Thermophilic fungi: Their physiology and enzymes

Thermophilic fungi are a small assemblage in mycota that have a minimum tem perature of growth at or above 20 degrees C and a maximum temperature of gr owth extending Itp to 60 to 62 degrees C. As the only representatives of eu karyotic organisms that can grow at temperatures above 45 degrees C, the th ermophilic fungi are valuable experimental systems for investigations of me chanisms that allow growth at moderately high temperature yet limit their g rowth beyond 60 to 62 degrees C. Although widespread in terrestrial habitat s, they have remained underexplored compared to thermophilic species of eub acteria and archaea. However, thermophilic fungi are potential sources of e nzymes with scientific and commercial interests. This review, for. the firs t time, compiles information on the physiology and enzymes of thermophilic fungi. Thermophilic fungi can be grown in minimal media with metabolic rate s and growth yields comparable to those of mesophilic fungi. Studies of the ir growth kinetics, respiration, mixed-substrate utilization, nutrient upta ke, and protein breakdown rate have provided some basic information not onl y on thermophilic fungi but also on filamentous fungi in general. Some spec ies have the ability to grow at ambient temperatures if cultures are initia ted with germinated spores or mycelial inoculum or if a nutritionally rich medium is used Thermophilic fungi have a powerful ability to degrade polysa ccharide constituents of biomass. The properties of their enzymes show diff erences not only among species but also among strains of the same species. Their extracellular enzymes display temperature optima for activity that ar e close to or above the optimum temperature for the growth of organism and, in general, are more heat stable than those of the mesophilic fungi. Some extracellular enzymes from thermophilic fungi are being produced commercial ly, and a few others have commercial prospects. Genes of thermophilic fungi encoding lipase, protease, xylanase, and cellulase have been cloned and ov erexpressed in heterologous fungi, and pure crystalline proteins have been obtained for elucidation of the mechanisms of their intrinsic thermostabili ty and catalysis. By contrast, the thermal stability of the few intracellul ar enzymes that have been purified is comparable to or; in some cases, lowe r than that of enzymes from the mesophilic fungi. Although rigorous data ar e lacking it appears that eukaryotic thermophily involves several mechanism s of stabilization of enzymes or optimization of their activity, with diffe rent mechanisms operating for different enzymes.