Concluding Remarks

Although widespread in terrestrial habitats, thermophilic fungi remain underexplored. Because they occur in habitats that are heterogeneous in temperature, the types and concentration of nutrients, competing species and other variables, they probably also adapted to factors other than a high temperature. They can be grown in minimal media with metabolic rates and growth yields comparable to those of mesophilic fungi. Studies of their growth kinetics, respiration, mixed-substrate utilization, nutrient uptake and protein breakdown rate have provided some basic information not only on thermophilic fungi but also on fungi in general. Thermophilic fungi have a powerful ability to degrade the polysaccharide constituents of biomass. The properties of their enzymes show differences not only among species but also among strains of the same species. Their extracellular enzymes display temperature optima for activity that is close to or above the optimum temperature for the growth of the organism and, in general, are more stable than those of mesophilic fungi. Genes of thermophilic fungi encoding lipase, protease, xylanase and cellulose have been cloned and overexpressed in heterologous fungi, and pure crystalline proteins have been obtained for elucidation of the mechanism of their intrinsic thermostability and catalysis. By contrast, the thermal stability of the few intracellular enzymes that have been purified is comparable to or, in some cases, lower than that of enzymes from the mesophilic fungi. The gain of thermostability in certain intracellular proteins may not be possible without a concomitant loss of catalytic activity, as shown by the example of invertase. In the thermophilic fungi, this enzyme is exceptionally unstable, cautioning against the generalization that thermophily in fungi is due to the thermostability of proteins. There is no single adaptive strategy in fungi inhabiting a hot environment; rather, a combination of mechanisms allow a thermophilic fungus to adapt to a hot environment: the intrinsic thermostability of macromolecules, interaction of proteins with ions and other cellular proteins including chaperonin molecules, self-aggregation, and possibly covalent or noncovalent interactions with the cell wall, the placement of unstable, inducible enzymes in the most strategic location of the hypha and their resynthesis.

Currently, enzymes from hyperthermophilic archaea that grow at temperatures beyond 80°C are being sought for applications in biotechnology. However, since flexibility of protein conformation is essential for catalysis, the enzymes from the hyperthermophiles will have optimal conformational flexibility at the temperatures for which they are adapted to grow but could become too rigid and have low catalytic rates at temperatures that range from 50 to 65 °C. Therefore, in most situations, enzymes from thermophilic fungi may be better suited in biotechnology than enzymes from hyperthermophilic archaea.

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