Ferritin is a self-assembled 24-mer spherical cage shaped iron storage protein which has an internal cavity of diameter 8 nm. The internal cage is connected with several symmetric pores and interestingly, small metal complexes and organic molecules can easily pass through these pores. The cage is very stable up to 80ºC but disassembles beyond the pH range of 2-11 which can reassemble again at neutral pH. This interesting feature is useful for incorporating larger molecules into the cage. Although the disassembly-reassembly mechanism of ferritin cage has been studied using various spectroscopic and analytical methods, it remains challenging to have a direct structural insight into this mechanism. We used high-speed AFM to observe the phenomena at single molecule level in real-time scale. Due to unique structural features, the ferritin cage is widely utilized for preparing uniform metal nanoparticles which have important catalytic and biomedical applications. For example, magnetic iron oxide nanoparticles grown inside ferritin cage can promote catalytic reactions in the cell and can target and visualize cancer cells. Despite many promising applications, it remains challenging to explore the basic structural features of protein-nanoparticle interactions. In this context, we promoted the chemical reduction of gold ions within a crystalline ferritin cage and analyzed the X-ray crystal structures at various steps. This gives a direct visualization on the formation of gold nanocluster in protein environment and is considered to be an intermediate stage of nanoparticle growth. In another aspect, multiple metal complexes were incorporated simultaneously into the ferritin cage to construct protein-based microcompartment similar to bacterial microcompartment where several enzymes work together for metabolic functions. We studied the X-ray crystal structures and performed the catalytic cascade reactions in confined ferritin environment as a proof of concept.
This presentation includes various structural aspects of metal immobilized ferritin protein cage with X-ray crystal structure analysis and their applications in catalysis as well as understanding the growth of metal nanoparticles in protein environment.
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