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Reconstituted IMPDH polymers accommodate both catalytically active and inactive conformations.

Sajitha A Anthony1, Anika L Burrell2, Matthew C Johnson2

  • 1Cancer Biology Program, Fox Chase Cancer Center, 333 Cottman Avenue, Philadelphia, PA 19111.

Molecular Biology of the Cell
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Summary
This summary is machine-generated.

Metabolic enzyme inosine monophosphate dehydrogenase (IMPDH) forms filaments, but polymerization does not alter its catalytic activity. Filaments accommodate multiple enzyme states, finely tuned by substrate and nucleotide levels.

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Area of Science:

  • Biochemistry
  • Cell Biology
  • Enzymology

Background:

  • Metabolic enzymes can form reversible macromolecular assemblies.
  • The function of these enzyme assemblies, like inosine monophosphate dehydrogenase (IMPDH), is often unclear.
  • IMPDH polymerization is regulated by purine nucleotides, but its effect on activity is unknown.

Purpose of the Study:

  • To investigate the effect of IMPDH polymerization on its catalytic activity.
  • To understand how polymerization influences enzyme conformation and nucleotide sensitivity.
  • To determine if IMPDH filaments stabilize specific active or inactive states.

Main Methods:

  • Generated human IMPDH2 point mutants to control polymerization.
  • Assessed catalytic activity, substrate affinity, and nucleotide sensitivity of polymerized and non-assembled IMPDH.
  • Utilized electron microscopy to visualize conformational states in filaments.
  • Validated findings in cellular contexts.

Main Results:

  • Polymerized and non-assembled IMPDH exhibit comparable catalytic activity, substrate affinity, and GTP sensitivity.
  • Electron microscopy showed that substrates and nucleotides shift conformational equilibria in both octamers and filaments.
  • IMPDH filaments accommodate multiple active and inactive states, unlike other metabolic filaments.
  • Polymerization may facilitate cooperative conformational transitions.

Conclusions:

  • IMPDH polymerization does not directly alter its catalytic activity or nucleotide regulation.
  • IMPDH filaments are conformationally dynamic, adapting to cellular metabolic cues.
  • This dynamic polymerization mechanism allows fine-tuning of enzyme function based on substrate and purine availability.