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An Epoxide Intermediate in Glycosidase Catalysis.

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This study reveals a novel mechanism for retaining glycoside hydrolases, demonstrating mannose 2-hydroxyl participation in glycosidic bond cleavage. This finding challenges previous understanding of enzymatic hydrolysis mechanisms.

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Area of Science:

  • Biochemistry
  • Enzymology
  • Structural Biology

Background:

  • Retaining glycoside hydrolases typically use enzymatic nucleophiles or substrate-borne groups for cleavage.
  • No mechanism involving the sugar 2-hydroxyl group has been previously reported.

Purpose of the Study:

  • To investigate and provide evidence for neighboring-group participation by a mannose 2-hydroxyl in glycoside hydrolase family 99 endo-α-1,2-mannanases.
  • To elucidate the unprecedented catalytic mechanism of these enzymes.

Main Methods:

  • X-ray crystallography to capture key reaction intermediates (Michaelis complex, intermediate mimics, product complex).
  • Quantum mechanics/molecular mechanics (QM/MM) modeling to predict the reaction pathway and transition state.
  • Kinetic isotope effect (KIE) studies using deuterium, carbon-13, and oxygen-18 labeling.

Main Results:

  • Crystallographic data revealed enzyme complexes with substrate and intermediate mimics (β-1,2-aziridine and β-1,2-epoxide).
  • QM/MM modeling predicted a 1,2-anhydro sugar intermediate with a flattened, envelope (E3) transition state.
  • KIEs confirmed an oxocarbenium ion-like transition state and directly implicated C2-hydroxyl participation.

Conclusions:

  • The study provides comprehensive evidence for a novel enzymatic mechanism involving mannose 2-hydroxyl participation.
  • This finding expands the known repertoire of glycoside hydrolase catalytic strategies.
  • The identified mechanism is unprecedented and previously only hypothesized.