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Two sulfur containing amino acids are __________ and __________.

Methionine and cystine

I am having trouble calculating the net charges on polypeptide chains. I understand the general rules concerning Pka and pH for each AA, but why is Lysine a +1 charge for pH 3 and 7 when its pI is 10.53? Why is Histidine a +1 charge at pH 3, but o for pH 7? Thanks.

Basically, what you need to remember is that pI is the pH at which amino acid has no net charge. At any pH below pI it will be positively charged and above that pH it will be negatively charged.

So, in your problem Lys pI is ~10, therefore, pH 3 and 7 are bellow pI and it will be positive. pI of the His is ~7, so at pH 3 it will be +ve and neutral at pH 7.

Alla wrote:Basically, what you need to remember is that pI is the pH at which amino acid has no net charge. At any pH below pI it will be positively charged and above that pH it will be negatively charged.

So, in your problem Lys pI is ~10, therefore, pH 3 and 7 are bellow pI and it will be positive. pI of the His is ~7, so at pH 3 it will be +ve and neutral at pH 7.

This rule seems only true for Lysine and Histidine. The pH is below the Pka, let's say for Tyr at pH 3 and 8, but both give me 0. At pH 11 it is -1. The pH 3 and 8 are below the pI, but they are not positive they are 0. Glu at pH 3 is 0, and at pH 8 and 11 they are both -1. I am really confused. What idea am I missing? Thank you very much.

When amino acid is part of the peptide, you only consider pKa of the side chain. For Tyr it's ~10. Then you look at the side chain itself. For Tys the affected part is -OH group. If pH is bellow pKa, the group will be -OH, above pKa - it will be -O-. For the Glu, the problem is similar... Remember that His and Lys side chains will not be negative either...

So, to sum up if the pI is greater than the given pH the charge will be 0 and if the pI is less than the given pH the charge is -1, but His and Lys are NEVER negative. Why then is His have a +1 charge at pH3, I would think the charge would be -1? Thanks a bunch!