SDN Members don't see this ad. (About Ads) So, for the non-basic isoelectric point, one must take the average of the pKa1 and pKa2. However, I noticed that cysteine and tyrosine have side chains with pKas. (1) Are they subject to deprotonation? (2) Do their pKa count as pKa2 while calculating isoelectric point? (3) Cysteine's side chain pKa is about 8 while tyrosine is 10. So I guess tyrosine's amino group would count as pKa2 and side chain as pKa3. Am I right?