Title : Palladium-mediated enzyme activation suggests multiphase initiation of glycogenesis
Abstract :
- Biosynthesis of glycogen, the essential glucose (and hence energy) storage molecule in humans, animals and fungi1, is initiated by the glycosyltransferase enzyme, glycogenin ( GYG )
- Deficiencies in glycogen formation cause neurodegenerative and metabolic disease2-4, and mouse knockout5 and inherited human mutations6 of GYG impair glycogen synthesis
- GYG acts as a 'seed core' for the formation of the glycogen particle by catalysing its own stepwise autoglucosylation to form a covalently bound gluco-oligosaccharide chain at initiation site Tyr 195
- Precise mechanistic studies have so far been prevented by an inability to access homogeneous glycoforms of this protein, which unusually acts as both catalyst and substrate
- Here we show that unprecedented direct access to different, homogeneously glucosylated states of GYG can be accomplished through a palladium-mediated enzyme activation 'shunt' process using on- protein C-C bond formation
- Careful mimicry of GYG intermediates recapitulates catalytic activity at distinct stages, which in turn allows discovery of triphasic kinetics and substrate plasticity in GYG 's use of sugar substrates
- This reveals a tolerant but 'proof-read' mechanism that underlies the precision of this metabolic process
- The present demonstration of direct, chemically controlled access to intermediate states of active enzymes suggests that such ligation-dependent activation could be a powerful tool in the study of mechanism