C-terminal S-(Gly-Gly)-L-Cys zwitterion residue
c-terminal s-(gly-gly)-l-cys zwitterion residue
A zwitterionic group obtained by transfer of a proton from the carboxy to the amino group of C-terminal <em>S</em>-(Gly-Gly)-<small>L</small>-Cys residue; major species at pH 7.3.