8.8 Sweeteners 437Fig. 8.8.A stereoscopic repre-
sentation of the conformation
of the peptide chains of mon-
ellin (top) and thaumatin
(bottom) (the location of tryp-
tic peptides that cross react
with heterologous antibodies
is indicated bythicker lines)
(according toKimet al., 1991)
Fig. 8.9.Monellin: schematic representation of the
A and B chains, showing the intra- and intermolecu-
lar hydrogen bonds (— —). Using genetic engineering
techniques, the two chains were linked via a peptide
bond (→) between the amino acids residues E (B50)
and R (A2) (according toKimet al., 1991)
was cloned and expressed inE. coliand yeast.
The protein (I) thus obtained was as sweet as
natural monellin (II). While the sweet taste of
II was completely quenched at pH 2 by heating
to 50◦C, I exhibited its full sweetness at room
temperature even after being heated to 100◦C.
The threshold value isfsac, g=3000. Based on
its low stability, slow triggering and slow fading
away of taste perception, monellin probably will
not succeed as a commercial sweetener.
8.8.5 Thaumatins
The fruit ofThaumatococcus danielliicontains
two sweet proteins: thaumatin I and II, with
fsac, g∼2000. There are also low amounts of
three other sweet proteins (thaumatin a, b and c).
The complete amino acid sequence and the
conformation (Fig. 8.7 and 8.8) of thaumatin I,
a peptide chain with 207 amino acid residues, has
been established (Table 8.5). As a result of cross
reactions with an anti-serum against monellin
(cf. 8.8.4), sequence Y(57)FD in aβ-turn is
regarded as the site of contact with the sweetness
receptor. It corresponds to sequence Y(A13)ASD
of monellin.
The sweet taste gets weaker with the increasing
acetylation of the 11ε-amino groups of the
protein, being lost already with four acetyl
groups. In contrast, up to 7ε-amino groups
could be modified by reductive methylation with
HCHO/NaBH 4 without reduction in the taste
intensity. The isoelectric point of the protein is
obviously of importance for its activity. Thau-
matin which is regarded as toxicologically safe
is used, e. g., in chewing gum and milk products.
Synergistic effects have been observed when
thaumatin is used in combination with saccharin
and acesulfame.