I am having difficulties solving this problem from class: A protein contains the

Question

I am having difficulties solving this problem from class:

A protein contains the following ionizable acids ( 4 Lys, 3 Arg, 8His, 5 Glu, and 3 asp). Estimate the pI for this peptide. Discussany assumptions you make. I don't understand the steps. Thefurthest that I can go is up to this point.
Count Pka
3 arg 12.5
4 lys   10.5
8 his   6.0
5 glu   4.3
3 asp 3.9

I know that I have to separate them to get the amount of positiveand negatives. How do I determine which ones to separate. Also Idon't really understand why I am using the pka from the side chain.Please help!!

Thanks a lot

Explanation / Answer

assume that all ionizable sidechains and carboxylate/amine groupshave all hydrogens. from there whatever charge the peptide has letsyou know which pka's to use to calculate pI. for example, if the charge is +3, you will then want to use thepka's 3 and 4. you then simply average those two pka's and you willhave your iso electric point. if you need more help let me know,but it might take me a little longer to type it out

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