moderate antibiotic and antifungal activities; the combined use of beauvericin with
ketoconazole (an antifungal drug) was found to enhance the antifungal effect,
suggesting the potential use of beauvericin as a co-drug for antifungal infections in
human [146, 147]. BEA has shown strong cytotoxicity to various human cancer cell
lines and induced the apoptosis of some cancer cell lines by activating calciumsensitive cell apoptotic pathways [148]. It also inhibits the directional cell motility
(haptotaxis) of cancer cells at subcytotoxic concentrations [147].
ENN production is catalyzed by large multidomain protein (M = 347 kDa) – the
nonribosomal peptide synthase (NRPS), known as enniatin synthetase (abbreviated
as Esyn1) [149]. As a family of related enzymes, the fungal NRPSs are all modularly
organized multienzyme complexes in which each module, located on the same
protein chain, is responsible for the initiation, elongation, and termination of growing polypeptide (in this case – by ring closure). Each module of the NRPS system
is composed of distinctly folded catalytic domains with highly conserved core
motifs, important for their catalytic activities. A minimal (inexactly) repeated unit
consists of three core domains in succession: an adenylation (A) domain which
recognizes and activates the substrate via adenylation with ATP and a thiolation/
transferase (T) or peptidyl carrier protein (PCP) domain which binds the activated
substrate to a 4
0 -phosphopantetheine (PP) cofactor via a thioester bond and transfers
the substrate to a condensation (C) domain which catalyzes peptide bond formation
between adjacent substrates on the megasynthase complex. Several other specialized
C-terminal domains involved in chain termination and release of the final peptide
product have also been identified. Optional domains include methyltransferase (M),
epimerization (E), heterocyclization (Cy), and oxidation (Ox) domains, which may
alter the enzyme-bound precursors or growing peptide intermediates at various
stages of the process. The full-length NRPS product is normally released by
a thioesterase (TE) domain giving rise to free acids, lactones, or lactams. Eukaryotic
NRPSs that synthesize cyclooligomer peptides assemble oligopeptide monomer
intermediates by the programmed iterative reuse of their modules, which
differs from the classical NRPS paradigm described in bacteria, and the resultant
monomers are frequently employed in further recursive oligomerization and cyclization process [129, 150–152].
The Esyn1-encoded protein was previously purified and characterized by Zocher
and co-workers (1982) from Fusarium oxysporum [153]. Biosynthesis proceeds
through the condensation of three dipeptidol units followed by ring closure.
The ENNs are synthesized from their primary precursors, i.e., valine, leucine,
or isoleucine, D-2-hydroxyisovaleric acid, and S-adenosylmethionine. The NRPS
domain architecture is composed of three functional modules: C-A-T-M (C, condensation domain; A, adenylation domain; T, thiolation/transferase domain; M,
methyltransferase domain). The two adenylation domains are responsible for the
specific activation of the primary substrates D-2-hydroxyisovaleric acid and L-amino
acid as acyl adenylate intermediates [153–158].
A genomic locus containing the gene cluster related to beauvericin (BEA)
biosynthesis in the entomopathogenic fungus, Beauveria bassiana, has also been
cloned. Beauvericin synthetase (bbBEAS) consists of a single polypeptide chain with
10 Fusarium Secondary Metabolism Biosynthetic Pathways: So Close but So. . .
225
ketoconazole (an antifungal drug) was found to enhance the antifungal effect,
suggesting the potential use of beauvericin as a co-drug for antifungal infections in
human [146, 147]. BEA has shown strong cytotoxicity to various human cancer cell
lines and induced the apoptosis of some cancer cell lines by activating calciumsensitive cell apoptotic pathways [148]. It also inhibits the directional cell motility
(haptotaxis) of cancer cells at subcytotoxic concentrations [147].
ENN production is catalyzed by large multidomain protein (M = 347 kDa) – the
nonribosomal peptide synthase (NRPS), known as enniatin synthetase (abbreviated
as Esyn1) [149]. As a family of related enzymes, the fungal NRPSs are all modularly
organized multienzyme complexes in which each module, located on the same
protein chain, is responsible for the initiation, elongation, and termination of growing polypeptide (in this case – by ring closure). Each module of the NRPS system
is composed of distinctly folded catalytic domains with highly conserved core
motifs, important for their catalytic activities. A minimal (inexactly) repeated unit
consists of three core domains in succession: an adenylation (A) domain which
recognizes and activates the substrate via adenylation with ATP and a thiolation/
transferase (T) or peptidyl carrier protein (PCP) domain which binds the activated
substrate to a 4
0 -phosphopantetheine (PP) cofactor via a thioester bond and transfers
the substrate to a condensation (C) domain which catalyzes peptide bond formation
between adjacent substrates on the megasynthase complex. Several other specialized
C-terminal domains involved in chain termination and release of the final peptide
product have also been identified. Optional domains include methyltransferase (M),
epimerization (E), heterocyclization (Cy), and oxidation (Ox) domains, which may
alter the enzyme-bound precursors or growing peptide intermediates at various
stages of the process. The full-length NRPS product is normally released by
a thioesterase (TE) domain giving rise to free acids, lactones, or lactams. Eukaryotic
NRPSs that synthesize cyclooligomer peptides assemble oligopeptide monomer
intermediates by the programmed iterative reuse of their modules, which
differs from the classical NRPS paradigm described in bacteria, and the resultant
monomers are frequently employed in further recursive oligomerization and cyclization process [129, 150–152].
The Esyn1-encoded protein was previously purified and characterized by Zocher
and co-workers (1982) from Fusarium oxysporum [153]. Biosynthesis proceeds
through the condensation of three dipeptidol units followed by ring closure.
The ENNs are synthesized from their primary precursors, i.e., valine, leucine,
or isoleucine, D-2-hydroxyisovaleric acid, and S-adenosylmethionine. The NRPS
domain architecture is composed of three functional modules: C-A-T-M (C, condensation domain; A, adenylation domain; T, thiolation/transferase domain; M,
methyltransferase domain). The two adenylation domains are responsible for the
specific activation of the primary substrates D-2-hydroxyisovaleric acid and L-amino
acid as acyl adenylate intermediates [153–158].
A genomic locus containing the gene cluster related to beauvericin (BEA)
biosynthesis in the entomopathogenic fungus, Beauveria bassiana, has also been
cloned. Beauvericin synthetase (bbBEAS) consists of a single polypeptide chain with
10 Fusarium Secondary Metabolism Biosynthetic Pathways: So Close but So. . .
225