Pseudomonas aeruginosa is the most common pathogen that persists in the cystic fibrosis lungs. Bacteria such as P. aeruginosa secrete siderophores (iron-chelating molecules) and the host limits bacterial growth by producing neutrophil-gelatinase-associated lipocalin (NGAL) that specifically scavenges bacterial siderophores, therefore preventing bacteria from establishing infection. P. aeruginosa produces a major siderophore known as pyoverdine, found to be important for bacterial virulence and biofilm development. We report that pyoverdine did not bind to NGAL, as measured by tryptophan fluorescence quenching, while enterobactin bound to NGAL effectively causing a strong response. The experimental data indicate that pyoverdine evades NGAL recognition. We then employed a molecular modeling approach to simulate the binding of pyoverdine to human NGAL using NGAL’s published crystal structures. The docking of pyoverdine to NGAL predicted nine different docking positions; however, neither apo- nor ferric forms of pyoverdine docked into the ligand-binding site in the calyx of NGAL where siderophores are known to bind. The molecular modeling results offer structural support that pyoverdine does not bind to NGAL, confirming the results obtained in the tryptophan quenching assay. The data suggest that pyoverdine is a stealth siderophore that evades NGAL recognition allowing P. aeruginosa to establish chronic infections in CF lungs. 1. Introduction Cystic fibrosis (CF) is an inherited disease resulting in the formation of thick, sticky mucus in the lungs and digestive pathways [1]. CF lungs are breeding grounds for a variety of bacterial pathogens. Pseudomonas aeruginosa is the most common bacterial isolate that persists in CF lungs by evading host defenses and clearance mechanisms, contributing to declining lung function [1, 2]. In order to grow and thrive, bacteria need iron as a cofactor for many metabolic enzymes. However, in the case of infection, iron is not freely available in the host and is predominantly found in iron-binding proteins such as transferrin, lactoferrin, and ferritin [3]. Thus, due to the low bioavailability of free iron, bacteria produce iron-chelating small molecules known as siderophores that are secreted in apo-form and imported as ferric complexes [1, 4]. P. aeruginosa produces two siderophores, pyoverdine (Figures 1(a), and 1(b)) and pyochelin, that are found to be important for biofilm development and bacterial virulence [4]. Figure 1: Molecular structures of ferric bacterial siderophores used in this study. (a) Chemical structure of
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