Perchlorate Salts Exert a Dominant, Deleterious Effect on the Structure, Stability, and Activity of α-Chymotrypsin

The presence of perchlorate ions on Mars raises the question of how these ions influence the biochemistry of any contaminant life introduced into the martian environment, or what selection pressures perchlorate ions exert on any environment that contains these ions, such as the Atacama Desert. In this study, we investigated the structure, stability, and enzyme activity of the model enzyme α-chymotrypsin in the presence of five Mars relevant salts, MgSO₄, MgCl₂, Mg(ClO₄)₂, Ca(ClO₄)₂, and NaClO₄. We found that all the perchlorate salts reduced the enzyme activity of α-chymotrypsin in a concentration-dependent manner, with Mg(ClO₄)₂ and Ca(ClO₄)₂ having the greatest effect. This observation extends to our structural studies, which show that 1 M Mg(ClO₄)₂ and Ca(ClO₄)₂ greatly alter the tertiary structural environment of α-chymotrypsin. We also found that all the perchlorate salts assayed reduced the melting temperature of α-chymotrypsin, whereas the sulfate and chloride salts were able to increase the protein melting temperature. We also demonstrated that a brine containing both perchlorate and sulfate ions exerts the same deleterious effects on α-chymotrypsin's melting temperature and enzyme activity as that of a perchlorate-only brine. This suggests that the perchlorate salts exert a dominant, deleterious effect on protein biochemistry. These results indicate that although perchlorate salts are beneficial to the presence of liquid water due to low eutectic points, they also hamper the habitability of their own environment. Life in such brines would, therefore, have to adapt its cellular machinery to the perchlorate ion's presence or find a way of excluding it from said machinery.