Structures of three crystal forms of the sweet protein thaumatin

Three crystal forms of the sweet-tasting protein thaumatin from the African berry Thaumatococcus daniellii have been grown. These include two naturally occurring isoforms, A and B, that differ by a single amino acid, and a recombinant form of isoform B expressed in yeast. The crystals are of space groups C2 with a = 117.7, b = 44.9, c = 38.0 Å, and β = 94.0°, P2₁2₁2₁ with a = 44.3, b = 63.7 and c = 72.7 Å, and a tetragonal form P4₁2₁2 with a = b = 58.6 and c = 151.8 Å. The structures of all three crystals have been solved by molecular replacement and subsequently refined to R factors of 0.184 for the monoclinic at 2.6 Å, 0.165 for the orthorhombic at 1.75 Å, and 0.181 for the tetragonal, also at 1.75 Å resolution. No solvent was included in the monoclinic crystal while 123 and 105 water molecules were included in the higher resolution orthorhombic and tetragonal structures, respectively. A bound tartrate molecule was also clearly visible in the tetragonal structure. The r.m.s. deviations between molecular structures in the three crystals range from 0.6 to 0.7 Å for Cα atoms, and 1.1 to 1.3 Å for all atoms. This is comparable to the r.m.s. deviation between the three structures and the starting model. Nevertheless, several peptide loops show particularly large variations from the initial model.