Extracellular iron reductases: identification of a new class of enzymes by siderophore-producing microorganisms

Arch Biochem Biophys. 1999 Apr 1;364(1):75-82. doi: 10.1006/abbi.1999.1109.


This study identifies extracellular iron reductases in culture supernatant fluids of the siderophore-producing microorganisms Escherichia coli and Pseudomonas aeruginosa. These enzymes were constitutively produced and reduced and released iron from a variety of ferric chelators. Dialyzable cofactors, necessary for the transfer of electrons in the enzymatic reduction of iron, were identified. The reductases were sensitive to treatment with proteinase K and guanidine-HCl, were not associated with siderophore activity, and were apparently released from the cell as extracellular enzymes. The acquisition of 59Fe2+ by cell suspensions of E. coli and P. aeruginosa was saturable, suggesting that the ferrous iron generated by these reductases can be bound and transported. Salmonella typhimurium mutants feoB, tonB, entB, and entBfeoB, deficient in numerous known iron uptake pathways, were found to exhibit substantial extracellular iron-reducing activities over that of the parent. A hypothesis is proposed in which the extracellular iron reductases excreted by siderophore-producing microorganisms may be responsible for the mobilization of iron during conditions of iron repletion when siderophores are repressed and may also function in concert with siderophores during periods of iron starvation.

Publication types

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

MeSH terms

  • Endopeptidase K / pharmacology
  • Escherichia coli / enzymology*
  • Extracellular Space / enzymology*
  • FMN Reductase*
  • Flavin Mononucleotide / metabolism
  • Guanidine / pharmacology
  • Iron / metabolism
  • Iron Radioisotopes
  • NAD / metabolism
  • NADH, NADPH Oxidoreductases / chemistry*
  • NADH, NADPH Oxidoreductases / drug effects
  • NADH, NADPH Oxidoreductases / metabolism*
  • Oxidation-Reduction
  • Pseudomonas aeruginosa / enzymology*
  • Salmonella typhimurium / enzymology
  • Salmonella typhimurium / genetics
  • Siderophores / metabolism
  • Sodium Dodecyl Sulfate / pharmacology


  • Iron Radioisotopes
  • Siderophores
  • NAD
  • Sodium Dodecyl Sulfate
  • Flavin Mononucleotide
  • Iron
  • FMN Reductase
  • NADH, NADPH Oxidoreductases
  • ferric citrate iron reductase
  • Endopeptidase K
  • Guanidine