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

Quaternary structure refers to the arrangement of multiple folded protein subunits in a multi-subunit complex. This level of structure is crucial for the function of many proteins, as the interactions between different polypeptide chains can affect the protein's stability, functionality, and regulatory properties.

In proteins with quaternary structure, each subunit may be identical or different, and their spatial arrangement is important for the overall activity of the protein. For example, hemoglobin has a quaternary structure consisting of four subunits that work together to transport oxygen in the bloodstream. Proper functioning often depends on these subunits being in the correct configuration, highlighting the significance of quaternary arrangements in biological processes.

The other options refer to different aspects of protein and molecular biology, such as the primary structure (the sequence of amino acids), nucleic acids (the sequence of nucleotides), and metal ion interactions, but they do not address the multi-subunit arrangement that characterizes quaternary structure.