Uropathogenic Escherichia coli wield enterobactin-derived catabolites as siderophores.

Uropathogenic Escherichia coli (UPEC) secrete multiple siderophore types to scavenge extracellular iron(III) ions during clinical urinary tract infections, despite the metabolic costs of biosynthesis. Here, we find the siderophore enterobactin (Ent) and its related products to be prominent components of the iron-responsive extracellular metabolome of a model UPEC strain. Using defined Ent biosynthesis and import mutants, we identify lower molecular weight dimeric exometabolites as products of incomplete siderophore catabolism, rather than prematurely released biosynthetic intermediates. In E. coli, iron acquisition from iron(III)-Ent complexes requires intracellular esterases that hydrolyze the siderophore. Although UPEC are equipped to consume the products of completely hydrolyzed Ent, we find that Ent and its derivatives may be incompletely hydrolyzed to yield products with retained siderophore activity. These results are consistent with catabolic inefficiency as means to obtain more than one iron ion per siderophore molecule. This is compatible with an evolved UPEC strategy to maximize the nutritional returns from metabolic investments in siderophore biosynthesis.

Uropathogenic Escherichia coli wield enterobactin-derived catabolites as siderophores.

Uropathogenic E. coli (UPEC) secrete multiple siderophore types to scavenge extracellular iron(III) ions during clinical urinary tract infections, despite the metabolic costs of biosynthesis. Here we find the siderophore enterobactin and its related products to be prominent components of the iron-responsive extracellular metabolome of a model UPEC strain. Using defined enterobactin biosynthesis and import mutants, we identify lower molecular weight, dimeric exometabolites as products of incomplete siderophore catabolism, rather than prematurely released biosynthetic intermediates. In E. coli, iron acquisition from iron(III)-enterobactin complexes requires intracellular esterases that hydrolyze the siderophore. Although UPEC are equipped to consume the products of completely hydrolyzed enterobactin, we find that enterobactin and its derivatives may be incompletely hydrolyzed to yield products with retained siderophore activity. These results are consistent with catabolic inefficiency as means to obtain more than one iron ion per siderophore molecule. This is compatible with an evolved UPEC strategy to maximize the nutritional returns from metabolic investments in siderophore biosynthesis.

Organic Solvents for Enhanced Proteolysis of Stable Proteins for Hydrogen-Deuterium Exchange Mass Spectrometry.

Protein digestion is a key challenge in mass spectrometry (MS)-based structural proteomics. Although using hydrogen-deuterium exchange kinetics with MS (HDX-MS) to interrogate the high-order structure of proteins is now established, it can be challenging for ß-barrel proteins, which are important in cellular transport. These proteins contain a continuous chain of H-bonds that impart stability, causing difficulty in digestion for bottom-up measurements. To overcome this impediment, we tested organic solvents as denaturants during on-line pepsin digestion of soluble ß-barrel proteins. We selected green fluorescent protein (GFP), siderocalin (Scn), and retinol-binding protein 4 (RBP4) as model proteins and screened six different polar-aprotic and polar-protic solvent combinations to disrupt the H-bonds and hydrophobic interactions holding together the ß-sheets. The use of organic solvents improves digestion, generating more peptides from the rigid ß-barrel regions, without compromising the ability to predict the retinol binding site on RBP4 when adopting this proteolysis with HDX.

Site-Specific Siderocalin Binding to Ferric and Ferric-Free Enterobactin As Revealed by Mass Spectrometry.

Both host and pathogen competitively manipulate coordination environments during bacterial infections. Human cells release the innate immune protein siderocalin (Scn, also known as lipocalin-2/Lcn2, neutrophil gelatinase-associated lipocalin/NGAL) that can inhibit bacterial growth by sequestering iron in a ferric complex with enterobactin (Ent), the ubiquitous Escherichia coli siderophore. Pathogenic E. coli use the virulence-associated esterase IroE to linearize the Ent cyclic trilactone to linear enterobactin (lin-Ent). We characterized lin-Ent interactions with Scn by using native mass spectrometry (MS) with hydrogen-deuterium exchange (HDX) and Lys/Arg specific covalent footprinting. These approaches support 1:1 binding of both Fe(III)-lin-Ent to Scn and iron-free lin-Ent to Scn. Both ferric and nonferric lin-Ent localize to all three pockets of the Scn calyx, consistent with Scn capture of lin-Ent both before and after Fe(III) chelation. These findings raise the possibility that Scn neutralizes both siderophores and siderophore-bound iron during infections. This integrated, MS-based approach circumvents the limitations that frustrate traditional structural approaches to examining Scn interactions with enterobactin-based ligands.

Excitation Energy Dependence of the Photoluminescence Quantum Yields of Core and Core/Shell Quantum Dots.

The photoluminescence (PL) intensity of semiconductor quantum dots (QDs) is routinely monitored to track the chemical and physical properties within a sample or device incorporating the QDs. A dependence of the PL quantum yields (QYs) on the excitation energy could lead to erroneous conclusions but is commonly not considered. We summarize previous evidence and present results from two methodologies that confirm the possibility of a dependence of the PL QYs on the excitation energy. The data presented indicate that PL QYs of CdSe and CdSe/ZnS QDs suspended in toluene are highest for excitation just above the band gap, Eg, of each. The PL QYs decrease with increasing excitation energies up to 1 eV above Eg. The PL intensity decay profiles recorded for these samples at varying emission and excitation energies indicate that the changes in the PL QYs result from the nonradiative relaxation pathways sampled as the charge carriers relax down to the band edge.

Bright core-shell semiconductor quantum wires.

Colloidal CdTe quantum wires are reported having ensemble photoluminescence efficiencies as high as 25% under low excitation-power densities. High photoluminescence efficiencies are achieved by formation of a monolayer CdS shell on the CdTe quantum wires. Like other semiconductor nanowires, the CdTe quantum wires may contain frequent wurtzite-zinc-blende structural alternations along their lengths. The present results demonstrate that the optical properties, emission-peak shape and photoluminescence efficiencies, are independent of the presence or absence of such structural alternations.