Chemical Senses Advance Access originally published online on March 1, 2005
Chemical Senses 2005 30(3):253-264; doi:10.1093/chemse/bji021
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Effects of Chemical Modification of Lysine Residues on the Sweetness of Lysozyme
Division of Food Science and Biotechnology, Graduate School of Agriculture, Kyoto University, Uji, Kyoto 611-0011, Japan
Correspondence to be sent to: Tetsuya Masuda, Division of Food Science and Biotechnology, Graduate School of Agriculture, Kyoto University, Uji, Kyoto 611-0011, Japan. e-mail: t2masuda{at}kais.kyoto-u.ac.jp
Lysozyme is a sweet-tasting protein with a sweetness threshold value of around 7 µM. To clarify the effect of basicity at the side chain of lysine residues on the threshold values of sweetness, charge-specific chemical modifications such as guanidination, acetylation and phosphopyridoxylation of lysine residues were performed. Sensory analysis showed that the sweetness threshold value of lysozyme was not changed by guanidination, whereas it was increased markedly by acetylation and phosphopyridoxylation. To confirm the importance of the basicity in the lysine residues in detail, purification of acetylated (Ac-) and phosphopyridoxylated (PLP-) lysozymes using SP-ion exchange column chromatography was performed. The threshold values were not changed by modification with fewer than two residues (
7 µM), whereas the threshold values significantly increased to 15 and 34 µM when tetra-Ac and tri-PLP, respectively. Furthermore, sweetness was not detected at 30 µM (hexa-, penta-Ac and tetra-PLP). It should be noted that removal of the negative charges of the phosphate groups in the tri-PLP lysozyme by acid phosphatase resulted in the recovery of sweetness (6.4 µM), indicating that basicity at the position of the lysine residues is responsible for lysozyme sweetness and that strict charge complementarities might be required for interaction to its putative receptor.
Key words: basicity, chemical modification, lysine, lysozyme, sweet-tasting protein
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