Biolology question

[chiddler]

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This is the first question of the EK in class exam. Disulfide isomerase is an enzyme that catalyzes the folding of proteins, specifically in the creation of disulfide bonds.

The answer is C. I would like a better explanation of this answer, please!

The following is the book explanation:

Protein disulfide isomerase is an enzyme. The function of any enzyme is to lower the activation energy of both the forward and reverse reactions. Enzymes cannot alter the equilibrium constant they can only increase the rate at which a reaction proceeds towards equilibrium.

I understand that they lower the activation energy, but why would they decrease the rate at which the bonds are broken?

thank yous!

 

[davcro]

I understand that they lower the activation energy, but why would they decrease the rate at which the bonds are broken?

They increase the rate that disulfide bonds are broken.

The enzyme makes it easier for the reaction to occur in both directions; thus the reaction speed is increased by the same amount in both directions. The forward direction creates bonds, the reverse destroys them. The speed that bonds are created (forward direction) increases by the same amount as they are destroyed (reverse direction).

 

[CoolWhipp]

Disulfide bonds are not always made correctly. My o-chem teacher would call the enzyme "promiscuous." Sometimes the wrong Cysteine groups are bonded. Protein disulfide isomerase also functions to "Rearrange" these bonds, thus breaking the incorrect ones.

This protein is unique because it functions to make disulfide bonds and break them.

 

[CoolWhipp]

They increase the rate that disulfide bonds are broken.

The enzyme makes it easier for the reaction to occur in both directions; thus the reaction speed is increased by the same amount in both directions. The forward direction creates bonds, the reverse destroys them. The speed that bonds are created (forward direction) increases by the same amount as they are destroyed (reverse direction).

I believe this is the better way to approach such a question then the way I had answered. Davcro used intuition, I used knowledge outside of the question. I would get this question correct once, davcro would get all similar questions correct.

 

[chiddler]

They increase the rate that disulfide bonds are broken.

The enzyme makes it easier for the reaction to occur in both directions; thus the reaction speed is increased by the same amount in both directions. The forward direction creates bonds, the reverse destroys them. The speed that bonds are created (forward direction) increases by the same amount as they are destroyed (reverse direction).

maaaan!

🙁

I have, YET AGAIN, misread the answer. A bit sleep deprived because body decides it's a good idea to wake up unprovoked at 6 am and not go back to sleep.

I understand completely, thanks very much for both responses.

 

[nabilesmail]

Well let's think about it. If the enzyme catalyzed and increased the rate of only the forward direction it would eventually pass equilibrium, however, if both occur (bonds broken and formed) then the enzyme gets the protein towards equilbrium quicker, bonds are still broken, however, atleast all the bonds formed that were necessary occured at a quicker pace.

 

[davcro]

I believe this is the better way to approach such a question then the way I had answered. Davcro used intuition, I used knowledge outside of the question. I would get this question correct once, davcro would get all similar questions correct.

Wow, that's very flattering. To be honest, I solved this one using the word isomerase. I assumed the word isomerase meant an enzyme that re-arranges molecules. Rearrangement would involve both breaking and making bonds.

The explanation I posted was the same as the authors. I think it is a better way to think of these problems.

 

[MedPR]

Not to beat a dead horse, but another way (simpler way, imo) to view the answer is this.

Enzymes (like all catalysts) do not change equilibrium. Therefore, it must lower the Ea for both the forward and reverse reaction.

(It took me a while to figure out the answer to this too when I was doing EK the other day).