why do R-side groups of amino acids like Tyrosine, Cysteine, Serine and Threonine have pKa values?

Question

since they are supposed to be uncharged amino acids? I always thought pKa always involve the change in charge between conjugate acids and bases. Can anyone enlighten me on this?

Answer 1

As free amino acids, all amino acids, regardless of R-group, have both a carboxy end and an amino end, each of which has it's own pKa. Hope that helps!

Answer 2

The sulfur of cysteine is capable of making disfulide bonds with the sulfur of an other cysteine. This is a covalent interaction (meaning it's strong), so it helps to keep the tertiary and quaternary structure of the protein stabilized.