Insulin has 3 disulfide bonds

Insulinhas 2 short polypeptide chains, and 3 disulfide bonds.Two disulfides form between the two chains. A third disulfide is within Chain A. Chain A is colored light blue and chain B is colored light green. The sulfurs of the cysteines are colored yellow.

Notice the precise geometry of a disulfide bond.

The structure of 2Zn pig insulin crystals at 1.5 A resolution.
Baker, E.N., Blundell, T.L., Cutfield, J.F., Cutfield, S.M., Dodson, E.J., Dodson, G.G., Hodgkin, D.M., Hubbard, R.E., Isaacs, N.W., Reynolds, C.D., al., et,
(1988) Philos.Trans.R.Soc.London,Ser.B 319: 369-456

(PubMed)

4ins (PDB)