Abstract
The invertase from Equisetum giganteum L., a lower vascular sporophytic plant, was purified to chromatographic and electrophoretic homogeneity. The enzyme appears to be a pentamer, M r 91,000, formed by identical subunits ( M r 18,000). An isoelectric point of 4.5 was found for the protein. The optimum pH was about 4.5 and the preferred substrate is sucrose, K m =10.4 mM. Glucose and fructose are classical non-competitive ( K i =120 mM) and competitive ( K i =96 mM) inhibitors, respectively. Proteins which behave as activators of the enzyme suppress the inhibitory action of the reaction products. The activation energy of the hydrolytic reaction is 18,000 cal/mol. The outstanding property of the invertase is a hysteretic behavior when the pH changes from 3.05 to 4.5. The lag time is independent of the enzyme concentration suggesting that slow conformational changes are induced by pH variation and not by different polymerization states.
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