The structure of proteins consists of long chains of amino acids linked by peptide bonds. These chains fold into specific three-dimensional shapes, allowing proteins to perform various functions in living organisms. Proteins are categorized based on their structure into four levels: primary (sequence of amino acids), secondary (alpha helices and beta sheets), tertiary (overall 3D shape), and quaternary (complex of multiple polypeptide chains). Understanding protein structure is crucial for elucidating their function in biochemical processes.