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Area of Science:

  • Biochemistry
  • Enzyme kinetics
  • Protein-cofactor interactions

Background:

  • Flavin cofactors enable diverse enzymatic catalysis.
  • Flavoproteins require precise control mechanisms to regulate their function.
  • The protein environment dictates flavin cofactor activity and substrate specificity.

Purpose of the Study:

  • To investigate the role of substrate functional groups in controlling iodotyrosine deiodinase activity.
  • To elucidate the mechanism by which substrates stabilize the flavin semiquinone intermediate.
  • To understand the structure-activity relationships governing reductive dehalogenation.

Main Methods:

  • Enzyme kinetics assays (kcat/Km) were performed with modified halotyrosine substrates.
  • Redox titrations were used to monitor flavin semiquinone accumulation.
  • Binding affinities were assessed using substrate analogues lacking key functional groups.

Main Results:

  • The alpha-ammonium group is essential for substrate binding and dehalogenation.
  • The carboxylate group significantly impacts dehalogenation efficiency (kcat/Km > 2000-fold suppression).
  • Flavin semiquinone stabilization requires coordination with both alpha-ammonium and carboxylate groups.

Conclusions:

  • Substrate coordination dictates the catalytic chemistry of iodotyrosine deiodinase.
  • The alpha-ammonium group is critical for enzyme-substrate complex formation and catalysis.
  • Perturbations to substrate functional groups reveal alternative catalytic activities, such as nitroreductase activity.