IRON UPTAKE BY MICROORGANISMS

IRON UPTAKE BY MICROORGANISMS

• Almost all microorganisms require iron for use in Cytochromes and many enzymes. Iron uptake is made difficult by the extreme insolubility of ferric iron (Fe³⁺) and its derivatives, which leaves little free iron available for transport.
• Many bacteria and fungi have overcome this difficulty by secreting Siderophores.
• Siderophores are low molecular weight organic molecules that are able to complex with ferric iron and supply it to the cell. These iron-transport molecules are normally either Hydroxamates or Phenolates - catecholates.
• Ferrichrome is a hydroxamate produced by many fungi; Enterobactin is the Catecholate formed by Escherichia coli. It appears that three siderophore groups complex with iron to form a six-coordinate, octahedral complex.
• Microorganisms secrete siderophores when iron is scarce in the medium. Once the iron-siderophore complex has reached the cell surface, it binds to a siderophore-receptor protein. Then the iron is either released to enter the cell directly or the whole iron siderophore complex is transported inside by an ABC transporter. In E. coli the siderophore receptor is in the outer membrane of the cell envelope; when the iron reaches the periplasmic space, it moves through the plasma membrane with the aid of the transporter.
• After the iron has entered the cell, it is reduced to the ferrous form (Fe²⁺).
• Iron is so crucial to microorganisms that they may use more than one route of iron uptake to ensure an adequate supply.