Proteins form one of the main classes of biological macromolecules. The wide range of functions that they perform reflects in the tremendous variety of their chemical structures and spatial forms. For description of proteins the terms "primary, secondary, tertiary, quaternary protein structure".
"Primary structure" means chemical formula of the protein in the form of a linear sequence of amino acid residues. The terms "secondary, tertiary, quaternary structure" describe the different levels of spatial arrangement of this linear sequence.