Iron Scavenging in Aspergillus Species: Structural and Biochemical Insights into Fungal Siderophore Esterases

Abstract

Fungi utilize high-affinity chelators termed siderophores with chemically diverse structures to scavenge the essential nutrient iron from their surroundings. Since they are among the strongest known Fe binding agents, intracellular release of the heavy metal atom is facilitated by the activity of specific hydrolases. In this work, we report the characterization and X-ray crystal structures of four siderophore esterases: AfEstB and AfSidJ from Aspergillus fumigatus, as well as AnEstB and AnEstA from Aspergillus nidulans. Even though they all display the conserved α/β-hydrolase fold, we found significant structural and enzymatic discrepancies in their adaption to both related and chemically diverse substrates. A structure of AfEstB in complex with its substrate triacetylfusarinine C gives insight into the active enzyme and shows tetrahedral coordination between the catalytic serine and the scissile ester bond.

Ilyonectria Root Rot of Ginseng Is Attenuated via Enzymatic Degradation of the Extracellular Fe3+-Bound Siderophore N,N′,N″-Triacetylfusarinine C

“…Kinetic assays using TAFC as the substrate demonstrated that the MBPtagged form of the enzyme was active, and LC−MS studies showed that the enzyme cleaves Fe 3+ -bound TAFC into its corresponding building blocks (Figure 3) as previously described. 20 Application of AfEstB to ginseng roots infected with I. morspanacis significantly reduced the severity of infection (Figures 4 and 5). These results show that enzymatic treatment decreases virulence and slows the growth of the fungus.…”

“…These results are in accordance with previously published values for the isolated recombinant enzyme. 20 We also monitored AfEstB activity by tracking the degradation of TAFC via high-resolution LC−MS (Figure 3C). AfEstB hydrolyzes the ester moieties that link the three acetylated ornithine units that comprise TAFC.…”

“…We determined K M and k cat values of 456 μM and of 3.7 s –1 , respectively, for our preparation of the enzyme. These results are in accordance with previously published values for the isolated recombinant enzyme …”

“…The result is not unexpected as AfEstB is highly specific for TAFC, and this extracellular siderophore is only critical for pathogenicity and not viability. 20 Our results suggest that continued and consistent application of the enzyme is required to prevent or delay infection, provided that the root is not already in an iron-rich environment. The experimental setup in this study represents an unusually severe I. mors-panacis infection, since it is unlikely for the ginseng root to be directly exposed to a concentrated mycelial mat of the fungus as in this experiment.…”

Synergistic Antifungal Effect of a Combination of Iron Deficiency and Calcium Supplementation

“… A. fumigatus has a virulence-required siderophore-mediated iron acquisition system ( 26 ). The major secreted siderophore is triacetylfusarinine C (TAFC) ( 27 , 28 ), which is inducibly secreted under iron-deficient conditions. In A. fumigatus , iron uptake, consumption and detoxification must be tightly controlled.…”