  The primary structure of proteins, that is their sequence, represents one of
the most abundant set of experimental data concerning biomolecules. The study
of correlations in families of co--evolving proteins by means of an inverse
Ising--model approach allows to obtain information on their native
conformation. Following up on a recent development along this line, we optimize
the algorithm to calculate effective energies between the residues, validating
the approach both back-calculating interaction energies in a model system, and
predicting the free energies associated to mutations in real systems. Making
use of these effective energies, we study the networks of interactions which
stabilizes the native conformation of some well--studied proteins, showing that
it display different properties than the associated contact network.
