004 - Bacterial Iron Transport: Coordination Properties of Azotobactin, the Highly Fluorescent Siderophore of Azotobacter vinelandii

In order to fix nitrogen, Azotobacter vinelandii needs a lot of iron for its nitrogenase enzyme. But in environments with oxygen, iron is usually found in insoluble, oxidized forms, so A. vinelandii secretes compounds called siderophores that bind/chelate the iron and make it easier to bring into the cell.

One of these in particular is a very pretty yellow-green compound that also can fluoresce blue under ultraviolet light, called azotobactin. The authors here wanted to study azotobactin properties, so they purified it, basically by filtering out bacterial cells and running the filtrate through columns.

They found that azotobactin has five sites that are involved in binding iron. This doesn't mean it binds five iron atoms though. Actually it seems only to bind one. It does seem to bind more strongly than another siderophore called aerobactin that Escherichia coli produces.

Once the cell takes iron-bound azotobactin back inside, it seems to reduce the complex, convert Fe(III) to Fe(II) and thus release the iron for other purposes (such as in nitrogenase).

The last interesting thing is that azotobactin is most fluorescent when it is not binding iron. Only one of the conformational states it goes through when bound to iron is slightly fluorescent.

Citation: Palanché, T., Blanc, S., Hennard, C., Abdallah, M. A. & Albrecht-Gary, A.-M. Bacterial Iron Transport: Coordination Properties of Azotobactin, the Highly Fluorescent Siderophore of Azotobacter vinelandii. Inorg. Chem. 43, 1137–1152 (2004).