Chemical and Functional Properties of Food Saccharides

© 2004 by CRC Press LLC
Secondary sweetness effects originate from the saccharide–receptor dispersion
interactions. They are represented by two 2→2# and 7→2# sugar molecular orbitals
(MO) excitations coupled with the HOMO → LUMO receptor transition (terms with
1556.877 and 1038.128 coefficients in Equation 5.6, respectively). For five of the
sweetest chlorosaccharides, the absolute values of the DISP energies are about 2.42
kJ/mole higher than the DISP energies for the remaining derivatives (Table 5.3).
The LCAO structure of the 2# empty orbital indicates a higher contribution of
fructofuranose 1′-CH 2 and 6′-CH 2 groups, which are identified as the G4 and G1
sweetener subsites (Figure 5.5).
The tertiary effects, abbreviated XH2, belong to the electrostatic interactions of
the QXH2 net charges. It is observed that deoxychlorination at the 6th position of
the glucopyranose moiety reduces sweetness. Thus, the less chlorinated 1′,4,6′triClG
is twice as sweet as the more chlorinated 1′,4,6,6′-tetraClG. Analogously,
1′,6′-diClS is twice as sweet as 1′4,6,6′-tetraClS. The substitution of the 6-OH group
with the weakly charged Cl-6 chlorine atom (Q ≈ −0.050) reduces the ELST energy
component by about 3 kJ/mole in 1′,4,6,6′-tetraClG.
5.9 BIOCHEMISTRY OF SWEET-TASTE TRANSDUCTION 13
The formation of a sugar–receptor complex is the most important phase in the first
stage of the mechanism of sweet-taste development. At this stage, energy stimulus
is produced in the receptor cell, which initiates a chain of biochemical reactions
essential for neurotransmitter release. Adsorption of neurotransmitter molecules on
sensory nerve fibers is a biochemical equivalent of the sweet-taste sensation.
A progress in the sweet-taste theory after 1990 was followed by a turning point
in the development of biochemistry of receptors and molecular biology with cloning
receptor DNA techniques. 13 Martin Rodbell and Alfred Gilman won the Nobel Prize
in 1994 for discovering G-coupled proteins, which fulfil an important part of the
transduction pathway of the sweet stimulus. Structure of the sweet-taste receptor is
considered similar to the structure of other G-protein receptors. 14
The receptor shows a polypeptide chain distinguished by seven transmembrane
domain segments, TM I to TM VII helices, forming a pocket in which the sweet
ligands are bound. The energy of formation of the ligand–receptor complex (about
21 kJ/mole for hydrogen bonds) is the origin of the stimulus for signal transduction.
Figure 5.8 presents a schematic representation of the transduction mechanism for
sweet taste. The sugar falls into a receptor box, which is coupled with a
heterotrimetric GTP-binding regulatory protein of the Gs type. Consequently, the αsubunit
of this Gs-protein presumably activates adenylyl cyclase (AC), which acting
on ATP increases concentration of the intracellular second messenger, cyclic AMP
(Figure 5.9).
The cAMP messenger may then stimulate a protein kinase A (PKA), which
phosphorylates ion channels, causing their depolarization and opens a Ca 2+ ionic
channel. The intracellular calcium ion (Ca 2+ ) activity increases, leading ultimately
to neurotransmitter release (Figure 5.8).
Nonsugar sweet substances have another transduction path mechanism also.
Artificial sweeteners induce the production of inositol 1,4,5-triphosphate (IP3). 15 In