A Novel C-Terminal Small Tail Provides Thermostability of FeSOD Implying a New Mechanism of Protein Heat Resistance

Superoxide dismutase (SOD) is found in a variety of organisms, including animals, plants, and microorganisms, and is widely used in medicine, food, and cosmetics. In this study, a novel heat-resistant SOD from Rhodothermus sp. XMH10 (RhSOD) has been found to have no loss of activity at 80 °C and exhibit high thermal stability across a temperature range from 20 °C to 80 °C. Unlike other reported SODs, RhSOD was found to have a unique small α-helix tail at the C-terminus, consisting of 11 amino acid residues. The absence of the C-terminal α-helix tail of RhSOD was shown to reduce its activity and thermal stability at 80 °C, suggesting that the C-terminal α-helix tail is crucial for the high thermal stability of RhSOD. Furthermore, the fusion of the C-terminal α-helix tail to the C-terminus of a thermophilic SOD from Anoxybacillus caldiproteolyticus (AcSOD) enhances its thermal stability at 70 °C and 80 °C. Circular dichroism (CD) spectral analysis further indicated that the C-terminal α-helix tail could improve the α-helix content, thus enhancing the structural stability of AcSOD. Thus, a novel C-terminal α-helix tail was firstly discovered, which could confer significant thermal stability to host proteins. This finding provides a new theoretical basis for the study of protein thermostability mechanism.