A residue is a single molecular unit within a polymer. Residue is thus another term for monomer. Although the term residue is most often used to refer to a specific amino acid within a polypeptide , it is also used to refer to sugars within a carbohydrate molecule and nucleotides within deoxyribonucleic acid ( DNA ) or ribonucleic acid (RNA).
A protein or a polypeptide is composed of amino acids linked together by peptide bonds, with amino acids as the monomeric units of the polypeptide. The order of amino acids in a protein is known as the primary structure of that protein. The specific sequence of amino acids in the protein determines its three-dimensional structure and ultimately its function. The amino acids are numbered sequentially, beginning at the amino terminus of the polypeptide. For example, the 45th amino acid in the sequence would be identified as residue 45. Most often, scientists refer to an individual residue using both the name of the amino acid and its position. Therefore, if residue 45 in a particular polypeptide sequence is serine, that residue would be referred to as serine-45.
Berg, Jeremy M.; Tymoczko, John L.; and Stryer, Lubert (2002). Biochemistry, 5th edition. New York: W. H. Freeman.
Voet, Donald; Voet, Judith G.; and Pratt, Charlotte W. (2002). Fundamentals of Biochemistry, updated edition. New York: Wiley.