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Clostridium difficile toxins A and B are cation-dependent UDP-glucose hydrolases with differing catalytic activities
1Department of Medicine, University of Virginia School of Medicine, Charlottesville, Virginia 22908, USA.
Abstract:
Toxins A and B of Clostridium difficile are UDP-glucose glucosyltransferases that exert their cellular toxicity primarily through their abilities to monoglucosylate, and thereby inactivate, Rho family small GTPases. Toxin A also hydrolyzes UDP-glucose, although this activity is not well characterized. In this study, we measured the kinetics of UDP-glucose hydrolysis by toxins A and B and found significant differences in the catalytic activities of these two structurally homologous toxins. The toxins displayed similar Michaelis constants (Km) for UDP-glucose, but the maximal velocity (Vmax) of toxin B was approximately 5-fold greater than that of toxin A. Toxins A and B exert their enzymatic actions intracellularly, and, interestingly, we found that each toxin absolutely required K+ for optimal hydrolase activity; Na+ was inactive. The toxins also required certain divalent cations for activity and exhibited a significantly greater Vmax and lower Km in the presence of Mn2+ as compared with Mg2+. We conclude that C. difficile toxins A and B are cation-dependent UDP-glucose hydrolases that differ significantly in their catalytic activities, a finding that may have important implications in understanding their different cytotoxic effects.
Insights
Clostridium difficile toxins A and B are cation-dependent UDP-glucose hydrolases. Toxin B exhibits a 5-fold higher maximal velocity than toxin A, impacting their cellular toxicity.
Area of Science:
- Biochemistry
- Microbiology
- Enzymology
Background:
- Clostridium difficile toxins A and B are key virulence factors.
- These toxins glucosylate Rho GTPases, leading to cellular dysfunction.
- UDP-glucose hydrolysis by Toxin A is poorly understood.
Purpose of the Study:
- To kinetically characterize UDP-glucose hydrolysis by C. difficile toxins A and B.
- To compare the catalytic activities and cofactor requirements of toxins A and B.
- To elucidate the role of cation cofactors in toxin hydrolase activity.
Main Methods:
- Enzyme kinetics assays were performed for UDP-glucose hydrolysis.
- Michaelis constants (Km) and maximal velocities (Vmax) were determined.
- The effects of various cations (K+, Na+, Mn2+, Mg2+) on enzyme activity were investigated.
Main Results:
- Toxins A and B are cation-dependent UDP-glucose hydrolases.
- Both toxins showed similar Km but toxin B had a 5-fold higher Vmax than toxin A.
- Optimal activity required K+ and was significantly enhanced by Mn2+ over Mg2+.
Conclusions:
- C. difficile toxins A and B possess distinct catalytic activities as UDP-glucose hydrolases.
- Differences in Vmax suggest varying intracellular hydrolysis rates.
- Cation dependency, particularly Mn2+, is crucial for hydrolase function, potentially influencing cytotoxic effects.