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Fig. 5. Model of trimethylamine N-oxide (TMAO), urea and pressure effects
on protein folding, based on basic pressure effects
(Siebenaller and Somero,
1989), counteracting effects
(Yancey et al., 1982) and
osmolyte physicochemical studies of Timasheff
(1992), Bolen and Baskakov
(2000) and Bennion and Daggett
(2004). Small spheres represent
water molecules. (A) An unfolded protein and/or substrate (S) with hydration
layers at a higher density than that of bulk water. (B) Thus, upon folding
and/or ligand binding, there is a net expansion (+
V) as water
molecules are released into bulk water during folding. If this is the case,
hydrostatic pressure will inhibit folding and/or binding (A). (C) Addition of
urea (U) enhances unfolding since that maximizes favorable binding
interactions. In B, TMAO (T) is surrounded by its own structured water layer,
which disfavors exposure of the protein's peptide backbone and of the
substrate to bulk water. TMAO thus favors folding and binding, reducing the
total order (higher in A and C).
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