Which of the following residues would most likely be chosen as a substitute for serine?

[WizzyWatkins]

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Which of the following residues would most likely be chosen as a substitute for serine?

A. Tyrosine
B. Glutamate
C. Alanine
D. Threonine

 

[aldol16]

Threonine.

 

[theonlytycrane]

D! Both ser and thr contain an alcohol group

 

[WizzyWatkins]

D! Both ser and thr contain an alcohol group

That's what I said. However the answer is not D, it's C. Apparently it has more to do with the molecular weight since serine and alanine are close to each other in regards to mass. I posted this question up because I thought something was screwy here.

 

[aldol16]

That's what I said. However the answer is not D, it's C. Apparently it has more to do with the molecular weight since serine and alanine are close to each other in regards to mass. I posted this question up because I thought something was screwy here.

Not correct at all. Serine and alanine are 16 molar mass units apart while serine and threonine are only 14 molar mass units apart. So the answer and explanation must be wrong. The only logical choice would be D.

 

[JoeCaputo]

Which of the following residues would most likely be chosen as a substitute for serine?

A. Tyrosine
B. Glutamate
C. Alanine
D. Threonine

Was this in the context of a passage? Are they trying to thwart a certain function of the protein?

 

[WizzyWatkins]

Was this in the context of a passage? Are they trying to thwart a certain function of the protein?

Yes, it was part of an MCAT passage. However the MCAT is usually written and has the answers presented in a way such that you can disregard everything else and just try to find similarities and/or differences between the answer choices given to you and what the question is asking to find the correct one (I find this to be especially true when asked about amino acids). However, this method didn't really work on this question, it was somewhat of an anomaly, and I didn't like the reasoning that was given for it.*

The reasoning behind the answer is as follows -

Alanine is similar in size to serine, cannot be phosphorylated, and will not add a positive or negative charge to the protein.

Can this reasoning still be applied to Threonine?

*If you guys need more information I suppose I could write everything out since the e-MCAT site doesn't allow the copying of content.

 

[aldol16]

The reasoning behind the answer is as follows -

Alanine is similar in size to serine, cannot be phosphorylated, and will not add a positive or negative charge to the protein.

Can this reasoning still be applied to Threonine?

It depends on what the question is asking. Is it asking for which one is most similar to Ser or which one would disturb the protein structure least while preventing it from being phosphorylated/activated/deactivated? There's a big difference here.

If it's asking about which one is most similar to serine, it would most definitely be threonine. It's closer in molar weight and it can be phosphorylated. If you use alanine instead, it can't be phosphorylated and if it's a key residue, then the protein is stuck either active or inactive. So if you were doing a site-directed mutagenesis experiment and didn't want to change protein function, alanine would be a terrible choice.

If it's asking about which one would disturb structure least and prevent the protein from being phosphorylated, then alanine is right since it's closest in molar weight to serine but lacks the key hydroxyl group that can be phosphorylated.

 

[JoeCaputo]

Yes, it was part of an MCAT passage. However the MCAT is usually written and has the answers presented in a way such that you can disregard everything else and just try to find similarities and/or differences between the answer choices given to you and what the question is asking to find the correct one (I find this to be especially true when asked about amino acids). However, this method didn't really work on this question, it was somewhat of an anomaly, and I didn't like the reasoning that was given for it.*

The reasoning behind the answer is as follows -

Alanine is similar in size to serine, cannot be phosphorylated, and will not add a positive or negative charge to the protein.

Can this reasoning still be applied to Threonine?

*If you guys need more information I suppose I could write everything out since the e-MCAT site doesn't allow the copying of content.

All the other choices have an OH group which can be phosphorylated, or are charged. When they say size I think they are referring to footprint and bond lengths, not MW.

 

[aldol16]

All the other choices have an OH group which can be phosphorylated. When they say size I think they are referring to footprint and bond lengths, not MW.

Glu cannot be phosphorylated.

 

[WizzyWatkins]

 

[aldol16]

Based on the question, you misread the problem. It's asking about a case where you're designing a mutant, CREB327-119, which is mutated at Ser119. Further, the question stem says that the variants cannot be phosphorylated at Ser119. Therefore, you want a residue that 1) cannot be phosphorylated and 2) is similar structurally and in terms of properties to serine so that it doesn't disturb the protein otherwise (you want to isolate one variable here and that's phosphorylation). So, since tyrosine and threonine can both be phosphorylated, they're out. Glutamate is also a bad choice because it is acidic and charged at physiological pH while serine is not so that introduces another variable. Alanine is the best choice because it is similar structurally to serine while having the property of not being able to be phosphorylated.

 

[basophilic]

Glu cannot be phosphorylated.

Yes it can. You only need a nucleophilic atom and the carboxylate O provides that.

 

[aldol16]

Yes it can. You only need a nucleophilic atom and the carboxylate O provides that.

No it can't. The carboxylate oxygen is deactivated by the carbonyl. I'm not talking about whether it can happen in theory. I am not aware of any residues that are actually phosphorylated at Glu. It's just too unreactive towards phosphorylation.

See: http://www.ncbi.nlm.nih.gov/pubmed/21623415 for a good description of what residues can be phosphorylated - they focus on the basic ones there. The acidic ones are not phosphorylated.

 

[JoeCaputo]

Glu cannot be phosphorylated.

Thank you for your input. It is also charged though, I don't think that is the answer.

 

[WizzyWatkins]

Based on the question, you misread the problem. It's asking about a case where you're designing a mutant, CREB327-119, which is mutated at Ser119. Further, the question stem says that the variants cannot be phosphorylated at Ser119. Therefore, you want a residue that 1) cannot be phosphorylated and 2) is similar structurally and in terms of properties to serine so that it doesn't disturb the protein otherwise (you want to isolate one variable here and that's phosphorylation). So, since tyrosine and threonine can both be phosphorylated, they're out. Glutamate is also a bad choice because it is acidic and charged at physiological pH while serine is not so that introduces another variable. Alanine is the best choice because it is similar structurally to serine while having the property of not being able to be phosphorylated.

Got it. I found this question to be especially tricky.

 

[aldol16]

Thank you for your input. It is also charged though, I don't think that is the answer.

Oh, it's not the answer - see OP's posting of the exact question. Just looks like he/she misread it. But your post about phosphorylation had other people thinking that it could be phosphorylated when in fact it cannot and is not. I was just clarifying so that people don't get confused even more.