Structure of monellin refined to 2.3 angstrom resolution in the orthorhombic crystal form

The structure of orthorhombic crystals of monellin, a sweet protein extracted from African serendipity berries, has been solved by molecular replacement and refined to 2.3 Angstrom resolution. The final R factor was 0.150 for a model with excellent geometry. A monellin molecule consists of two peptides that are non-covalently bound, with chain A composed of three beta-strands interconnected by loop regions and chain B composed of two beta-strands interconnected by an alpha-helix. The N terminus of chain A is in close proximity to the C terminus of chain B. The two molecules in the asymmetric unit are related by a non-crystallographic twofold axis and form a dimer, similar to those previously observed in other crystal forms of both natural and single-chain monellin. The r.m.s. deviation between the C alpha atoms in the two independent molecules is 0.60 Angstrom, while the deviations from the individual molecules in the previously reported monoclinic crystals are 0.50-0.57 Angstrom. This result proves that the structure of monellin is not significantly influenced by crystal packing forces.