Why is glycine often seen in beta turns?

Glycine is frequently found in beta turns due to its small size, which allows for greater flexibility and adaptability in protein structure. The small side chain of glycine consists of only a hydrogen atom, enabling it to fit into tighter spaces that are often present in the turns of polypeptide chains. Beta turns are structural elements in proteins that involve a sharp change in direction of the backbone, and glycine's unique properties facilitate this bending.

In contrast, larger amino acid side chains could create steric hindrance, making it difficult for the backbone to make the necessary turns. This property of glycine makes it an ideal candidate for these short, sharp bends in protein structures, where the volume of the side chain must be minimized to maintain the integrity of the turn.