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

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 Fe³⁺ 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.