Which structural level of proteins contains non-covalent and covalent disulfide bonds to form multimers?

The structural level of proteins that contains both non-covalent interactions and covalent disulfide bonds to form multimers is the quaternary structure. Proteins at this level are composed of more than one polypeptide chain, which are referred to as subunits. These subunits can be identical or different and are held together by various types of interactions.

In quaternary structures, non-covalent interactions, such as hydrogen bonds, ionic bonds, and hydrophobic interactions, play a significant role in stabilizing the assembly of the subunits. Additionally, disulfide bonds, which are covalent links between the sulfur atoms of cysteine residues, contribute to the overall stability and structural integrity of the protein complex.

This level is crucial for the functionality of many proteins, as the arrangement and interactions between multiple subunits can directly influence the protein's biological activity. For instance, hemoglobin, which carries oxygen in the blood, is a classic example of a protein that exhibits quaternary structure through the assembly of four polypeptide chains, including both non-covalent interactions and disulfide bonds.