Salt bridges are believed to have an important role in stabilisation of native protein structure. The assessment of their contribution to the electrostatic term of the free energy is a yet unsolved task. One can point out a number of reasons: beginning with conceptual issues, such as the complex nature of the interplay between the different types of non-covalent interactions and going to details, such as the interactions of the participating groups with their environments. Here we focus on the interplay between electrostatic interactions the functional groups forming salt bridges are involved in and the conformational flexibility of the protein molecule. We show that the connection between these two factors appears to be one of the keys for a better understanding of the forces determining stability of proteins.