ABSTRACT
A protein is a linear sequence of amino acids linked together by peptide bonds. The peptide bond is a covalent bond between the α-amino group of one amino acid and the α-carboxyl group of another. The primary level of structure in a protein is the linear sequence of amino acids as joined together by peptide bonds. This sequence is determined by the sequence of nucleotide bases in the gene encoding the protein. The secondary level of structure in a protein is the regular folding of regions of the polypeptide chain. The two most common types of protein fold are the α-helix and the β-pleated sheet. The third level of structure found in proteins, tertiary structure, refers to the spatial arrangement of amino acids that are far apart in the linear sequence as well as those residues that are adjacent. Proteins containing more than one polypeptide chain, such as hemoglobin, exhibit a fourth level of protein structure called quaternary structure.
