  An all-atom model of proteins is used to show that the same sequence of amino
acids can have many alternative structures, that are very distant from, and
that can be as stable as, the corresponding native structure. Such alternative
structures are not easily rationalized as belonging to the native basin and
indicate instead that the free energy landscape of proteins is
multi-funnel-shaped and that Anfinsen's thermodynamic hypothesis alone cannot
explain protein folding. An alternative two-step process for folding is
proposed and its consistency with the experimental evidence available is
discussed.
