To clarify the structural basis for the sweetness of thaumatin I, lysine-mo
dified derivatives and carboxyl-group-modified derivatives were prepared by
chemical modification followed by chromatographic purification. The sweetn
ess of derivatives was evaluated by sensory analysis. Phosphopyridoxylation
of lysine residues Lys78, Lys97, Lys106, Lys137 and Lys187 markedly reduce
d sweetness. The intensity of sweetness was returned to that of native thau
matin by dephosphorylation of these phosphopyridoxylated lysine residues ex
cept Lys106. Pyridoxamine modification of the carboxyl group of Asp21, Glu4
2, Asp60, Asp129 or Ala207 (C-terminal) did not markedly change sweetness.
Analysis by far-UV circular dichroism spectroscopy indicated that the secon
dary structure of all derivatives remained unchanged, suggesting that the l
oss of sweetness was not a result of major disruption in protein structure.
The five lysine residues, modification of which affected sweetness, are se
parate and spread over a broad surface region on one side of the thaumatin
I molecule. These lysine residues exist in thaumatin, but not in non-sweet
thaumatin-like proteins, suggesting that these lysine residues are required
for sweetness. These lysine residues may play an important role in sweetne
ss through a multipoint interaction with a putative thaumatin receptor.