  A protein model with the pairwise interaction energies varying as local
environment changes, i.e., including some kinds of collective effect between
the contacts, is proposed. Lattice Monte Carlo simulations on the
thermodynamical characteristics and free energy profile show a well-defined
two-state behavior and cooperativity of folding for such a model. As a
comparison, related simulations for the usual G\={o} model, where the
interaction energies are independent of the local conformations, are also made.
Our results indicate that the evolution of interactions during the folding
process plays an important role in the two-state cooperativity in protein
folding.
