The Pneumococcal Iron Uptake Protein A (PiuA) Specifically Recognizes Tetradentate Fe III bis - and Mono-Catechol Complexes

Streptococcus pneumoniae (Spn) is an important Gram-positive human pathogen that causes millions of infections worldwide with an increasing occurrence of antibiotic resistance. Fe acquisition is a crucial virulence determinant in Spn ; further, Spn relies on exogenous Fe III -siderophore scavenging to meet nutritional Fe needs. Recent studies suggest that the human catecholamine stress hormone, norepinephrine (NE), facilitates Fe acquisition in Spn under conditions of transferrin-mediated Fe starvation. Here we show that the solute binding lipoprotein PiuA from the piu Fe acquisition ABC transporter PiuBCDA, previously described as an Fe-hemin binding protein, binds tetradentate catechol Fe III complexes, including NE and the hydrolysis products of enterobactin. Two protein-derived ligands (H238, Y300) create a coordinately saturated Fe III complex, which parallel recent studies in the Gram-negative intestinal pathogen Campylobacter jejuni. Our in vitro studies using NMR spectroscopy and 54 Fe LC-ICP-MS confirm the Fe III can move from transferrin to apo-PiuA in an NE-dependent manner. Structural analysis of PiuA Fe III - bis -catechol and Ga III - bis -catechol and Ga III -(NE) 2 complexes by NMR spectroscopy reveals only localized structural perturbations in PiuA upon ligand binding, largely consistent with recent descriptions of other solute binding proteins of type II ABC transporters. We speculate that tetradentate Fe III complexes formed by mono- and bis -catechol species are important Fe sources in Gram-positive human pathogens, since PiuA functions in the same way as SstD from Staphylococcus aureus.