Autor/es:
ISSOGLIO, F.M.; CARRIZO, M.E.; ROMERO, J.M.; CURTINO, J.A.
Resumen:
The auto-glucopolymerization of glycogenin (GN),
which results in formation of a GN-bound, Tyr194-linked
á-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.