The auto-glucopolymerization extent of monomeric and dimeric glycogenin

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

Congreso; 3er Congreso de la Sociedad Latinoamericana de Proteinas (LAPS) y XXXIX Reunion Anual de la Sociedad Argentina de Biofisica (SAB); 2010

The auto-glucopolymerization of glycogenin (GN), which results in formation of a GN-bound, Tyr194-linked alpha-1,4-oligoglucan, is required to prime glycogen biosynthesis. It has been considered that the dimeric form of glycogenin, as it exists in solution, is the auto-glucopolymerizable enzyme form. The kinetics of auto-glucosylation and the activity exhibited by a heterodimer formed by two inactive GN mutants, one affecting the active site and the other the auto-acceptor capacity, sustained a proposed intersubunit reaction mechanism. We described that monomeric glycogenin, at sub-micromolar concentration, is also able to catalyze its intramolecular glucopolymerization. Two new questions arise from the above results. One refers to which GN form, monomer or dimer, would better accomplish the primer synthesis. Determination of the pd of the acquired Tyr-linked oligoglucan, would be indicative of optimum glucopolymerization extent and probable best priming capacity. The other refers to whether both, intrasubunit and intersubunit reactions might occur in dimer autoglucosylation. In the present work we determined the specific autoglucopolymerization extent (SAE) of monomeric and homodimeric glycogenin and of heterodimers formed by forcing picomolar active GN to form heterodimer, by mixing with micromolar GN mutants having lost the enzyme activity or both, activity and acceptor capacity. The results show that the monomer reaches about twice the SAE of the dimer. Both, intrasubunit and intersubunit glucosylation take place in the dimer. The lower glucose polymerization produced by the dimer, deduced from its lower SAE, might be due to steric restrictions, judging from the SAE of heterodimers which carried out intra- or inter-subunit glucosylation only.