Contrasting the denaturing effect of guanidinium chloride with the stabilizing effect of guanidinium sulfate

Abstract

Guanidinium chloride, GdmCl, is a strong denaturing agent of globular proteins, whereas guanidinium sulfate, Gdm(2)SO(4), is a stabilizing agent of globular proteins. The stabilizing activity of Gdm(2)SO(4) is unexpected because the denaturant capability of GdmCl is due to direct interactions of Gdm(+) ions with protein surface groups. It is shown that the statistical thermodynamic approach devised to explain the molecular origin of cold denaturation [G. Graziano, Phys. Chem. Chem. Phys., 2010, 12, 14245-14252] can provide a rationalization of the different behaviour of GdmCl and Gdm(2)SO(4) towards globular proteins. The fundamental quantity is the reversible work to create in the aqueous solution a cavity suitable to host the D-state and a cavity suitable to host the N-state. In aqueous GdmCl solutions, this contribution is not large enough to overwhelm the conformational entropy gain upon unfolding and the direct attractions between Gdm(+) ions and protein surface groups; in aqueous Gdm(2)SO(4) solutions, it is so large that it overwhelms the two destabilizing contributions. Sulfate ions, due to their high charge density, interact strongly with water molecules producing a number density increase, that, in turn, renders the cavity creation process very costly, reversing the denaturing power of Gdm(+) ions and stabilizing the N-state of globular proteins.

How does sucrose stabilize the native state of globular proteins?

“…(10). Adopting the same calculation procedure used in previous applications of the present theoretical approach [9][10][11][12], I have assumed that: (1) the N-state can be represented as a simple sphere, whereas the D-state can be represented as a prolate spherocylinder, possessing the same V vdW of the sphere representing the N-state, but a markedly larger WASA; (2) specifically, the N-state is a sphere of radius a = 10Å, V vdW = 4189Å 3 and WASA = 1633Å 2 (calculated using for water molecules the customary radius of 1.4Å [24]), whereas the Dstate is a prolate spherocylinder of radius a = 4Å, cylindrical length l = 78Å, V vdW = 4189Å 3 and WASA = 3013Å 2 ; these numbers are reliable for a 50-residue globular protein [9]; (3) the G c contribution can be estimated by calculating the reversible work to create in water and in aqueous sucrose solution the corresponding cavities, by assuming that: (a) water can be treated as a hard sphere fluid possessing the experimental density of water at the desired temperature; (b) aqueous sucrose solutions can be treated as hard sphere fluid mixtures possessing the experimental density of aqueous sucrose solutions at the desired temperature (note, however, that sucrose molecules are not spherical).…”

“…The G c quantity is purely entropic in all liquids and accounts for the configurational/translational entropy loss of liquid molecules due to the solvent-excluded volume associated with cavity creation [8,22,23]; see also Appendix A. The G c (H 2 O) term stabilizes the N-state, because it accounts for the configurational/translational entropy gain of water molecules associated with the decrease in solvent-excluded volume upon folding [8][9][10][11][12]. The latter corresponds to the decrease in water accessible surface area [24], WASA, upon folding; WASA is defined as the area over which a probe water-oxygen centre can be rolled in order to make van der Waals contacts with protein atoms [24].…”

“…Suitable analysis of the following thermodynamic cycle sheds light on many features of the conformational stability of globular proteins [9][10][11][12], and can clarify the mechanism by which sucrose stabilizes the native state of globular proteins: …”

“…I have developed a statistical thermodynamic approach that incorporates in a correct manner the solvent-excluded volume effect and has been able to rationalize the occurrence of cold denaturation [9,10], the contrasting role of GdmCl and Gdm 2 SO 4 [11], and the TMAO counteraction of urea denaturing activity [12]. I would like to show that the same theoretical approach can clarify the mechanism by which sucrose stabilizes the N-state of globular proteins.…”

A view on the dogma of hydrophobic imperialism in protein folding

“…classic scaled particle theory (Reiss, 1966), that is grounded in statistical geometry (Graziano, 2007), does appear to be well suited for this task]. The application of this theoretical model has provided a rationalization for the occurrence of cold denaturation (Graziano, 2010;Riccio & Graziano, 2011), the contrasting activity of guanidinium chloride and guanidinium sulfate (Graziano, 2011a), the ability of trimethylamine N-oxide to counteract the denaturing effect of urea (Graziano, 2011b), and the stabilizing effect of sucrose (Graziano, 2012).…”