In terms of protein structure, what does quaternary structure refer to?

Quaternary structure in proteins refers specifically to the arrangement and interaction of multiple folded polypeptide chains, or subunits, that come together to form a functional protein complex. This level of structure is distinct because it involves the association of two or more polypeptide chains, which may be identical or different, into a larger functional unit.

The stability and functionality of proteins at this level often rely on various interactions such as hydrogen bonds, ionic interactions, hydrophobic forces, and van der Waals attractions between the subunits. Some well-known examples of proteins that exhibit quaternary structure include hemoglobin and immunoglobulins, where the arrangement of different polypeptide chains is crucial for their biological activity.

The other options do not accurately define quaternary structure. The number of amino acids pertains to primary structure, the sequence of nucleotides relates to nucleic acids and their primary structure, and the binding of metal ions does not encapsulate the concept of multiple subunits coming together in a protein. Instead, it can play a role in enzyme activity or structure, but it is not a defining feature of quaternary organization.