What characterizes a type I β-turn in protein structures?

A type I β-turn in protein structures is characterized primarily by the presence of proline and glycine. In this configuration, proline plays a crucial role as it introduces a distinctive kink in the peptide backbone due to its rigid ring structure, which helps to facilitate the turn. Glycine, being the smallest amino acid, offers flexibility and allows for tighter turns because of its lack of a bulky side chain.

These specific residues are critical for the formation of the β-turn, which typically occurs between two strands of a β-sheet and helps to provide a compact, stable structure within the protein. The arrangement of proline followed by glycine is particularly suited for these turns, allowing the chains to fold back on themselves while maintaining structural integrity.

In summary, the combination of proline and glycine is fundamental in creating the unique characteristics of a type I β-turn, enabling the compact folding patterns necessary for proper protein function.