105
ing. This type of siderophore is termed as ‘carboxylate siderophore’ well known as
‘rhizoferrin’ which is isolated from Rhizopus spp. using ion-exchange column chromatography. Structurally, rhizoferrin and its analogues contain 1,4-diaminobutane
symmetrically acylated to the terminal carboxylate of citric acid through amide
bonds (Drechsel et al. 1995).
7.4.3 Catecholate Siderophore
This class of siderophores is known to have phenolate or 2,3-dihydroxybenzoate
(DHB) as a binding moieties. Catechol (also called as pyrocatechol), naturally
occurring organic, colourless compound, is the ortho-isomer of the three isomeric
benzenediols and found in trace amounts. Azotobacter vinelandii, in iron-deficient
medium, forms various types of catecholate-based siderophores such as monocatecholate aminochelin, dicatecholate azotochelin and tri-catecholate protochelin
(Baakza et al. 2004). Basically, all these types of naturally occurring siderophores
contain negatively charged oxygen donors as hard Lewis base which binds with Fe
3+
which act as hard Lewis acid as per its chemical nature.
7.4.4 Mixed Ligand Siderophores
This class of siderophores, also called heterobactins, contains combined donor
groups of hydroxamate and catecholate together. Siderophore of this type includes
mixed ligand of lysine, ornithine and histamine derivatives. For instance, mycobactins (Mycobacterium spp.) contain hydroxamate and phenolate donor groups as chelating ligands (Mohammad et al. 2011).
7.5 Mechanism of Binding of Iron in Cell
In fungi, siderophore-mediated iron (Fe
3+
) uptake can be regulated by four mechanisms, namely, shuttle, hydrolytic, taxicab and reductive. Three of them, i.e. shuttle,
taxicab and hydrolytic mechanisms, depend upon the specific recognition of several
siderophore. (a) In shuttle mechanism, the iron (Fe
3+
)-siderophore complex initially
enters the cell membrane, and soon after that, it releases the metal from ligand (e.g.
ferrichrome in fungal species, viz. Ustilago sphaerogena and Ustilago maydis)
which ultimately leads to excretion of free siderophore (Sah and Singh 2015). This
mechanism is depicted in Fig. 7.3.
The hydrolytic mechanism involves the transportation of iron (Fe
3+
)-siderophore
complex in its intact form into the cell (e.g. uptake of ferric triacetyl fusarinine C in
mycelia sterilia). Simultaneously, both reductive and degradative steps occur inside
7 Contrasting Role of Fungal Siderophore in Metal Ion Complex Formation
ing. This type of siderophore is termed as ‘carboxylate siderophore’ well known as
‘rhizoferrin’ which is isolated from Rhizopus spp. using ion-exchange column chromatography. Structurally, rhizoferrin and its analogues contain 1,4-diaminobutane
symmetrically acylated to the terminal carboxylate of citric acid through amide
bonds (Drechsel et al. 1995).
7.4.3 Catecholate Siderophore
This class of siderophores is known to have phenolate or 2,3-dihydroxybenzoate
(DHB) as a binding moieties. Catechol (also called as pyrocatechol), naturally
occurring organic, colourless compound, is the ortho-isomer of the three isomeric
benzenediols and found in trace amounts. Azotobacter vinelandii, in iron-deficient
medium, forms various types of catecholate-based siderophores such as monocatecholate aminochelin, dicatecholate azotochelin and tri-catecholate protochelin
(Baakza et al. 2004). Basically, all these types of naturally occurring siderophores
contain negatively charged oxygen donors as hard Lewis base which binds with Fe
3+
which act as hard Lewis acid as per its chemical nature.
7.4.4 Mixed Ligand Siderophores
This class of siderophores, also called heterobactins, contains combined donor
groups of hydroxamate and catecholate together. Siderophore of this type includes
mixed ligand of lysine, ornithine and histamine derivatives. For instance, mycobactins (Mycobacterium spp.) contain hydroxamate and phenolate donor groups as chelating ligands (Mohammad et al. 2011).
7.5 Mechanism of Binding of Iron in Cell
In fungi, siderophore-mediated iron (Fe
3+
) uptake can be regulated by four mechanisms, namely, shuttle, hydrolytic, taxicab and reductive. Three of them, i.e. shuttle,
taxicab and hydrolytic mechanisms, depend upon the specific recognition of several
siderophore. (a) In shuttle mechanism, the iron (Fe
3+
)-siderophore complex initially
enters the cell membrane, and soon after that, it releases the metal from ligand (e.g.
ferrichrome in fungal species, viz. Ustilago sphaerogena and Ustilago maydis)
which ultimately leads to excretion of free siderophore (Sah and Singh 2015). This
mechanism is depicted in Fig. 7.3.
The hydrolytic mechanism involves the transportation of iron (Fe
3+
)-siderophore
complex in its intact form into the cell (e.g. uptake of ferric triacetyl fusarinine C in
mycelia sterilia). Simultaneously, both reductive and degradative steps occur inside
7 Contrasting Role of Fungal Siderophore in Metal Ion Complex Formation