Potato carboxypeptidase inhibitor protein:  at top is the C-terminal end that  binds in the carboxypeptidase active site and would normally be the first enzymatic substrate in progressive cleavage of the polypeptide chain.  However, the inhibitor C-terminus is SS-linked to the helix through a tight ring of SS bonds, so that the enzyme cannot suck in the next residue for cleavage.  Alpha helix is  in red, beta strands in blue, and SS bonds in yellow.  Ribbon diagram hand drawn and colored by Jane Richardson between 1982 and 1984, from PDB file 4CPA.
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