Question regarding SDS Page

[Alpha-ketogluterate]

---

Members do not see this ad.

Hi,

Question 92 in the B/B Section banks presents us with an enzyme called Glycerol Phosphorylase (GK), which is a "homo dimer". "assume that GK maintains a single state in solution"

The info above is all that is needed to answer the question.

The question asks to predict what would be observed on native vs SDS page.

The answer which I picked was 1 band for both which is the correct answer BUT, their explanation makes me have another question which is why I'm here.

My reasoning: Since both native and SDS page are done under NON reducing conditions (since it is not mentioned that a reductant was used or not, it is safe to assume non reducing), then if there exists any disulfide bonds they would remain intact and so our homodimer would travel as one in both cases albeit it would be unraveled in the SDS page, so, 1 band for both.

AAMC reasoning: Since it is a homodimer, if we disrupt the quaternary structure in SDS page they would travel as 1 band.

Okay this makes sense too, but why are they assuming that the SDS page would separate the sub-units? This would only happen if there were no disulfide bonds. The passage does not mention disulfide bonds at all (it does not rule them out, nor confirm them), but they are very common in quaternary structures to my limited knowledge.

Is it the case if that if no disulfide bonds are mentioned in the passage, we should assume that they do not exist, and that SDS page will always separate subunits of a multi dimeric protein?

 

[WIMNFamilyMed]

A homodimer, being two of the same, is made of 2 subunits that are the same size, and would show as one band.
I feel like SDS disrupting the subunits was a common assumption in my biochem courses and test prep.
Hope this addresses your confusion, sorry, your post got kind of lengthy for me to sort out your question(s).

 

[WIMNFamilyMed]

A homodimer, being two of the same, is made of 2 subunits that are the same size, and would show as one band.
I feel like SDS disrupting the subunits was a common assumption in my biochem courses and test prep.
Hope this addresses your confusion, sorry, your post got kind of lengthy for me to sort out your question(s).

reddit native vs sds page on section bank bb q91 hope link displays. This might help too 🙂

 

[Alpha-ketogluterate]

A homodimer, being two of the same, is made of 2 subunits that are the same size, and would show as one band.
I feel like SDS disrupting the subunits was a common assumption in my biochem courses and test prep.
Hope this addresses your confusion, sorry, your post got kind of lengthy for me to sort out your question(s).

Thanks for answering. I understand their reasoning, and I do not argue that it would be 1 band no matter what. I am just questioning the assumption that SDS Page would 100% separate subunits. The disruption of the structure is a given, but a dimer would still show as 1 band even if it was a heterodimer in my opinion under non reducing SDS page because the negative charge imparted by SDS will not break disulfide links.

So here's my reasoning:
1) SDS Page WILL disrupt the structure but will not separate dimeric proteins unless it is done under reductive circumstances (such as 2MercaptoEthanol)
2) Disulfide bonds are pretty darn common in tertiary and quarternary structures, so we can assume that subunits will stick together unless a reductant is used or unless the passage explicitly says there are no disulfide bonds.

Or do you believe that if the passage does NOT mention disulfide bonds, we are to assume that none exist, and thus assume that SDS page will separate the sub-units?