STRUCTURES AND FUNCTIONS OF FUNGAL SIDEROPHORES CONTAINING HYDROXAMATE AND COMPLEXONE TYPE IRON-BINDING LIGANDS

A variety of fungi are able to synthesize iron chelating compounds (siderophores) which are excreted into the cultivation medium for iron sequestering purposes. Most ascomycetous and basidomycetous fungi synthesize hydroxamate-type siderophores containing N(delta)hydroxy-N(delta)-acyl-L-ornithine residues which constitute the iron binding ligands. After complexing of ferric ions, octahedral complexes are formed. Uptake of iron via siderophores involves the recognition of siderophores by the cognate transport systems which is dependent on the configuration of the metal centre (LAMBDA/DELTA) and its surrounding functionalities. Transport across the membrane requires metabolic energy and can be inhibited by respiratory poisons, uncouplers, low temperatures, anaerobiosis and is functioning in a pH range of pH 2-7. Four classes of hydroxamate siderophores have so far been isolated: ferrichromes, fusarinines, coprogens and rhodotorulic acid. Besides their function in solubilizing and transport of iron, hydroxamate siderophores are also involved in iron storage. Recently, a novel complexone type siderophore (rhizoferrin) has been discovered which seem to be widespread in the Mucorales. Interestingly the Mucorales are the only fungal group where ferritin has been detected so far, suggesting that the substitution of hydroxamate siderophores by complexone siderophores has led to the formation of ferritins as the eucaryotic iron storage proteins.