Intrinsically disordered proteins (IDPs) represent a big group of proteins, whose structure is more flexible compared to globular proteins. Antitoxin Phd is an IDP and originates from a toxin-antitoxin (TA) module of bacteriophage P1. The binding of antitoxin to toxin Doc inhibits the Doc activity. The disordered C-terminal domain of antitoxin Phd forms an α-helix upon binding. Free toxin Doc inhibits protein synthesis and slows down the metabolism of the cell. Apart from the binding of toxin Doc, other factors, like the environment in which the antitoxin is present, affect its structure. Using CD-spectroscopy and DSC-calorimetry we examined the affects of protein concentration, salt concentration and temperature on the protein structure and evaluated thermodynamic parameters, that accompany structural changes of Phd. High concentrations of protein and salt as well as low temperature move the equilibrium towards a structured dimer. Low concentrations of protein and salt as well as high temperature move the equilibrium towards a disordered monomer. Structuring of the protein increases its thermodynamic stability. These findings are important for future in vitro experiments, because we can change the protein structure with changing the environmental conditions of the solution. The changes in protein structure of Phd, caused by changes in temperature and salt concentration, can be compared with those of physiologically relevant toxin binding.
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