Atomic structure of the sweet-tasting protein thaumatin I at pH 8.0 reveals the large disulfide-rich region in domain II to be sensitive to a pH change.
Thaumatin, an intensely sweet-tasting plant protein, elicits a sweet taste at 50 nM. Although the sweetness remains when thaumatin is heated at 80 °C for 4h under acid conditions, it rapidly declines when heating at a pH above 6.5. To clarify the structural difference at high pH, the atomic structure of a recombinant thaumatin I at pH 8.0 was determined at a resolution of 1.0Å. Comparison to the crystal structure of thaumatin at pH 7.3 and 7.0 revealed the root-mean square deviation value of a Cα atom to be substantially greater in the large disulfide-rich region of domain II, especially residues 154-164, suggesting that a loop region in domain II to be affected by solvent conditions. Furthermore, B-factors of Lys137, Lys163, and Lys187 were significantly affected by pH change, suggesting that a striking increase in the mobility of these lysine residues, which could facilitate a reaction with a free sulfhydryl residue produced via the β-elimination of disulfide bonds by heating at a pH above 7.0. The increase in mobility of lysine residues as well as a loop region in domain II might play an important role in the heat-induced aggregation of thaumatin above pH 7.0.
Division of Food Science and Biotechnology, Graduate School of Agriculture, Kyoto University, Gokasho, Uji, Kyoto 611-0011, Japan. t2masuda@kais.kyoto-u.ac.jp
Tetsuya
T
Masuda
Keisuke
K
Ohta
Bunzo
B
Mikami
Naofumi
N
Kitabatake
Fumito
F
Tani
D006863:Hydrogen-Ion Concentration; D008956:Models, Chemical; D010940:Plant Proteins / Q000737:chemistry*; D017434:Protein Structure, Tertiary; D013549:Sweetening Agents / Q000737:chemistry*; D013649:Taste
D010940:Plant Proteins; D013549:Sweetening Agents; C003427:thaumatin protein, plant