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Thioglycoside hydrolysis catalyzed by beta-glucosidase
1Department of Chemistry, Tulane University, New Orleans, LA 70118, USA.
Biochemical and Biophysical Research Communications
|August 31, 2007
Summary
Sweet almond beta-glucosidase exhibits thioglycohydrolase activity, with significantly lower catalytic rates for S-glycosides compared to O-glycosides. The enzyme mechanism for thioglucoside hydrolysis does not involve general acid catalysis.
Area of Science:
- Enzymology
- Biochemistry
Background:
- Sweet almond beta-glucosidase (EC 3.2.1.21) possesses thioglycohydrolase activity.
- Enzyme kinetics reveal differences in substrate hydrolysis rates for S- and O-glycosides.
Purpose of the Study:
- To investigate the catalytic mechanism of sweet almond beta-glucosidase on thioglycosides.
- To compare the kinetic parameters and pH-dependence of S- and O-glycoside hydrolysis.
Main Methods:
- Enzyme kinetic assays were performed using p-nitrophenyl thioglucoside (pNPSG) and its O-glycoside analogue.
- pH-profile analysis of catalytic efficiency (kcat/Km) was conducted.
- Solvent kinetic isotope effect studies were employed.
Main Results:
- Km values for S- and O-glycosides were comparable, but kcat values for S-glycosides were approximately 1000-fold lower.
- The pH-dependence of kcat/Km for pNPSG hydrolysis mirrored that of the O-glycoside, indicating shared catalytic group requirements.
- No solvent kinetic isotope effect was observed for pNPSG hydrolysis.
Conclusions:
- Sweet almond beta-glucosidase catalyzes thioglucoside hydrolysis via a mechanism distinct from general acid catalysis.
- A deprotonated carboxylate (pKa 4.5) and a protonated group (pKa 6.7) are crucial for thioglucoside hydrolysis, similar to O-glycoside hydrolysis.
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