58
The NRPS chain extension unit contains three chains:
1. Adenylation domain (A): Adenylation domain specifically identifies the substrate and catalyzes the adenylation of its carboxyl group.
2. Thiolation domain (T): Thiolation domain is also called as the peptidyl carrier
protein (PCP) domain. It utilizes terminal thiol of a post-translationally installed
phosphopantetheine arm to bind the activated carboxyl group of the adenylate.
3. Condensation domain (C): It catalyses acylation of the resulting thioester with
activated acyl group attached to the T domains in upstream unit. In some NRPS
unit, C domain is replaced by a heterocyclization (CY) domain that catalyzes the
heterocycle formation by a reaction of β-amino thiol group in the substrate
attached to the T domain of the upstream unit.
The chain initiation unit contains only A and T domains. The growing chain is
covalently bound to the T domain in successive unit throughout the assembly
process.
Thioesterase domain (TE): This domain is usually present in the final unit.
Hydrolysis or cyclization result in the release of assembled chain from the
NRPS. Some TE domains catalyze NADH-dependent cleavage of the acyl thioester
attached to the T domain.
4.3.2.2 NRPS-Independent Biosynthesis
Compared with NRPS-dependent pathway, enzymology of non-ribosomal-peptidesynthetases-independent siderophore (NSI) biosynthesis has been neglected for
nearly three decades. Neilands and colleagues reported the first genetic
characterization of the NIS biosynthetic pathway of aerobactin 1980 (Challis 2005;
de Lorenzo et al. 1986). Aerobactin siderophore is a common metabolic product of
different bacteria such as Vibrio, Yersinia, Salmonella, and E. (Challis 2005). The
enzymes involved in aerobactin biosynthesis are LucD, a mono-oxygenasedependent FADH2 that converts L-lysine L-N6-huydroyl-lysine using molecular
oxygen as a co-substrate (Challis 2005). The enzymes involved have specific properties of the substrate according to biochemical characterization. Several NIS synthetases were reported, for instance, DesD, a cluster of gene des ABCD, pubC, and
put AB (Barona-Gomez et al. 2006; Challis 2005).
In contrast with most other genes encoding fungal NRPSs, NPS6 is widely conserved among filamentous ascomycetes (Lee et al. 2005). The authors also demonstrated that the application of exogenous Fe3+ restored the mutant NPS6 virulence in
exposed plants and that the gene encodes the extracellular hydroxamate siderophore.
NPS6 appears to be an orthologue of sidD, indicating that sidD (or peptide sidD
product) can partially mediate resistance to oxidative stress in fumigatus and contribute to the virulence of the fungus. Furthermore, the removal of sidC in A. nidulans led
to the inability to produce ferricrocin siderophore and upregulation of some antioxidant enzymes (e.g., catalase and superoxide dismutase) (Eisendle et al. 2006). More
S. Ashraf et al.
The NRPS chain extension unit contains three chains:
1. Adenylation domain (A): Adenylation domain specifically identifies the substrate and catalyzes the adenylation of its carboxyl group.
2. Thiolation domain (T): Thiolation domain is also called as the peptidyl carrier
protein (PCP) domain. It utilizes terminal thiol of a post-translationally installed
phosphopantetheine arm to bind the activated carboxyl group of the adenylate.
3. Condensation domain (C): It catalyses acylation of the resulting thioester with
activated acyl group attached to the T domains in upstream unit. In some NRPS
unit, C domain is replaced by a heterocyclization (CY) domain that catalyzes the
heterocycle formation by a reaction of β-amino thiol group in the substrate
attached to the T domain of the upstream unit.
The chain initiation unit contains only A and T domains. The growing chain is
covalently bound to the T domain in successive unit throughout the assembly
process.
Thioesterase domain (TE): This domain is usually present in the final unit.
Hydrolysis or cyclization result in the release of assembled chain from the
NRPS. Some TE domains catalyze NADH-dependent cleavage of the acyl thioester
attached to the T domain.
4.3.2.2 NRPS-Independent Biosynthesis
Compared with NRPS-dependent pathway, enzymology of non-ribosomal-peptidesynthetases-independent siderophore (NSI) biosynthesis has been neglected for
nearly three decades. Neilands and colleagues reported the first genetic
characterization of the NIS biosynthetic pathway of aerobactin 1980 (Challis 2005;
de Lorenzo et al. 1986). Aerobactin siderophore is a common metabolic product of
different bacteria such as Vibrio, Yersinia, Salmonella, and E. (Challis 2005). The
enzymes involved in aerobactin biosynthesis are LucD, a mono-oxygenasedependent FADH2 that converts L-lysine L-N6-huydroyl-lysine using molecular
oxygen as a co-substrate (Challis 2005). The enzymes involved have specific properties of the substrate according to biochemical characterization. Several NIS synthetases were reported, for instance, DesD, a cluster of gene des ABCD, pubC, and
put AB (Barona-Gomez et al. 2006; Challis 2005).
In contrast with most other genes encoding fungal NRPSs, NPS6 is widely conserved among filamentous ascomycetes (Lee et al. 2005). The authors also demonstrated that the application of exogenous Fe3+ restored the mutant NPS6 virulence in
exposed plants and that the gene encodes the extracellular hydroxamate siderophore.
NPS6 appears to be an orthologue of sidD, indicating that sidD (or peptide sidD
product) can partially mediate resistance to oxidative stress in fumigatus and contribute to the virulence of the fungus. Furthermore, the removal of sidC in A. nidulans led
to the inability to produce ferricrocin siderophore and upregulation of some antioxidant enzymes (e.g., catalase and superoxide dismutase) (Eisendle et al. 2006). More
S. Ashraf et al.