Glycogenin: arriving to a conformational change via MD simulations

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

San Javier, Tucumán

Congreso; XLI Reunión Anual de la Sociedad Argentina de Biofísica (SAB); 2012

Sociedad Argentina de Biofísica (SAB)

Glycogen is a branched glucose polysaccharide that serves as energetic reserve in many organisms. Its biosynthesis starts with glycogenin autoglucosylation1. This enzyme synthesizes a 13 unit alpha-1,4-oligoglucan, attached to its Tyr195 residue in an alpha-1,4-glycosidic linkage. Glycogenin belongs to the retaining glycosyltransferase family, and its proposed catalytic mechanism is an internal nucleophilic substitution "Sni" (similar to Sn1). The most recent crystallographic study on human glycogenin2 reveals a conformational change, analogous to that reported in other glycosyltransferases that use sugar-nucleotides as donor substrates, that seems to be essential for the enzyme activity. This conformational change includes the movement of a lid segment over the substrate donor binding site, adopting an "open" or a "closed" structure. Furthermore, the authors determined the position of a nascent glucose chain in which the nonreducing end serves as an acceptor substrate for the elongation of the oligosaccharide chain. The positioning of the oligosaccharide was only possible in the closed conformation, suggesting that this structure is required for the stabilization of the acceptor. Moreover, Tyr195 is positioned at 1.2-1.5 nm from the anomeric carbon of the donor substrate, indicating that a conformational change would be necessary for Tyr195 to attack the UDPG glucose moiety, resulting in the first glucose incorporation. In this work we will try to decipher the conformational change for Tyr195 to attack the anomeric C´1 atom on the glucose donor, during glycogenin autoglucosylation first reaction, by using molecular dynamics simulations and the "umbrella sampling" technique.