Glycogenin initiates the biosynthesis of proteoglycogen, the mammalian glycogenin-bound glycogen, by intramolecular autoglucosylation. The incubation of glycogenin with UDP-glucose results in formation of a tylosine-bound maltosaccharide, reaching maximum polymerization degree of 13 glucose units at cessation of the reaction. No exhaustion of the substrate donor occurred at the autoglucosylation end and the full autoglucosylated enzyme continued catalytically active for transglucosylation of the alternative substrate dodecyl-maltose. Even the autoglucosylation cessation once glycogenin acquired a mature maltosaccharide moiety, proteoglycogen and glycogenin species ranging rM 47-200 kDa, derived from proteoglycogen, showed to be autoglucosylable. The results describe for the first time the ability of polysaccharide-bound glycogenin for intramolecular autoglucosylation, providing evidence for cessation of the glucose polymerization initiated into the tyrosine residue, by inaccessibility of the acquired maltosaccharide moiety to further autoglucosylation. (c) 2008 Elsevier Inc. All rights reserved.