Impact of Phosphate on Iron Mineralization and Mobilization in Nonheme Bacterioferritin B from <i>Mycobacterium tuberculosis</i>

Abstract

Ferritins are supramolecular nanocage proteins, which synthesize hydrated ferric oxyhydroxide mineral via protein mediated rapid Fe2+ sequestration and ferroxidase reactions. Ferritin minerals are also associated with a significant amount of phosphate, which contribute toward their structure and reactivity. Like iron, phosphate also regulates the pathogenesis of Mycobacterium tuberculosis (Mtb), which expresses two types of ferritin: heme binding bacterioferritin A (BfrA) and nonheme binding bacterioferritin B (BfrB). Unlike Mtb BfrA, the rapid kinetics and mechanism of ferroxidase activity, and the mineral core formation/dissolution in Mtb BfrB are not well explored. Moreover, the effect of physiological levels of phosphate (0–10 mM) on the synthesis, structure, and reactivity of ferritin mineral core also require investigation in detail. Therefore, the stopped-flow rapid kinetics of ferroxidase activity (ΔA650/Δt) of Mtb BfrB was carried out, which detected a transient intermediate similar to diferric peroxo species as observed in frog and human ferritins. Increasing phosphate concentration increased the initial rate of iron mineralization (ΔA350/Δt) and dissolved O2 consumption (both ∼1.5–2-fold). Phosphate not only decreased the amount of iron loading and size of the BfrB mineral core (both up to 2-fold) but also decreased its crystallinity, resembling the variations observed in the core morphology of different native ferritins. In addition, phosphate inhibited the kinetics of reductive iron mobilization (∼6–8-fold) indicating its influence on the stability of the iron mineral core. Hence, the current work provides the kinetic/mechanistic insight toward the ferroxidase activity in Mtb BfrB, apart from demonstrating the role of phosphate toward the structure/reactivity of its iron mineral.