can you please explain the quaternary structure of protein.

When multiple polypeptide chains assemble together to form a multiple subunit protein complex, it is known as quaternary structure of protein. It is the clustering of several polypeptides to form a complex protein with a specific structure. This assembling of polypeptide chains is possible through several bonds like hydrogen bond, salt bridges and disulfide bonds. The examples of proteins with quaternary structure are haemoglobin, collagen, insulin, DNA polymerase, nucleosome etc.