Which protein structure is typically described as being functional on its own?

A domain is a distinct functional and structural unit of a protein that can often perform its function independently of the other parts of the protein. Domains are typically composed of a stable fold that can evolve, function, and exist independently, allowing them to participate in various biological processes, including enzyme activity and molecular recognition.

This autonomy in function is crucial in biochemistry, as various proteins can share similar domains, suggesting a common evolutionary origin and functionality. Domains also contribute to the overall three-dimensional structure of proteins while maintaining their individual functional capabilities.

In contrast, motifs refer to short sequences of amino acids that often form part of a larger structural unit and do not usually function independently. Subunits are individual polypeptide chains that come together to form a larger protein complex, and they typically rely on interactions with other subunits to be functional. A β-turn is a specific type of secondary structure found in proteins that contributes to the overall folding but does not carry out a specific functional role independently of the overall protein structure.