulp and paper industry were discussed.