| Autor: |
Grouselle, Michel, Abdul Aziz Thiam, Pudles, Julio |
| Předmět: |
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| Zdroj: |
European Journal of Biochemistry; 1973, Vol. 39, p431-441, 11p |
| Abstrakt: |
Specific acylation of the histidine residues of yeast hexokinase with diethylpyrocarbonate at pH 6.1 or 7.5 leads to a partial and reversible inactivation of the enzyme. The carboethoxylation of the histidine residues at pH 7.5 was faster than at 6.1 and the loss in activity was also higher. However the difference in the amount of inactivation was related to a greater instability of the carboethoxylated enzyme at pH 7.5. When the carboethoxylation was carried out at pH 7.5 and at 0 °C, only the histidines residues were modified (nine residues per enzyme subunit) and still 40% of the activity was left. D-Glucose protects the enzyme activity in the course of the carboethoxylation at pH 7.5, but has no effect at pH 6.1. However the rate and number of histidines acylated were the same as when the experiments were carried in absence of the substrate. All these results clearly indicate that the histidine residues are not implicated either in the catalytic or in the substrate binding site of yeast hexokinase. This was also confirmed by showing that the 60% inactivated enzyme had the same Km as the native enzyme for both substrates D-glucose and Mg · ATP. Only v was affected. The inactivation effect of the carboethoxylation was not related to a displacement of the pH optimum of the activity, since the enzymes species having five and eight histidine residues modified-respectively in absence and presence of D-glucose at pH 7.5 gave the same pH profile of activity as the native enzyme. Absorption spectroscopy, fluorescence and ultracentrifugation studies have shown that the loss of enzymic activity in the course of the modification of the histidine residues is only related to changes of the protein conformation. [ABSTRACT FROM AUTHOR] |
| Databáze: |
Complementary Index |
| Externí odkaz: |
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