A model of the small protein, Ribonuclease A, is shown below. Hydrogens are omitted for clarity. The protein is folded to create a cleft for binding the RNA substrate. This cleft is shown on the left side in the initial presentation, and is evident in the backbone display. Three α-helices exist in the bottom layer of the protein, as shown, and seven β-sheet strands lie above them. All these features, together with several β-hairpin turns may be examined by clicking appropriate buttons. Four disulfide bonds help to maintain this structure. A model RNA strand, tetrameric thymidylic acid, will be shown bound in the cleft, by clicking the last button.
Nisin is a polypeptide (34 amino acids) made by the bacterium Lactococcus lactis. Of the 34 amino acids composing nisin, eight are not created by ribosome translation. but involve post-translational reactions. Primary among these are dehydration of serine and threonine to acrylic and crotonic acid derivatives. Conjugative addition of a nearby cysteine sulfhydryl group then leads to the five sulfide rings in nisin. These are defined by clicking the third button below. Nisin kills gram positive bacteria by binding to their membranes and targeting lipid II, an essential precursor of cell wall synthesis. the fourth and fifth buttons demonstrate these features.