Figure 4.24 The structure of the protein collagen. (A) Space-filling model of collagen. Each strand is shown in a different color. (B) Cross section of a model of collagen. Each strand is hydrogen bonded to the other two strands. The atom of a glycine residue is identified by the letter G. Every third residue must be glycine because there is no space in the center of the helix. Notice that the pyrrolidine rings are on the outside.