Define and compare the four levels of protein organization.

Question: Describe the four levels of protein organization and compare their structures and functions. Answer: The four levels of protein organization are primary, secondary, tertiary, and quaternary structures. The primary structure refers to the linear sequence of amino acids in a polypeptide chain, which determines the protein's shape and function. Secondary structure includes local conformations such as alpha-helices and beta-sheets, which are stabilized by hydrogen bonds. Tertiary structure is the overall 3D arrangement of the polypeptide chain and results from folding secondary structures into a compact shape driven by interactions between amino acid side chains. Finally, quaternary structure refers to the arrangement of multiple polypeptide chains (subunits) in a protein complex, stabilized by noncovalent interactions. These structures build upon one another, creating more complex and functional proteins as they advance from primary to quaternary levels.