Topographical Characterization of Ferritin Heteropolymers by AFM

Ferritin is the iron storage protein that plays a central role in critical cellular processes to maintain iron homeostasis in living organisms. It consists of an iron biomineral core of ferrihydrite surrounded by a spherical hetero-polymeric shell composed of 24 amino acid subunits of two types, H and L. There are H-rich ferritins and L-rich ferritins, meaning that the respective amino acids are more abundant depending on the organ tissue. Due to the nanoscale size of the ferritin molecule, ca. 12-nm outer shell diameter, it is challenging to analyze its topographical properties. Atomic Force Microscopy (AFM) utilizes a cantilever and tip to create an image of the surface of a substrate on the nanometer scale. Using AFM, we were able to obtain topographical data by acquiring height profiles of four different ferritin samples: L-rich apoferritin, H-rich apoferritin, L-rich holoferritin, and H-rich holoferritin. Using the pinpoint mode of a Parks' System AFM, we were also able to get a measure of the stiffness of the heteropolymers by acquiring Deformation and Young's Modulus data for each sample.