Inhibitory effect of the partially resolved coordination isomers of chromic desferricoprogen on coprogen uptake in Neurospora crassa

Acquisition, transport, and storage of iron by pathogenic fungi

Iron is required by most living systems. A great variety of means of acquisition, avenues of uptake, and methods of storage are used by pathogenic fungi to ensure a supply of the essential metal. Solubilization of insoluble iron polymers is the first step in iron assimilation. The two methods most commonly used by microorganisms for solubilization of iron are reduction and chelation. Reduction of ferric iron to ferrous iron by enzymatic or nonenzymatic means is a common mechanism among pathogenic yeasts. Under conditions of iron starvation, many fungi synthesize iron chelators known as siderophores. Two classes of compounds that function in iron gathering are commonly observed: hydroxamates and polycarboxylates. Two major responses to iron stress in fungi are a high-affinity ferric iron reductase and siderophore synthesis. Regulation of these two mechanisms at the molecular level has received attention. Uptake of siderophores is a diverse process, which varies among the different classes of compounds. Since free iron is toxic, it must be stored for further metabolic use. Polyphosphates, ferritins, and siderophores themselves have been described as storage molecules. The iron-gathering mechanisms used by a pathogen in an infected host are largely unknown and can only be posited on the basis of in vitro studies at present.

Peptide siderophores

Siderophores are low molecular weight iron chelators, produced by virtually all bacteria, fungi and some plants. They serve to deliver the essential element iron, barely soluble under aerobic conditions, into microbial cells. Siderophores are therefore important secondary metabolites which are very often based on amino acids and their derivatives. Biosynthesis, transport, regulation and chemical synthesis of natural siderophores and their analogues is of considerable interest for the protein and peptide chemist. This review gives an overview of the structural classes of peptidic siderophores, along with data on their biosynthesis. On a number of representative examples, strategies and schemes of their chemical synthesis are described.

Coordination Chemistry of the Carboxylate Type Siderophore Rhizoferrin: The Iron(III) Complex and Its Metal Analogs

Rhizoferrin is a member of a new class of siderophores (microbial iron transport compounds) based on carboxylate and hydroxy donor groups rather than the commonly encountered hydroxamates and catecholates. We have studied the coordination chemistry of rhizoferrin (Rf), as a representative of this group, with Fe(3+), Rh(3+), Cr(3+), Al(3+), Ga(3+), VO(2+), and Cu(2+). The metal complexes have been studied by UV-vis, CD, NMR, and EPR spectroscopies and mass spectrometry. The formation constants for the iron complex have also been measured and yield a log K(LFe) of 25.3. The Rh and Cr rhizoferrin complexes are unusual in that they appear to adopt a chirality about the metal center that is the opposite of the native iron analog. Several of the alternative metal ion complexes are found to have biological activity toward Morganella morganii in a plate type assay.

Ferricrocin functions as the main intracellular iron-storage compound in mycelia of Neurospora crassa

Neurospora crassa produces several structurally distinct siderophores: coprogen, ferricrocin, ferrichrome C and some minor unknown compounds. Under conditions of iron starvation, desferricoprogen is the major extracellular siderophore whereas desferriferricrocin and desferriferichrome C are predominantly found intracellularly. Mössbauer spectroscopic analyses revealed that coprogen-bound iron is rapidly released after uptake in mycelia of the wild-type N. crassa 74A. The major intracellular target of iron distribution is desferriferricrocin. No ferritin-like iron pools could be detected. Ferricrocin functions as the main intracellular iron-storage peptide in mycelia of N. crassa. After uptake of ferricrocin in both the wild-type N. crassa 74A and the siderophore-free mutant N. crassa arg-5 ota aga, surprisingly little metabolization (11%) could be observed. Since ferricrocin is the main iron-storage compound in spores of N. crassa, we suggest that ferricrocin is stored in mycelia for inclusion into conidiospores.

Molecular recognition of siderophores in fungi: role of iron-surrounding N-acyl residues and the peptide backbone during membrane transport in Neurospora crassa

Recognition of ferric siderophores in Neurospora crassa was found to depend on the number and kind of N-acyl residues that surrounded the iron coordination center. In the coprogen series, uptake decreased in the order of coprogen, neocoprogen I, and neocoprogen II, indicating that gradual replacement of the N-transanhydromevalonyl groups by N-acetyl groups had an adverse effect on uptake. The reverse effect was observed in the ferrichrome series, where uptake decreased in the order of ferrichrysin, asperchrome D1, asperchrome B1, and ferrirubin. Configuration of the anhydromevalonyl group (cis or trans) in ferrichromes was also an important determinant in the recognition process. On the basis of uptake and inhibition studies, it is proposed that in ferrichromes part of the molecule (iron configuration and the N-acyl groups) is responsible for binding, whereas another (cyclic peptide ring) is involved in the subsequent process of transport.