TBR CBT #4 Question 111 - Amino Acid & pH

[thomasfx10]

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I guessed at this question and got it right. Are we really suppose to know each side chain to determine the pH? ...

I am still not understanding this completely ... Look at side chain to see if it has a carboxyl group if yes then -1.

Then look at the others to find one that is protonated ... +1 ... Add them up and you will get the charge? Thanks!

111 At pH = 7, the charge on the eleven-amino acid polypeptide would be:


Cys-Thr-Met-Ser-Lys-Gly-Met,Phe,Ala,Pro,Trp


A. –3.
B. 0.
C. +1.
D. +4.


B is the best answer. Of the eleven amino acids in the protein, none has an active proton on the side chain. To determine the charge of the entire polypeptide, only the charge on the two terminals must be known. The amino terminal is basic, so at pH = 7, the pH < pKa (RNH3+). Thus, the amino terminal is protonated and carries a positive charge. The carboxyl terminal is acidic, so at pH = 7, the pH > pKa (RCO2H). Thus, the carboxyl terminal is deprotonated and carries a negative charge. The amino terminal is positive and the carboxyl terminal is negative, meaning that the polypeptide is a zwitterion and has a net charge of zero. The best answer is B.

 

[loveoforganic]

This seems beyond the scope of the MCAT, and I think the answer is wrong. Lysine has an amine R-chain, so the polypeptide should be +1.

As far as determining, yes, you would need to be able to know what the 3 letter prefixes stood for and what general functionality each R-group had. Overall, a polypeptide has a carboxyl terminus and an amino terminus, always. The amino terminus is protonated, the carboxyl terminus deprotonated, so you don't have to consider them - they cancel each other out. So you look at the R-group of each AA, one by one, and if that R group is an amine functionality, charge +1, because it's protonated, and vice versa for carboxyl functionalities (deprotonated). Of course, there are other functionalities on R groups (guanidino, imidazole, etc)

 

[thomasfx10]

Thanks ... I did hear BR sometimes goes into too much detail. This is good info to know!

 

[BerkReviewTeach]

This seems beyond the scope of the MCAT, and I think the answer is wrong. Lysine has an amine R-chain, so the polypeptide should be +1.

Absolute true that it would be +1 with lysine. But the problem didn't have any of the basic or acidic amino acids. The eleven were:

Ser, Thr, Gly, Met, Phe, Ala, Pro, Trp, Met, Thr, and Cys​

So it would be neutral pH = 7. And given the amino acids in the question, having no sidechains to worry about, I tend to think this was reasonable for the MCAT.

As far as determining, yes, you would need to be able to know what the 3 letter prefixes stood for and what general functionality each R-group had. Overall, a polypeptide has a carboxyl terminus and an amino terminus, always. The amino terminus is protonated, the carboxyl terminus deprotonated, so you don't have to consider them - they cancel each other out. So you look at the R-group of each AA, one by one, and if that R group is an amine functionality, charge +1, because it's protonated, and vice versa for carboxyl functionalities (deprotonated). Of course, there are other functionalities on R groups (guanidino, imidazole, etc)

Excellent explanation.

Thanks ... I did hear BR sometimes goes into too much detail. This is good info to know!

Unfortunately, the MCAT from time to time does the same thing. Know all you can within reason. For about three to five detailed questions on your exam, you'll have to do the best you can to make an educated guess, pretty much no matter what you know.