A protein folds spontaneously in phosphate-buffered saline (PBS). Guanidinium chloride is gradually titrated into the PBS-buffered protein solution, causing the protein to denature. During the titration, the ΔG° of protein folding is:

Question

A protein folds spontaneously in phosphate-buffered saline (PBS).  Guanidinium chloride is gradually titrated into the PBS-buffered protein solution, causing the protein to denature.  During the titration, the ΔG° of protein folding is:

Quizplus · Accepted Answer

11ecba2d_0f0a_bfbc_a3bf_6fc56c82adc7_MD0008_00 The standard Gibbs free energy (ΔG°) describes whether a reaction favors products or reactants. Mathematically, it is expressed as ΔG° = −RT ln(K_eq) where R is the gas constant, T is the temperature in Kelvin, and K_eq is the equilibrium ratio of products to reactants. When the reaction favors products, K_eq is greater than 1 and ln(K_eq) is positive. Multiplying this positive number by −RT yields a negative ΔG°. Conversely, when the reaction favors reactants, K_eq is less than 1, making ln(K_eq) negative and ΔG° positive. Under conditions that mimic a physiological setting, such as phosphate-buffered saline (PBS) solutions, most proteins spontaneously fold into a conformation that minimizes interactions between hydrophobic residues and water (hydrophobic effect). Because the equilibrium of the protein in PBS lies toward the folded form (products), ΔG° of folding is negative under these conditions. Addition of guanidinium chloride causes proteins to denature, indicating that it shifts the equilibrium to favor the unfolded state (reactants). Because the equilibrium lies toward reactants in the presence of guanidinium chloride, the ΔG° of folding becomes positive. Therefore, during the titration, the ΔG° of folding is initially negative and becomes positive. (Choice B) The ΔG° of unfolding is initially positive and changes to negative, but the ΔG° of folding is the opposite. (Choices C and D) The ΔG° of folding is initially negative but becomes positive. Educational objective: The standard Gibbs free energy (ΔG°) describes whether a reaction favors products or reactants. A negative ΔG° indicates that products are favored, whereas a positive ΔG° indicates that reactants are favored. The equilibrium of a process such as protein folding may be shifted by denaturing agents such that the reaction shifts from favoring folded proteins (products, negative ΔG°) to favoring unfolded proteins (reactants, positive ΔG°).

A Protein Folds Spontaneously in Phosphate-Buffered Saline (PBS)