PMID:4721625
| Citation |
Case, GS, Sinnott, ML and Tenu, JP (1973) The role of magnesium ions in beta-galactosidase hydrolyses. Studies on charge and shape of the beta-galactopyranosyl binding site. Biochem. J. 133:99-104 |
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| Abstract |
1. beta-d-Galactopyranosyl trimethylammonium bromide is a competitive inhibitor of beta-galactosidase, K(i)=1.4+/-0.2mm at 25 degrees C. 2. Tetramethylammonium bromide is not an inhibitor (K(i)>0.2m). 3. The kinetics of deactivation of Mg(2+)-saturated, and of inhibitor-and Mg(2+)-saturated, enzyme in 10mm-EDTA are similar. 4. The apparent K(i) for the glycosylammonium salt is approx. 2.2mm in the absence of Mg(2+). 5. It is therefore concluded that Mg(2+) and the inhibitor bind independently, and that the Mg(2+) does not act as an electrophilic catalyst. 6. Complexant fluorescence measurements indicate binding of 1 Mg(2+) ion per 135000-dalton protomer. 7. This stoicheiometry is confirmed by equilibrium dialysis. 8. 1,6-Anhydrogalactopyranose is neither a substrate (k(cat.)/K(m)< 3x10(-2)m(-1).S(-1)) nor an inhibitor (K(i)>0.2m). 9. Considerations of conformations available to the cationic inhibitor and to the anhydrogalactose indicate that the substrate is bound with the pyranose ring in a conformation not greatly different from the normal chair (C1) conformation. |
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| Keywords |
Binding Sites; Bromine; Dialysis; Galactose; Galactosidases/antagonists & inhibitors; Kinetics; Magnesium; Molecular Conformation; Protein Binding; Protein Conformation; Pyrans; Quaternary Ammonium Compounds; Spectrometry, Fluorescence; Spectrophotometry; Spectrophotometry, Ultraviolet |
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