Structural and stereochemical aspects of iron transport in fungi

A variety of fungi are known to overproduce and excrete desferri-siderophores under iron limitation. After complexing with ferric iron, octahedral complexes are formed and taken up by siderophore-specific transport systems. These systems represent energy consuming systems as inferred from their sensitivity to respiratory inhibitors, uncouplers and changes of the membrane potential and are able to recognize structure and stereochemical configuration of the various siderophore molecules. Ferrichromes, the most common siderophores in fungi, are generally recognized as Lambda-cis coordination complexes. Triacetylfusarinins, although prevailing as Delta-cis optical isomers in aqueous solution, are assumed to be taken up after isomerization to the corresponding Lambda-cis complexes. However, coprogens which also show a predominant Delta-absolute configuration in solution seem to be transported without prior isomerization. When both, ferrichromes as well as triacetylfusarines or coprogens are taken up, competition during uptake is observed, suggesting the presence of a common transport system during translocation of siderophores across the fungal plasma membrane.