On the left, you can see an insulin molecule.
Reset the insulin molecule.

Here are the two chains in insulin. The blue chain with two alpha helices is chain a, and the sea green chain with one alpha helix is chain b.

Now look at the disulfide bonds connecting the two peptide chains. The sulfide bonds are colored purple.

The amino end of chain b is mostly hydrophilic because it has charged amino acids like arganine and lycine. These amino acids have similar properties to the binding site of the human insulin recepter protein. Click the button to highlight the amino terminus of chain b in red.

Now lets take a look at the middle of theinsulin molecule. I have highlighted some important amino acids that are all carbon rich and hydrophobic in blue.

It is important to remember that the two peptide chains are one in the same before they get procesed in the endoplasmic reticulum. Chain is is at the carboxyl terminus of proinsulin and chain b is at the amino terminus. I have highlighted the two amino acids that would be next to the cut peptide chain. In yellow is the n-terminus of chain a and in blue is the c-terminus of chain b. Notice their position relatvie to eachother. With this information we can deduce that when insulin is processed in the ER it is looped before the center peptide chain is cut out.