Glycogen primer: Glycogenin autoglucosylation mechanism

BAZÁN, SOLEDAD; ISSOGLIO, FEDERICO M.; CARRIZO, MARÍA E.; CURTINO, JUAN A.

Mar del Plata, Buenos Aires, Argentina

Congreso; XLIII Reunión Anual de la Sociedad Argentina de Investigación Bioquímica y Biología Molecular (SAIB).; 2007

Sociedad Argentina de Investigación Bioquímica y Biología Molecular (SAIB).

The rate and mechanism of incorporation of the first sugar molecule into the tyrosine residue of glycogenin (Gn) was studied for the first time by autoxylosylation from UDP-xylose. The molecular aggregation of the enzyme and the kinetics of the subsequent autoglycosylation was also studied, using non-glucosylated Gn (apo-31) and partially glucosylated Gn (wt-31) truncated at residue 270. Gn eluted as dimer when loaded at high protein concentration and monomer loaded at low concentration. The first sugar was incorporated into the tyrosine residue by an intramolecular reaction, with slightly higher catalytic efficiency by monomeric than dimeric Gn. The intermediate growth was best achieved by intramolecular transglucosylation between the subunits of dimeric Gn. The final growth of the Gn-bound maltosaccharide occurred at a high specific reaction rate by intramolecular glucosylation of monomeric Gn. The whole autoglucosylation of apo-31 showed high specific glucosylations at the low concentrations at which the enzyme existed as monomer. Higher concentrations resulted in formation of an immature Gn-bound maltosaccharide, reported to be unable to serve as primer. This concentration dependent selfmodulation of the full autoglucosylation is a heretofore not described property by which Gn might be involved in the regulation of the biosynthesis of glycogen.