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Which of the following shows the correct changes in thermodynamic properties for a chemical reaction in which amino acids are linked to form a protein?

A) +ΔH, +ΔS, +ΔG
B) +ΔH, -ΔS, -ΔG
C) +ΔH, -ΔS, +ΔG
D) -ΔH, -ΔS, +ΔG
E) -ΔH, +ΔS, +ΔG
Answer: C

I know that dehydration is endergonic (+ΔG).

I have two questions:

  1. Why is entropy decreasing? In the beginning of the reaction, we have two amino acids. By dehydration, we have at the end two molecules: the two amino acids together and the H2O molecule. The same number of particles before and after.

  2. Why is enthalpy change positive? You made a bond between the two amino acids. That is exothermic.

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  • $\begingroup$ I don't think C is the answer. Answer should be D. There is a net lowering of enthalpy because it takes energy to break a peptide bond. $\endgroup$
    – WYSIWYG
    Feb 2, 2015 at 7:25

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The energy used to catalyze the peptidyl transferase reaction is from the breakage of the bond between the amino acid in question, and the aminoacyl-tRNA it's attached to. The two reactions are coupled by the ribosome. The ribosome can then lower the entropy by positioning of the molecules (including water) in the active site as described here.

So we have our reaction ΔG = ΔH - TΔS

Your ΔG is positive because your reaction is endergonic, and the ΔH is positive because the peptide bond is the system, and you're absorbing energy to form it. The entropy is decreasing as such (removing enerygy = removing heat from outside the system), and the actual reaction for the formation of a peptide bond is unfavorable. As I mention above, this unfavorable reaction is coupled to a favorable reaction in the hydrolysis of an aminoacyl-tRNA to make it possible. What I forgot to mention above is that the ribosome is making this more favorable as well by decreasing the activation entropy for the reaction, and that's what they describe in the linked journal.

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  • $\begingroup$ Please answer both questions to make the answer complete. $\endgroup$
    – yolo123
    Jan 31, 2015 at 19:03
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    $\begingroup$ @yolo made what i thought were sensible edits. $\endgroup$
    – CKM
    Jan 31, 2015 at 19:51

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