  The mechanical unfolding of proteins is investigated by extending the
Wako-Saito-Munoz-Eaton model, a simplified protein model with binary degrees of
freedom, which has proved successful in describing the kinetics of protein
folding. Such a model is generalized by including the effect of an external
force, and its thermodynamics turns out to be exactly solvable. We consider two
molecules, the 27th immunoglobulin domain of titin and protein PIN1. In the
case of titin we determine equilibrium force-extension curves and study
nonequilibrium phenomena in the frameworks of dynamic loading and force clamp
protocols, verifying theoretical laws and finding the position of the kinetic
barrier which hinders the unfolding of the molecule. The PIN1 molecule is used
to check the possibility of computing the free energy landscape as a function
of the molecule length by means of an extended form of the Jarzynski equality.
