Alpha-hemoglobin stabilizing protein (AHSP) markedly decreases the redox potential and reactivity of alpha subunits of human HbA with hydrogen peroxide

Background: AHSP modifies redox properties of bound α subunits. Results: Isolated hemoglobin subunits exhibit significantly different redox properties compared to HbA. A significant decrease in the reduction potential of α subunits bound to AHSP results in preferential binding of ferric α. Conclusion: AHSP:α subunit complexes do not participate in ferric-ferryl heme redox cycling. Significance: AHSP binding to α subunits inhibits subunit pseudoperoxidase activity

Bordetella pertussis FbpA Binds Both Unchelated Iron and Iron Siderophore Complexes

Bordetella pertussis is the causative agent of whooping cough. This pathogenic bacterium can obtain the essential nutrient iron using its native alcaligin siderophore and by utilizing xeno-siderophores such as desferrioxamine B, ferrichrome, and enterobactin. Previous genome-wide expression profiling identified an iron repressible B. pertussis gene encoding a periplasmic protein (FbpA_Bp). A previously reported crystal structure shows significant similarity between FbpA_Bp and previously characterized bacterial iron binding proteins, and established its iron-binding ability. Bordetella growth studies determined that FbpA_Bp was required for utilization of not only unchelated iron, but also utilization of iron bound to both native and xeno-siderophores. In this <i>in vitro</i> solution study, we quantified the binding of unchelated ferric iron to FbpA_Bp in the presence of various anions and importantly, we demonstrated that FbpA_Bp binds all the ferric siderophores tested (native and xeno) with μM affinity. <i>In silico</i> modeling augmented solution data. FbpA_Bp was incapable of iron removal from ferric xeno-siderophores <i>in vitro</i>. However, when FbpA_Bp was reacted with native ferric-alcaligin, it elicited a pronounced change in the iron coordination environment, which may signify an early step in FbpA_Bp-mediated iron removal from the native siderophore. To our knowledge, this is the first time the periplasmic component of an iron uptake system has been shown to bind iron directly as Fe³⁺ and indirectly as a ferric siderophore complex

Borate as a Synergistic Anion for <i>Marinobacter algicola</i> Ferric Binding Protein, FbpA: A Role for Boron in Iron Transport in Marine Life

Boron in the ocean is generally considered a nonbiological element due to its relatively high concentration (0.4 mM) and depth independent concentration profile. Here we report an unexpected role for boron in the iron transport system of the marine bacterium <i>Marinobacter algicola</i>. Proteome analysis under varying boron concentrations revealed that the periplasmic ferric binding protein (Mb-FbpA) was among the proteins whose expression was most affected, strongly implicating the involvement of boron in iron utilization. Here we show that boron facilitates Fe³⁺ sequestration by Mb-FbpA at pH 8 (oceanic pH) by acting as a synergistic anion (B­(OH)₄^1–). Fe³⁺ sequestration does not occur at pH 6.5 where boric acid (B­(OH)₃; p<i>K</i>_a = 8.55) is the predominant species. Borate anion is also shown to bind to apo-Mb-FbpA with mM affinity at pH 8, consistent with the biological relevance implied from boron’s oceanic concentration (0.4 mM). Borate is among those synergistic anions tested which support the strongest Fe³⁺ binding to Mb-FbpA, where the range of anion dependent affinity constants is log <i>K′</i>_eff = 21–22. Since the p<i>K</i>_a of boric acid (8.55) lies near the pH of ocean water, changes in oceanic pH, as a consequence of fluctuations in atmospheric CO₂, may perturb iron uptake in many marine heterotrophic bacteria due to a decrease in oceanic borate anion concentration