Insulin molecules have a tendency to form dimers in solution due to hydrogen-bonding between the C-termini of B chains. Additionally, in the presence of zinc ions, insulin dimers associate into hexamers. Insulin is stored in the body as a hexamer, but the active form is the monomer. Monomers and dimers readily diffuse into blood, whereas hexamers diffuse poorly. A monomer unit and a corresponding dimer unit may be examined by clicking the designated buttons
The hormone insulin may be examined by way of the buttons below the model. Insulin has two peptide chains, A and B. The fairly compact A chain, contains 2 sections of alpha helix (A2 Ile - A8 Thr and A13 Leu - A19 Tyr), lying alongside one another. The B chain curves around the A chain. It consists of a larger section of alpha-helix (B9 Ser - B19Cys). The smaller glycine residues at 20 and 23 allow it to fold into V shape. The three-dimensional structure of insulin is further stabilized by disulphide bridges. There are 6 cysteines, so 3 disulphide bridges are formed: 2 between the A and B chains (between A7 & B7, and A20 & B19), and one within the A chain (A6 & A11). The model is first displayed by balls & sticks. The two chains are distinguished by color (A=blue, B=red) by the fourth button. The last two buttons shows the disulfide cysteine units and remove hydrogen atoms (for clearer viewing).
HIV-1 protease is a protein made by the HIV virus that is crucial to the virus's infectious capacity. The protein is composed of two symmetrically related subunits, having 99 amino acids each. Clicking the second button identifies the subunits, and a second click displays a cartoon representation, highlighting the secondary structure. The subunits come together in such as way as to form a tunnel where they meet. The protein to be cleaved sits in this tunnel, at the middle of which is the active site of the protease. This site consists of two Asp-Thr-Gly catalytic triads (residue numbers 25, 26, and 27 on one chain and 125, 126, and 127 on the second). Other buttons display various features of this enzyme, including the binding of an inhibitor.