Thermal stability and enzymatic activity of quinoin, a type 1 ribosome inactivating protein (RIP) from quinoa seeds

Quinoin is a type 1 ribosome inactivating protein (RIP) isolated from Chenopodium quinoa seeds exhibiting enzymatic features, such as polynucleotide:adenosine glycosylase (PNAG) activity on DNA and a broad spectrum of biological functions, including antiviral, antifungal, and antitumour activities. In this study, we investigated the effects of pH and ionic strength on the enzymatic activity and thermal stability of quinoin to better understand its functional versatility and potential for biotechnological applications. We found that quinoin PNAG activity was enhanced under acidic conditions and in the presence of physiological salt concentration. To provide a link between its activity and structural stability, we performed a thorough biophysical analysis using circular dichroism (CD) and differential scanning calorimetry (DSC). Our data revealed that: i) quinoin is well-folded and thermally stable over the pH range from 7.0 to 3.0; and ii) thermal stability is influenced by pH and ionic strength, pointed out at the role of charge–charge interactions. These results suggest that the enhanced PNAG activity under acidic and high ionic strength conditions is due to the increase in structural dynamics and modulation of the charge interactions in the active site. Overall, this study provides new insights into the structure–activity relationship of quinoin and highlights its potential to operate beyond physiological pH with important biological and biotechnological applications.