Entry retrieved from the Protein Data Bank at RCSB.
1XY2 Hormone. Date: Jun 05, 1987.
Authors: S.P.Wood, I.J.Tickle, A.M.Treharne, J.E.Pitts, Y.Mascarenhas, J.Y.Li, J.Husain, S.Cooper, T.L.Blundell, V.J.Hruby, A.Buku, A.J.Fischman, H.R.Wyssbrod
Compound: Beta-Mercaptopropionate-Oxytocin (Dry Form)
Method: X-Ray Diffraction. Resolution 1.20 �.

Oxytocin is a hormone that induces labor in pregnant woman and stimulates milk production in nursing mothers.

This is a beta-mercaptopropinate of oxytocin. Beta-mercaptopropionic acid is thiopropionic acid and has the formula HS-CH₂CH₂COOH. H-atoms are not shown. The free floating O-atoms are water molecules (even in the dry form). To really "see" the structure, first find the amide end of Gly. It is bonded to Leu (the iso-butyl group is easily recognized), Pro (the 5-membered ring), and Cys (the one with the CH₂SH side chain). Now, in regular oxytocin one would find another Cys on the other side of the -S-S- bridge. That Cys has now been replaced by HS-CH₂CH₂COOH. Note how the acid is connected to the amine end of Tyr (the one with CH₂-Ph-OH). One last thing: The -S-S- bridge is odd; there should be 2 S-atoms but there seem to be 3. Also, there seem to be 3 C-atoms when the thio acid should have only 2 between the carboxyl-C and its S-atom. For this region of the protein the structure is not fully solved, two positions of a -CH₂-S- piece are possible and both are shown.