102
7.3 Fungal Iron Regulation
In fungi, regulation of iron uptake is necessary to maintain iron homeostatic processes. For this, four different mechanisms of iron uptake have been suggested in
fungi at molecular level by different mechanistic-based studies (Haas 2014). These
four mechanisms involve (a) ferric iron (Fe
3+
) uptake through siderophores, (b)
reductive iron assimilation, (c) heme uptake and d) direct iron uptake. These four
mechanisms are explained as follows.
7.3.1 Iron (Fe
3+
) Uptake Through Siderophore
Each and every fungal species exhibit siderophore-iron transporter (SIT)-mediated
extracellular iron uptake mechanism. SIT constitute majorly facilitator protein family, which acts as a proton-coupled symporters potentiated by plasma membrane. In
addition, high solubility and high energy factors render iron-chelated siderophore to
combat during microbial growth. On the other hand, triacetyl fusarinine (TAFC)
and fusarinine C (FsC) facilitate intercellular release of iron by partial hydrolysis by
esterase (Estb) enzyme (Howard 1999).
7.3.2 Reductive Iron Assimilation (RIA)
In fungi, iron acquisition with high affinity is usually achieved either by secreted
siderophores (iron chelators) or by reductive iron assimilation (RIA) mechanisms
(Fatima et al. 2017). In order to start iron uptake, iron is first reduced from ferric
(Fe
3+
) to more soluble ferrous (Fe
2+
) form by localized ferrireductases present in the
plasma membrane of fungal species. Soon after this, the ferrous iron is re-ionized
which is further imported by protein complex consisting of iron permease (FtrA)
and ferrioxidase (Fetc) genes. The protein such as permease also transports metals
other than iron such as copper and zinc.
7.3.3 Heme Uptake
In contrast to bacteria, binding and uptake of iron, in fungi, are done only with heme
component. For instance, Candida albicans heme uptake mechanism involves the
glycosylphosphatidylinositol (GPI)-anchored cell surface mannoprotein, namely,
Rbt5P, but the details regarding its transport mechanism are still unknown. In order
to utilize heme-iron complex, the uptake requires intercellular degradation of heme
with heme oxygenase (Hmx1p, localized in endoplasmic reticulum).
S. Bhardwaj et al.
7.3 Fungal Iron Regulation
In fungi, regulation of iron uptake is necessary to maintain iron homeostatic processes. For this, four different mechanisms of iron uptake have been suggested in
fungi at molecular level by different mechanistic-based studies (Haas 2014). These
four mechanisms involve (a) ferric iron (Fe
3+
) uptake through siderophores, (b)
reductive iron assimilation, (c) heme uptake and d) direct iron uptake. These four
mechanisms are explained as follows.
7.3.1 Iron (Fe
3+
) Uptake Through Siderophore
Each and every fungal species exhibit siderophore-iron transporter (SIT)-mediated
extracellular iron uptake mechanism. SIT constitute majorly facilitator protein family, which acts as a proton-coupled symporters potentiated by plasma membrane. In
addition, high solubility and high energy factors render iron-chelated siderophore to
combat during microbial growth. On the other hand, triacetyl fusarinine (TAFC)
and fusarinine C (FsC) facilitate intercellular release of iron by partial hydrolysis by
esterase (Estb) enzyme (Howard 1999).
7.3.2 Reductive Iron Assimilation (RIA)
In fungi, iron acquisition with high affinity is usually achieved either by secreted
siderophores (iron chelators) or by reductive iron assimilation (RIA) mechanisms
(Fatima et al. 2017). In order to start iron uptake, iron is first reduced from ferric
(Fe
3+
) to more soluble ferrous (Fe
2+
) form by localized ferrireductases present in the
plasma membrane of fungal species. Soon after this, the ferrous iron is re-ionized
which is further imported by protein complex consisting of iron permease (FtrA)
and ferrioxidase (Fetc) genes. The protein such as permease also transports metals
other than iron such as copper and zinc.
7.3.3 Heme Uptake
In contrast to bacteria, binding and uptake of iron, in fungi, are done only with heme
component. For instance, Candida albicans heme uptake mechanism involves the
glycosylphosphatidylinositol (GPI)-anchored cell surface mannoprotein, namely,
Rbt5P, but the details regarding its transport mechanism are still unknown. In order
to utilize heme-iron complex, the uptake requires intercellular degradation of heme
with heme oxygenase (Hmx1p, localized in endoplasmic reticulum).
S. Bhardwaj et al.