pI and pH

[MedPR]

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This is from TBR Biology passage 9.1 questions 2 and 3.

According to the explanation for question 2, if you lessen the negative charge, you increase the pI. Therefore, HbS, which has a valine instead of a glutamate (compared to HbA, has a higher pI than HbA. This makes sense.

However, in the explanation for question 3... "Because HbS has a valine residue instead of a glutamate residue... it is missing one negative charge. We know that the pI for HbS is slightlyless than the pI for HbA."

Which one is correct?

 

[Ssina]

maybe in the question 2 they are talking about the whole protein (beta 1-8) and in question 3 only the amino acids by themselves (or vice versa, don't have the book with me right now)! I just did that passage last night.

For 2, I actually just calculated the pIs separately... valine 2+9/2=5.5ish, glutamate 4+2/2=3ish and said slightly greater pI for valine! I'll be going over this passage tonight so let me know if you figure it out by then

 

[MedPR]

maybe in the question 2 they are talking about the whole protein (beta 1-8) and in question 3 only the amino acids by themselves (or vice versa, don't have the book with me right now)! I just did that passage last night.

For 2, I actually just calculated the pIs separately... valine 2+9/2=5.5ish, glutamate 4+2/2=3ish and said slightly greater pI for valine! I'll be going over this passage tonight so let me know if you figure it out by then

I haven't really tried to figure it out yet because for question #3 it doesn't matter how the pI changes. Both HbA and HbS are acidic in pH 9 solution, so both are going to act as anions, though the HbS (the one with Valine) less-so. Since they are both acidic (and anionic) they will both be attracted to the positive node and both will move in the same direction, but the HbA will move faster/further since it is more anionic (has 1 more acidic residue). Thus, you can tell that the one closer to the anode (I think, I don't have the book open anymore) is the HbA and also that the anode is the positive end.

So I understand question 3, and I know the answer for question 3 is correct in the book, but I want to be sure that the answer for question 2 is correct too. I haven't come across anything in content review so far that tells you how or what the isoelectronic point is (I don't remember it from undergrad either).