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'Thermitase' (EC 3.4.21.14), a thermostable extracellular serine protease from
Thermoactinomyces vulgaris, binds one calcium ion with a dissociation constant
of about 10-4M at 25 degrees C and pH 7.5 to 3.5. In addition, two calcium ions
are bound more tightly to the enzyme, as shown by experiments with a
calcium-selective electrode. The single most weakly bound calcium ion causes a
slight quenching of the protein fluorescence emission, accompanied by a
stabilization against thermal denaturation or autolysis and an increase of
esterolytic activity of approx. 10%. The tightly bound calcium ions have only a
slight influence on activity or on thermal denaturation or autolytic
degradation. The activation parameters of thermal denaturation indicate that
'thermitase' belongs to the class of thermostable enzymes with a high intrinsic
stability.
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