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Interaction of sweet proteins with their receptor
Humans detect taste with taste receptor cells. These are
clustered in taste buds. Each taste bud has a pore that
opens out to the surface of the tongue enabling molecules
and ions taken into the mouth to reach the receptor cells
inside. There are five primary taste sensations salty, sour,
sweet, bitter and umami. Sweet and umami (the taste of
monosodium glutamate) are the main pleasant tastes in
humans. T1Rs are mammalian taste receptors that assemble
two heteromeric G-protein-coupled receptor complexes
T1R1+T1R3, an umami sensor, and T1R2+T1R3, a
sweet receptor [50].
Sweet and taste-modifying proteins interact with the
T1R2-T1R3 receptor with a different mechanism compared
to small molecular weight compounds [51].
Recently, it has been shown that the T1R2-T1R3 receptor
has many characteristics similar to the mGluR [52], apart
from some minor differences in the active site region.
The major work by Kunishima et al. [52] solving the crystal
structure of the N-terminal active site region of the subtype
1 of mGluR both free and complexed with glutamate
has helped a lot in understanding the mechanism of interaction
between ligand and T1R2-T1R3 receptor. Their
structural work on mGluR and its N-terminal domain
[52,53] showing considerable conformational change
induced by the glutamate complexation. The 'Active' and
'resting' conformations of m1-LBR, an extracellular ligand
binding region of mGluR, is modulated by the dimer
interface. The protomer can form 'open' or 'closed' confirmations
and is made up of two domains namely LB1 and
LB2. The population of active conformers depends on ligand
binding, i.e. the so called 'closed-open_A'. The ligandfree
receptor exists as two different structures, free form I
(open-open_R), the 'resting' conformation with two open
protomers and free form II (closed-open_A), nearly identical
to the complexed form (Figure 1, references 52, 54).
The mechanism suggested by these structures is that the
receptor is in dynamic equilibrium, and that ligand binding
stabilizes the 'active' dimer. There are thus two ways,
in principle, to activate the receptor: first, to complexate
form I with the proper ligand (glutamate for the mGluR,
aspartame or any other small molecular weight sweetener
for the T1R2-T1R3 receptor) and second, by shift the equilibrium
equilibrium
between free form I and free form II in favor of
free form II.