Thermostability of Heteropolymer Ferritin with Different H to L Subunit Ratios Studied by Differential Scanning Calorimetry (2022)

Ferritin is an iron storage protein composed of two distinct subunits, H and L. The two subunits co-assemble to form a shell-like protein structure of twenty-four subunits within which a stable inorganic iron hydroxide is stored. Though homopolymers exist in nature, mammalian ferritins exist as heteropolymers of different subunit ratios. It is believed that the L subunit confers stability to the protein shell due to salt bridges and inter-subunit contacts between residues along three- and four-fold protein channels. We took advantage of an engineered plasmid design, recently developed in our lab, to synthesize and purify several heteropolymer ferritins and studied their thermostability and unfolding thermograms using a differential scanning calorimeter. The DSC data demonstrate that more L-subunits confer maximal protein stability, reaching a high melting temperature of 115°C for a ferritin composed of 90%L and 10%H ferritin, and that homopolymer H ferritin showed less thermostability than any of the heteropolymers. Our results are consistent with the way that nature has designed these highly stable protein nanostructures with a preferred interaction between H and L over H-H or L-L .