TWO MOLECULAR ENZYME FORMS AND REACTION MECHANISMS FOR THE GLYCOGENIN AUTOGLUCOPOLYMERIZATION

ISSOGLIO, FEDERICO M.; CARRIZO MARÍA E.; ROMERO, JORGE M.; CURTINO, JUAN A.

Puerto Madrin, Chubut

Congreso; XLVI Reunión Anual Sociedad Argentina de Investigación en Bioquímica y Biología Molecular; 2010

The formation of glycogenin (Gn)-bound α-1,4-oligoglucan primeris required for the de novo biosynthesis of glycogen (G). DimericGn, as it exists in solution and in the enzyme crystals, wasconsidered to be the molecular form which synthesizes the primer byintersubunit glucosylation mechanism. We have described howeverthat monomeric Gn is also able to catalyze its intramolecularglucopolymerization. In considering which Gn form actually primesG biosynthesis, this might be determined by the polymerizationdegree (pd) of the Tyr-linked oligoglucan the Gn form can produce,having the size required by glycogen synthase and branchingenzyme for further elongation and branching. A pd of 12 wasreported for the oligoglucan produced by Gn dimer; however theauto-glucopolymerization extent capacity of the monomer wasunknown. Now we determined the glucopolymerization degree offully autoglucosylated monomeric and homodimeric Gn and ofheterodimers formed by mixing 1) a Gn mutant lacking its tyrosineacceptor with a mutant containing the tyrosine acceptor but lackingglucosylation activity, and 2) the wild type enzyme with a mutantwhich lacked both, glucosylating activity and tyrosine acceptor. Theresults show that besides the intersubunit glucosylation of Gn dimer,the intramolecular glucosylation of Gn monomer can produce theoligo-glucopolymer primer for G biosynthesis.