Thermodynamic Stability of the Dimeric Toxin CcdB

The toxin-antitoxin module ccd located on Escherichia coli plasmid F encodes the antitoxin CcdA and the toxin CcdB. When not complexed with CcdA, CcdB attacks its cellular target gyrase and kills the cell by causing inhibition of both transcription and replication. At physiological conditions CcdB exists as a homodimer. Here we present a study of CcdB unfolding that is focused on the characterization of the structure-thermodynamics relationship needed for understanding the stability and function of CcdB at the molecular level. In this light, thermodynamic parameters of unfolding obtained by global analysis of urea-induced unfolding curves measured at various temperatures by circular dichroism spectroscopy were parsed into the contributions arising from the differences in intra- and inter-molecular interactions of CcdB in the folded dimeric and unfolded monomeric state. According to this parsing the unfolded monomers retain about 30% of the residual structure indicating that the urea-denatured state of CcdB is not a completely unfolded state.