Thermodynamics of intersubunit interactions in cholera toxin upon binding to the oligosaccharide portion of its cell surface receptor, ganglioside GM1

Biochemistry. 1989 Jun 13;28(12):5019-24. doi: 10.1021/bi00438a017.


The binding and the energetics of the interaction of cholera toxin with the oligosaccharide portion of ganglioside GM1 (oligo-GM1), the toxin cell surface receptor, have been studied by high-sensitivity isothermal titration calorimetry and differential scanning calorimetry. Previously, we have shown that the association of cholera toxin to ganglioside GM1 enhances the cooperative interactions between subunits in the B-subunit pentamer [Goins, B., & Freire, E. (1988) Biochemistry 27, 2046-2052]. New experiments presented in this paper reveal that the oligosaccharide portion of the receptor is by itself able to enhance the intersubunit cooperative interactions within the B pentamer. This effect is seen in the protein unfolding transition as a shift from independent unfolding of the B promoters toward a cooperative unfolding. To identify the origin of this effect, the binding of cholera toxin to oligo-GM1 has been measured calorimetrically under isothermal conditions. The binding curve at 37 degrees C is sigmoidal, indicating cooperative binding. The binding data can be described in terms of a nearest-neighbor cooperative interaction binding model. In terms of this model, the association of a oligo-GM1 molecule to a B protomer affects the association to adjacent B promoters within the pentameric ring. The measured intrinsic binding enthalpy per protomer is -22 kcal/mol and the cooperative interaction enthalpy -11 kcal/mol. The intrinsic binding constant determined calorimetrically is 1.05 x 10(6) M-1 at 37 degrees C and the cooperative Gibbs free energy equal to -850 cal/mol.(ABSTRACT TRUNCATED AT 250 WORDS)

Publication types

  • Research Support, U.S. Gov't, P.H.S.

MeSH terms

  • Binding Sites
  • Binding, Competitive
  • Calorimetry
  • Calorimetry, Differential Scanning
  • Cholera Toxin / analysis*
  • Energy Transfer
  • G(M1) Ganglioside / analysis*
  • Membrane Proteins / metabolism*
  • Oligosaccharides / analysis*
  • Temperature
  • Thermodynamics


  • Membrane Proteins
  • Oligosaccharides
  • G(M1) Ganglioside
  • Cholera Toxin