Unravelling the Role of Heme in Bacterioferritin and Regulating Ferritin Self-Assembly: Implication Towards Health and Diseases

Abstract

Iron behaves as a double-edged sword, i.e., beneficial as well as harmful, for both host and pathogens. During the host-pathogen battle for iron, host secretes H2O2 to kill pathogens but pathogens employ counter mechanisms/ antioxidative proteins to detoxify it. Mycobacterium tuberculosis (Mtb), the causative pathogen of tuberculosis, expresses two maxi-ferritins: a heme binding bacterioferritin (BfrA) and a non-heme ferritin (BfrB) but not the mini-ferritin, i.e. Dps protein (which protects DNA from H2O2 induced oxidative stress). In the absence of Dps, Mtb must have evolved with other defensive strategies. Moreover, the expression of BfrA along with BfrB possibly suggests the existence of some additional functions for BfrA/heme. The current study on Mtb BfrA revealed its two new functions (catalase and Dps-like DNA protection activity) along with rapid ferroxidase activity. The presence of heme in Mtb BfrA contributes to its enhanced cage stability and reductive iron mobilization. Ferritin proteins are hollow, spherical, nanocaged structures, which are self-assembled from 24-polypeptide subunits and are capable of storing ~4500 iron atoms as ferrihydrite mineral. Although the factors that drive self-assembly process and control its kinetics are little investigated, the inherent reversibility of this phenomena has been recently exploited in cellular imaging and targeted drug delivery. The laser light scattering studies on ferritin self-assembly identified subunit monomers/dimers as starting materials and revealed the factors that altered the kinetics whereas electrostatics predicted the critical amino acid residues. In addition, the ferritin environment was modified by a biopolymer (guar gum) to repress iron mobilization by inhibiting the cage disintegration under acidic conditions, possibly suggesting its implication in oral iron formulations. Therefore, the current dissertation not only strengthens the understanding of the new functions of Mtb BfrA along with the role of heme, to possibly serve as a future platform to curb tuberculosis, but also provides insights on ferritin self-assembly to optimize the loading/unloading of drugs and nanomaterials for biomedical applications.