Solved: The protein ribonuclease A in its native, or most

Chapter 7, Problem 101IE

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The protein ribonuclease A in its native, or most stable, form is folded into a compact globular shape:

(a) Does the native form have a lower or higher free energy than the denatured form, in which the protein is an extended chain? (b) What is the sign of the entropy change in going from the denatured to the folded form? (c) In the native form, the molecule has four bonds that bridge parts of the chain. What effect do you predict these four \(-\mathrm{S}-\mathrm{S}-\)linkages to have on the free energy and entropy of the native form relative to the free energy and entropy of a hypothetical folded structure that does not have any \(-\mathrm{S}-\mathrm{S}-\) linkages? Explain. (d) A gentle reducing agent converts the four \(-\mathrm{S}-\mathrm{S}-\) linkages in ribonuclease A to eight \(-\mathrm{S}-\mathrm{H}\) bonds. What effect do you predict this conversion to have on the tertiary structure and entropy of the protein? (e) Which amino acid must be present for \(-\mathrm{SH}\) bonds to exist in ribonuclease A?