A model of this 73 amino acid polypeptide is displayed here. The N-terminal unit is Thr; the C-terminal unit is His. Displays of the coiled backbone and hydrogen bonding may be turned on and off by the appropriate buttons. The hydrogen bonds are shown as dashed magenta colored lines. Zooming into the model (Shift-Click) may be necessary to see them clearly. Secondary structures identified as beta-sheets and alpa-helices are shown by clicking the fifth button; a second click shows the β-turn. Proline units introduce kinks in the chain, and disufide links stabilize the tertiary structure. These features are displayed by the last two buttons.
In the following model, backbone carbon atoms are dark gray, hydrogen is cyan, nitrogen is blue and oxygen is red. The N-terminal end is on the left, and the C-terminus on the right. The amino acid configurations are L. . Hydrogen bonds, a backbone trace and an alpha-helix ribbon will be displayed by clicking the appropriate buttons. The alpha-helix is a common structural motif in many proteins. The hormone insulin is one example, which may be examined by way of the buttons in the blue shaded row 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 stabilised 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). Insulin is initially displayed as a stick model. The two chains are distinguished by color (A=blue, B=green) through the second button. The last two buttons shows the disulfide cysteine units and remove hydrogen atoms (for clearer viewing) . All the buttons in the upper section will also act on the insulin model.
The following model shows a short peptide chain (18 amino acids), which is bent to form antiparallel β-sheets. Hydrogen atoms are not shown. Carbon atoms are dark gray, nitrogen is blue and oxygen is red. By clicking the labeled buttons, various displays may be viewed.The buttons in the blue shaded row below this model activate a display of human retinol binding protein, which has a cavity formed by eight β-sheet strands ( a beta-barrel ). This feature will be presented by clicking the Show Beta Sheets button. The cavity accepts a retinol (vitamin A) substrate, which may be seen by clicking the last button. In addition to the beta-barrel, an alpha-helix region has been identified and may be seen as a green ribbon.