Protein structure refers to the specific arrangement of amino acids in a protein, which determines its function. Proteins are made up of long chains of amino acids that fold into unique shapes. There are four levels of protein structure: primary, secondary, tertiary, and quaternary. The primary structure is the sequence of amino acids, while secondary structure includes local folding patterns like alpha helices and beta sheets. The tertiary structure is the overall 3D shape of a single protein molecule, and quaternary structure involves the assembly of multiple protein subunits. The specific structure of a protein is crucial for its biological activity, as it influences how the protein interacts with other molecules in the body.