Selected enzymes from extreme thermophiles with applications in biotechnology

Enzymes from extreme thermophiles that grow above 70 °C have a number of attractions in industrial applications. They are often highly resistant to denaturing conditions and are stable at elevated temperatures and over a range of pH values. There has been a widespread search for micro-organisms producing novel enzymes and where found, most publications (and research grant applications) promise that their superior properties would be suited to particular industries that operate at elevated temperatures - for example, bleaching of kraft pulp in the pulp and paper industry. Yet examination of the academic and patent literature reveals few of these proteins adopted in industrial enzymology. Most employed successfully have been as laboratory reagents, particularly Thermus aquaticus DNA polymerase, which made the polymerase chain reaction possible and revolutionized gene manipulation at a laboratory level. Extremozymes marketed for the laboratory are lower volume / higher value products and have been cloned and expressed (usually) in Escherichia coli. This review examines the characterization and application of thermophilic enzymes for several activities that have been identified and (usually) produced in recombinant bacteria. DNA polymerases, glycosyl hydrolases, lipases and proteases from extreme thermophiles are described and evaluated for their potential and actual applications in biotechnology. Some of the barriers to widespread industrial acceptance are described. Emphasis is placed on a number of examples illustrated by the anaerobic extreme thermophiles Caldicellulosiruptor sp. and Dictyoglomus sp. with which the authors are familiar. A number of attractive enzymes are not scalable economically in Escherichia coli or the organism from which the gene has been isolated is an obligate anaerobe and enzyme yields from the native organism are low. Consequently, attention has turned to commonly used ‘cell factories’ to provide suitable yields of enzymes used as bulk chemicals. These 'factories' are usually fungi such as Saccharomyces cerevisiae and Trichoderma reesei or bacteria such as Bacillus sp. A number of challenges, such as codon usage incompatibility, must be overcome to achieve economic yields.