One Protein, Two Enzymes Revisited: A Structural Entropy Switch Interconverts the Two Isoforms of Acireductone Dioxygenase

J Mol Biol. 2006 Nov 3;363(4):823-34. doi: 10.1016/j.jmb.2006.08.060. Epub 2006 Aug 26.


Acireductone dioxygenase (ARD) catalyzes different reactions between O2 and 1,2-dihydroxy-3-oxo-5-(methylthio)pent-1-ene (acireductone) depending upon the metal bound in the active site. Ni2+ -ARD cleaves acireductone to formate, CO and methylthiopropionate. If Fe2+ is bound (ARD'), the same substrates yield methylthioketobutyrate and formate. The two forms differ in structure, and are chromatographically separable. Paramagnetism of Fe2+ renders the active site of ARD' inaccessible to standard NMR methods. The structure of ARD' has been determined using Fe2+ binding parameters determined by X-ray absorption spectroscopy and NMR restraints from H98S ARD, a metal-free diamagnetic protein that is isostructural with ARD'. ARD' retains the beta-sandwich fold of ARD, but a structural entropy switch increases order at one end of a two-helix system that bisects the beta-sandwich and decreases order at the other upon interconversion of ARD and ARD', causing loss of the C-terminal helix in ARD' and rearrangements of residues involved in substrate orientation in the active site.

Publication types

  • Research Support, N.I.H., Extramural
  • Research Support, Non-U.S. Gov't
  • Research Support, U.S. Gov't, Non-P.H.S.

MeSH terms

  • Binding Sites
  • Conserved Sequence
  • Dioxygenases / chemistry*
  • Dioxygenases / metabolism*
  • Entropy*
  • Iron / metabolism
  • Isoenzymes / chemistry
  • Isoenzymes / metabolism
  • Klebsiella / enzymology*
  • Models, Biological
  • Mutant Proteins / chemistry
  • Peptides / chemistry
  • Protein Structure, Secondary
  • Stereoisomerism
  • Structure-Activity Relationship


  • Isoenzymes
  • Mutant Proteins
  • Peptides
  • Iron
  • Dioxygenases
  • aci-reductone oxidase (CO-forming)

Associated data

  • PDB/2HJI