Crystal Structure of a Sweet Protein, Monellin, at 5.5-Å Resolution

A 5.5-Å resolution crystal structure of an intensely sweet protein, monellin, has been determined on the basis of four isomorphous heavy-atom derivatives. The structure reveals many protruded features unlike most globular proteins. There are four monellin molecules in the asymmetric unit of the crystal and two dimer molecules related by a noncrystallographic twofold axis.