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Nature. 2018 Nov;563(7730):235-240. doi: 10.1038/s41586-018-0644-7. Epub 2018 Oct 24.

Palladium-mediated enzyme activation suggests multiphase initiation of glycogenesis.

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Department of Chemistry, University of Oxford, Oxford, UK.
Structural Genomics Consortium, University of Oxford, Oxford, UK.
Departament de Química Inorgànica i Orgànica (Secció de Química Orgànica) and Institut de Química Teòrica i Computacional (IQTC), Universitat de Barcelona, Barcelona, Spain.
Institut de Química Computacional i Catalisi and Departament de Química, Universitat de Girona, Girona, Spain.
School of Chemistry, University of Southampton, Southampton, UK.
Institució Catalana de Recerca i Estudis Avançats (ICREA), Barcelona, Spain.
Structural Genomics Consortium, University of Oxford, Oxford, UK.
Department of Chemistry, University of Oxford, Oxford, UK.


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.

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