A glycosaminoglycan formed by alternating residues of d-glucosamine and either muramic acid {2-amino-3-O-[(S)-1-carboxyethyl]-2-deoxy-d-glucose} or l-talosaminuronic acid (2-amino-2-deoxy-l-taluronic acid), which are usually N-acetylated or N-glycoloylated. The carboxyl group of the muramic acid is commonly substituted by a peptide containing residues of both l- and d-amino acids, whereas that of l-talosaminuronic acid is substituted by a peptide consisting of l-amino acids only.