The biochemistry of thermophilic micro-organisms: The nature and control of the enzymes of the glyoxylate cycle in a thermophilic bacillus.
thesisposted on 19.11.2015, 09:08 by Mansel William. Griffiths
The isocitrate lyase from a thermophilic Bacillus is activated about threefold by a variety of salts. Such strong stimulation of activity is not seen with isooitrate lyase from the mesophiles, Bacillus licheniformis, Bacillus megaterium, Escherichia coli, and Aspergillus nidulans. The salt activation is markedly pH-dependent. At pH values above 8.6, salt (KC1) indeed inhibits the enzyme activity. Potassium chloride also causes a significant shift of the pH optimum of the enzyme towards the acid side. As the temperature of the enzyme reaction is raised, activation becomes progressively weaker. Potassium chloride also affords considerable protection against enzyme denaturation at 55 C. The activation and the stabilisation, however, appear to be independent effects. Of six other enzymes in the thermophile that were examined, isocitrate dehydrogenase was equally strongly activated by KC1 and malate synthase was less strongly, but significantly activated; citrate synthase, malate dehydrogenase, glutamate dehydrogenase, lactate dehydrogenase were unaffected or slightly inhibited by KC1. The property of being strongly activated by salt appears to be a peculiar characteristic of the thermophile isocitrate lyase and possibly evolved concomitantly with its thermostability. As well as differences in the properties of the thermophile isocitrate lyase and the enzyme from mesophiles, there appears to be a difference in the way in which the enzyme is controlled in the thermophile and the rnesophile, E. coli. The synthesis of isocitrate lyase in the thermophile seems to be induced by isocitrate. A mutant, NG-15, of the thermophile is deficient in isocitrate dehydrogenase, and concomitantly there is a large pool of isocitrate in cells of this mutant. This mutant is able to synthesise isocitrate lyase under most growth conditions, due to the high level of isocitrate in its cells. Further evidence is presented to support this conclusion. There is no evidence in the literature to suggest that the enzyme from E. coli is controlled in this manner. The intermediates of the tricarboxylic acid cycle appear to play an important role in the fine control of the isocitrate lyase from the thermophile, but there is evidence that they are not as important for the fine control of the enzyme from the mesophile, E. coli. Differences have been shown to exist in the properties and mode of control of an enzyme from thermophilic and from mesophilic sources.