17
contrast to this, α-carboxylate- based siderophores are produced by the fungal
group Rhizopus microsporus and Zygomycetes which synthesized rhizoferrin
(Baakza et al. 2004). However, rhizoferrin represents a fairly simple molecule
consisting of citric acid linked to diaminobutane but possesses potential metalbinding properties.
The requirement of iron uptake and virulence of one of the major opportunistic
fungi A. fumigatus is also dependent on the production of siderophore molecules
(Schrettl et al. 2004). It produces varied siderophores for acquisition of iron from
extracellular and intracellular environment. A. fumigatus synthesize fusarinine type of
siderophores for procurement of iron from extracellular location, viz., fusarinine C
and its acetylated derivative triacetylfusarinine C (TAFC) (Miethke and Marahie
2007; Hissen et al. 2005). However, ferricrocin [a ferrichrome (FC) representative]
fulfils the intracellular iron requirement of A. fumigatus (hyphae) for carrying out
distribution and storage of iron inside the cell. The major enzymes involved in the
biosynthesis of hydroxamate-type siderophores are produced by non-ribosomal peptide synthetase (NRPS) multienzymes (Mootz et al. 2002; Eisendle et al. 2003). NRPS
is a large multifunctional enzyme which is capable of synthesizing peptides in absence
of mRNA and ribosome by linking various amino acids (proteogenic and non-proteogenic) through peptide or thioester bond. Mostly, fungal siderophores are comprised
of hydroxamate motifs which are synthesized using non-proteogenic amino acids
(e.g., L-ornithine) which are further processed by undergoing several reactions on the
multiple domains of NRPS enzyme. Some siderophores like TAFC and hydroxyferricrocin need post-synthetic modifications after their release from NRPS domains
(Eisendle et al. 2003).
The first step involved in the biosynthesis of hydroxamate-based fungal siderophores is hydroxylation at N5 position of L-ornithine (provided from mitochondrial
pool) to form N5-hydroxylated ornithine which is catalyzed by ornithine-N5monooxygenase (NMO) as represented by a flow diagram in Fig. 2.3. This oxygenation process utilizes the O 2 , NADPH, and FAD as cofactor. The enzyme NMO is
encoded by SidA gene of A. fumigatus which plays a crucial role in biosynthesis of
siderophore. This gene is also involved in spreading the virulence of pathogen as
confirmed through gene deletion studies in mice (Hissen et al. 2005). The next step
involves acylation at N5 position by a transfer of acyl group (acetyl or anhydromevalonyl) from acyl coenzyme A and its derivatives. The enzyme involved in acylation is
anhydromevalonyl or acetyl coenzyme A-N5 -transacylase. These enzymes are
encoded by SidF (upregulated by iron starvation) and SidL gene (which is constitutively expressed) and lead to synthesis of TAFC and FC type of siderophores, respectively (Eisendle et al. 2003; Blatzer et al. 2011). The mevalonate biosynthetic pathway
is linked to siderophore biosynthetic pathway via anhydromevalonyl- CoA. The genes
responsible for the synthesis of anhydromevalonyl-CoA are SidI (encodes acyl-CoA
synthase) and SidH (encodes enoyl-CoA hydratase). The next step involves the NRPS
enzymes, which link the different hydroxamate units generated in previous steps
through ester or peptide bonds and, thus, synthesize macrocyclic peptides which are
capable of chelating iron for nutritional requirement. These NRPS enzymes are co2 Inhibition of Siderophores in Blocking Fungal Infection
contrast to this, α-carboxylate- based siderophores are produced by the fungal
group Rhizopus microsporus and Zygomycetes which synthesized rhizoferrin
(Baakza et al. 2004). However, rhizoferrin represents a fairly simple molecule
consisting of citric acid linked to diaminobutane but possesses potential metalbinding properties.
The requirement of iron uptake and virulence of one of the major opportunistic
fungi A. fumigatus is also dependent on the production of siderophore molecules
(Schrettl et al. 2004). It produces varied siderophores for acquisition of iron from
extracellular and intracellular environment. A. fumigatus synthesize fusarinine type of
siderophores for procurement of iron from extracellular location, viz., fusarinine C
and its acetylated derivative triacetylfusarinine C (TAFC) (Miethke and Marahie
2007; Hissen et al. 2005). However, ferricrocin [a ferrichrome (FC) representative]
fulfils the intracellular iron requirement of A. fumigatus (hyphae) for carrying out
distribution and storage of iron inside the cell. The major enzymes involved in the
biosynthesis of hydroxamate-type siderophores are produced by non-ribosomal peptide synthetase (NRPS) multienzymes (Mootz et al. 2002; Eisendle et al. 2003). NRPS
is a large multifunctional enzyme which is capable of synthesizing peptides in absence
of mRNA and ribosome by linking various amino acids (proteogenic and non-proteogenic) through peptide or thioester bond. Mostly, fungal siderophores are comprised
of hydroxamate motifs which are synthesized using non-proteogenic amino acids
(e.g., L-ornithine) which are further processed by undergoing several reactions on the
multiple domains of NRPS enzyme. Some siderophores like TAFC and hydroxyferricrocin need post-synthetic modifications after their release from NRPS domains
(Eisendle et al. 2003).
The first step involved in the biosynthesis of hydroxamate-based fungal siderophores is hydroxylation at N5 position of L-ornithine (provided from mitochondrial
pool) to form N5-hydroxylated ornithine which is catalyzed by ornithine-N5monooxygenase (NMO) as represented by a flow diagram in Fig. 2.3. This oxygenation process utilizes the O 2 , NADPH, and FAD as cofactor. The enzyme NMO is
encoded by SidA gene of A. fumigatus which plays a crucial role in biosynthesis of
siderophore. This gene is also involved in spreading the virulence of pathogen as
confirmed through gene deletion studies in mice (Hissen et al. 2005). The next step
involves acylation at N5 position by a transfer of acyl group (acetyl or anhydromevalonyl) from acyl coenzyme A and its derivatives. The enzyme involved in acylation is
anhydromevalonyl or acetyl coenzyme A-N5 -transacylase. These enzymes are
encoded by SidF (upregulated by iron starvation) and SidL gene (which is constitutively expressed) and lead to synthesis of TAFC and FC type of siderophores, respectively (Eisendle et al. 2003; Blatzer et al. 2011). The mevalonate biosynthetic pathway
is linked to siderophore biosynthetic pathway via anhydromevalonyl- CoA. The genes
responsible for the synthesis of anhydromevalonyl-CoA are SidI (encodes acyl-CoA
synthase) and SidH (encodes enoyl-CoA hydratase). The next step involves the NRPS
enzymes, which link the different hydroxamate units generated in previous steps
through ester or peptide bonds and, thus, synthesize macrocyclic peptides which are
capable of chelating iron for nutritional requirement. These NRPS enzymes are co2 Inhibition of Siderophores in Blocking Fungal Infection