Glycogen biosynthesis: glycogenin-bound maltosaccharide inaccessibility to further autoglucosylation

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

Carlos Paz, Córdoba, Argentina

Congreso; XLIV Reunión Anual de la Sociedad Argentina de Investigación Bioquímica y Biología Molecular (SAIB); 2008

Sociedad Argentina de Investigación Bioquímica y Biología Molecular (SAIB)

The intramolecular autoglucosylation of glycogenin (Gn) initiates the de novo biosynthesis of proteoglycogen, with formation of the Gn-bound maltosaccharide primer required by glycogen synthase for subsequent glucose polymerization. The tyrosine-bound maltosaccharide, produced by incubation of Gn with UDP-glucose, reaches a maximum polymerization degree of 13 glucose units at cessation of the reaction. No exhaustion of the substrate donor occurred at the autoglucosylation end and the full autoglucosylated enzyme continued catalytically active for transglucosylation of the alternative substrate dodecyl-beta-maltoside (DBM). Even though the autoglucosylation ceases once Gn acquired the mature maltosaccharide moiety, Gn species ranging rM 47 to 200 kDa, isolated from purified proteoglycogen after controlled amylolytic digestion, as well as proteoglycogen, were able to autoglucosylate. It was described that alpha-1,4-linked glucoses adopts a left-handed helical structure in maltoheptaose. Such a helical structure adopted by the mature Gn-bound maltosaccharide might leave the last incorporated glucose spatially far from the glucosylation site, a restriction which might be overcome by branched-bound alpha-1,4-glucans. Our work provides the first evidence to date for Gn intramolecular autoglucosylation ending by inaccessibility of the built maltosaccharide to the glucosylating site of the enzyme.