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pH-induced transitions in cholera toxin conformation: a fluorescence study
M J De Wolf1, G A Van Dessel, A R Lagrou
1RUCA-Laboratory for Human Biochemistry, University of Antwerp, Belgium.
Biochemistry
|June 30, 1987
Summary
Cholera toxin (CT) subunit A binds detergents weakly, while subunits CT and CT B show increased detergent binding at lower pH. Receptor binding does not affect CT or CT B detergent interactions.
Area of Science:
- Biochemistry
- Molecular Biology
Background:
- Cholera toxin (CT) is a bacterial toxin composed of an active A subunit (CT A) and a binding B subunit (CT B).
- Understanding the biophysical properties of CT subunits is crucial for elucidating toxin mechanisms.
Purpose of the Study:
- To investigate the detergent binding affinities of cholera toxin (CT) and its subunits (CT A and CT B).
- To explore the influence of pH, receptor binding, and chemical modifications on detergent interactions.
Main Methods:
- Intrinsic fluorescence measurements using dibrominated Bry 96.
- Assessing detergent binding at varying pH, salt concentrations, and in the presence of receptor analogs (GM1/oligo-GM1).
- Investigating the effect of dithiothreitol activation and solute quenching (acrylamide).
Main Results:
- CT A exhibits a higher affinity for the detergent Bry 96 compared to intact CT and CT B.
- Receptor binding and dithiothreitol activation do not significantly alter detergent binding for CT and CT B.
- Detergent binding to CT and CT B increases significantly as pH decreases, particularly between pH 6.5 and 4.0.
- Lowering pH exposes the Trp-88 residue in CT B, indicating conformational changes.
Conclusions:
- CT A possesses weak hydrophobic properties, influencing its detergent interaction.
- The B subunit of CT undergoes pH-dependent conformational changes affecting detergent binding.
- These findings provide insights into the biophysical behavior of cholera toxin subunits under different environmental conditions.