Chemistry Reference
In-Depth Information
must adapt (Yancey et al. 1982). Osmolytes have been utilized to regulate intracel-
lular pressure in all kingdoms of life, providing a standard example of convergent
evolution. There are two classes of osmolytes, protecting and denaturing osmolytes.
Protecting osmolytes stabilize proteins thermodynamically by maintaining the
native structure and conformational interactions. Denaturing osmolytes shift the
native structure toward the denatured or unfolded state by disrupting native contacts
and conformations. Osmolytes also affect the solution phase stability of proteins and
thus have a direct effect on their solubility.
12.1 THERMODYNAMIC BACKGROUND
Considerable insight into the mechanisms of both solution stability and protein sta-
bility has been gained by studying the interactions between osmolytes and proteins
(Auton and Bolen 2004, 2005, 2007). This has provided new models of the protein
folding landscape (Rose et al. 2006). Thermodynamic measurements carried out on
proteins and osmolyte aqueous solutions have determined that an important driv-
ing force responsible for osmolyte activity is the thermodynamic interactions of the
osmolyte with the peptide backbone and thus the solution density distributions, fluc-
tuations, and correlations (Liu and Bolen 1995; Auton and Bolen 2004).
Classically we think of osmolyte-induced stability as brought about by a thermo-
dynamically unfavorable interaction with the protein (Lee and Timasheff 1981;
Arakawa and Timasheff 1985a). We understand that if we decompose a protein into
side chains and backbone that stability induced by such an osmolyte results in cor-
relations or solution fluctuations, which thermodynamically yield a response less
favorable than that with water (the osmophobic effect) (Rösgen, Pettitt, and Bolen
2004). Similarly, we consider solution stabilization or protein denaturation as hav-
ing a substantial component that occurs by favorable interactions with the osmo-
lyte (Pace 1986; Bolen and Baskakov 2001). More recently, it has become clear that
the thermodynamics is not overwhelmingly dominated by the interactions with the
side chains, but rather the backbone makes a major, perhaps dominant contribution
in many cases (Auton, Holthauzen, and Bolen 2007; Auton et al. 2011). The pre-
cise nature of the balance is dependent on the concentration of osmolyte as recently
shown (Auton et al. 2011) Some less concentrated solutions of osmolyte solutions do
not show the backbone contributions as purely dominating the side chains. However,
the backbone shows a classically surprising contribution to stability in all cases; the
side chains are not responsible alone.
Bearing this in mind, the Tanford model of protein stability must be largely
amended because of its relationship to the hydrophobic interactions between osmo-
lytes and the amino acid side chains. The data for the systems with the strongest
osmolyte effects clearly points to a de-emphasis of the role of the amino acid side
chains (Auton et al. 2011). That data also show a sliding scale. The group transfer
free energies of Tanford were apparent free energies and not corrected for activ-
ity and solubility (Nozaki and Tanford 1963) and as a result the importance of the
protein backbone for protein folding, as envisioned at the onset of protein structural
biophysics (Pauling, Corey, and Branson 1951), is now emphasized.
