Facilitating Iron Acquisition from Ferritin Protein Nanocage by Reductive Pathway: Role of Electron Transfer Mediators and Chelators

Abstract

Iron acts as a double-edged sword i.e. beneficial as well as harmful, depending upon its form (free or bound) and its cellular level. Ferritin detoxifies excess of free Fe(II) and concentrates it in the form of ferrihydrite (Fe2O3·xH2O) mineral. When in need, ferritin iron is released for cellular metabolic activities. In vivo, what kind of cytoplasmic signals and regulation exists for iron mobilizations from ferritin mineral core is not well known. Moreover, some pathogen can acquire iron from ferritin nanocage by unknown mechanism. However, in vitro, the low solubility of ferric mineral at neutral pH and its encapsulation by stable protein nanocage limits ferritin iron release. Physiological reducing agent, NADH (E1/2= −330 mV) was inefficient in releasing the ferritin iron (E1/2= +183 ± 20 mV), when used alone. Therefore, the current work employed phenazines, phenothiazines, phenoxazines, flavins as electron transfer (ET) mediators and also exploited the bifunctional character of phendione (as ET mediator cum chelator) during the iron mobilization from ferritin. Smaller sized, neutral ET mediators (PYO and RF) released more iron from ferritin by their efficient electron relay ability. The presence of dissolved O2, resulting in initial lag phase and low iron release in flavins, had little impact in case of other mediators. In comparison to neutral pH, acidic pH enhanced iron mobilization, which explains iron release inside the lysosome. The molecular modelling as well as fluorescence studies provided further structural insight towards interaction of redox mediators on ferritin surface for electron relay. Applying Marcus ET theory, our current observations suggest that dyes with E1/2 values well separated from those of the reducing agents and of the ferritin mineral can be exploited not only to facilitate ET in methemoglobinemia and iron removal during biological iron overload conditions but also to check microbial growth.