What characterizes β-turns in protein structures?

In the context of protein structures, β-turns are specific types of secondary structures that enable proteins to fold back on themselves, facilitating a compact configuration. One of the defining features of β-turns is their characteristic use of specific amino acids to stabilize the turn. While glycine is indeed often found at position 3 of a β-turn due to its small size and flexibility, it is not a strict requirement for all β-turns. Rather, specific types of β-turns accommodate various amino acids at different positions, but glycine is frequently observed in many types of β-turns due to the steric flexibility it provides.

The answer that emphasizes the requirement of glycine at position 3 is a general observation, capturing the tendency for its inclusion in these structural motifs. Glycine's unique side chain (or lack thereof) enables tighter turns than many other amino acids. This contributes to the overall stability of β-turn structures, which typically include four residues, allowing for sharp turns in the polypeptide backbone.

Understanding this context clarifies why other options do not correctly characterize β-turns. While flexibility and some specific amino acids might play roles in other aspects of protein folding or types of turns, they do not encapsulate the essential features of β-turn