Invertase, an enzyme from yeast studied extensively that played an important role in the development of biochemistry, is synthesized constitutively in the mesophilic fungi, wherein its activity increases with an increase in growth (biomass). By contrast, it is an inducible enzyme in the two thermophilic fungi studied, i.e., it is produced only in response to the availability of sucrose in the growth medium (Table 10.1). Strangely, the induced enzyme activity begins to diminish before any substantial quantity of sucrose is utilized or an appreciable amount of biomass is formed (Chaudhuri et al., 1999). Paradoxically, despite this pattern of development of invertase, the final mycelial yields are not affected. An investigation of this unusual pattern of development of invertase activity in a thermophilic fungus, T. lanuginosus, has given some insight into various strategies in the biochemical adaptation of thermophilic fungi.

It seems likely that invertase in thermophilic fungi is localized in the hyphal tip. In the early stages of growth the number of tips per unit mass of mycelium is maximal and, correspondingly, the invertase activity is maximal. At later times, the number of tips per unit mass of mycelium decreases as the increase in mass is contributed mainly by cell elongation and cell wall thickening. Consequently, invertase-specific activity shows an apparent decline with growth. In contrast, in the mesophilic fungus N. crassa, invertase is distributed all along the hypha, an unquantitated amount being bound to the wall. Consequently, its specific activity steadily increases in proportion with the increase in growth measured as an increase in biomass.

Section 10.2 referred to an early finding of incongruity in the metabolic rate of thermophilic fungi and the van't Hoff rule (Noack, 1920). The invertase example hints on the nature of molecular evolution—not all proteins evolved thermostability at the

Table 10.1 Distinctive Properties of Invertase of a Thermophilic and a Mesophilic Fungus

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