Two molecular enzyme forms and reaction mechanisms for the glycogenin autoglucopolymerization

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

Congreso; XLVI Reunión Anual de la Sociedad Argentina de Investigación Bioquímica y Biología Molecular (SAIB); 2010

Sociedad Argentina de Investigación Bioquímica y Biología Molecular (SAIB)

The formation of glycogenin (Gn)-bound alpha-1,4-oligoglucan primer is required for the de novo biosynthesis of glycogen (G). Dimeric Gn, as it exists in solution and in the enzyme crystals, was considered to be the molecular form which synthesizes the primer by intersubunit glucosylation mechanism. We have described however that monomeric Gn is also able to catalyze its intramolecular glucopolymerization. In considering which Gn form actually primes G biosynthesis, this might be determined by the polymerization degree (pd) of the Tyr-linked oligoglucan the Gn form can produce, having the size required by glycogen synthase and branching enzyme for further elongation and branching. A pd of 12 was reported for the oligoglucan produced by Gn dimer; however the auto-glucopolymerization extent capacity of the monomer was unknown. Now we determined the glucopolymerization degree of fully autoglucosylated monomeric and homodimeric Gn and of heterodimers formed by mixing 1) a Gn mutant lacking its tyrosine acceptor with a mutant containing the tyrosine acceptor but lacking glucosylation activity, and 2) the wild type enzyme with a mutant which lacked both, glucosylating activity and tyrosine acceptor. The results show that besides the intersubunit glucosylation of Gn dimer, the intramolecular glucosylation of Gn monomer can produce the oligo-glucopolymer primer for G biosynthesis.