In my last blog post I talked about how high pressures can increase enzyme activity even in the presence of perchlorate salts. Well now it is time to talk about the study which led to those results, which, ironically has only just been published. Peer review, what can you do?
My latest paper explored the effects of perchlorate salts on the structure, stability, and activity of my model enzyme α-chymotrypsin. These results essentially formed the benchmark for my understanding of how perchlorate salts affect α-chymotrypsin. This gave me a solid platform from which I can relate all future work back to as a reference point. So what did I find?
Perchlorate salts are bad for biochemistry, but not equally so. The three perchlorate salts examined were sodium, magnesium, and calcium perchlorate. I found that all perchlorate salts reduced the activity and the melting temperature of my enzyme in a concentration dependent manner. In fact the enzyme totally unfolds at room temperature in high enough concentrations of magnesium and calcium perchlorate. The extent to which these salts exerted a deleterious effect followed a fairly standard Hofmeister series, namely that sodium perchlorate exhibits the least deleterious effects, with calcium perchlorate exhibiting the most, and magnesium perchlorate exhibiting intermediate values. However, it is hard to draw solid comparisons between sodium perchlorate and magnesium and calcium perchlorate as they are divalent cations. Due to their stoichiometry, a one molar solution of magnesium or calcium perchlorate has twice as many perchlorate anions in solution as does a one molar solution of sodium perchlorate. You can of course double the amount of sodium perchlorate to equal out the anion concentrations, but now you have twice as many sodium cations in solution as you have magnesium or calcium. Ah the fun times we have trying to understand ionic effects in salt studies… This is why I prefer to look at these effects from a “whole salt” perspective.
Another feature explored in this study was the fact that the perchlorate effect appears to be dominant. On Mars, it is unlikely that an environment would contain only one type of salt, so it makes sense to examine combinations to assess whether they negate, or exacerbate each other. As magnesium sulphate is fairly widespread on Mars, it made sense to include it alongside the perchlorates. I found that magnesium sulphate increased the melting temperature of α-chymotrypsin, i.e. it stabilised the enzyme, but could it still exert this effect in the presence of perchlorates, or at least mitigate some of the perchlorate nastiness? Interestingly the answer was no, the perchlorate salt effect was dominant. This suggests that even if there are stabilising salts in a perchlorate containing solution, the deleterious effect of the perchlorates will still come out on top and exert its effect on biochemistry. This result was a little surprising as it has been shown previously that stabilising agents can generally negate the effects of a range of deleterious compounds. It is hard to explain why the perchlorate effect is dominant without understanding what is happening at the molecular level, which unfortunately is beyond us for now. If I were to speculate, I would suggest that the mechanism through which perchlorates exert these destabilising effects is not simply just the opposite of how stabilising molecules exert their effects.
This research builds on a long history of deleterious salt effects, otherwise known as chaotropicity. The deleterious nature of perchlorate salts agrees with many other studies, however the dominant nature of these effects was unexpected. Altogether this paints a rather bleak picture for the potential habitability of Mars if we cannot have functioning biochemistry in low molal concentrations of perchlorates. However, my research is all about the weird things that happen when we look at more than just one environmental parameter. We already saw that high pressures increased enzyme activity when in the presence of perchlorates, and I can only hint that we have found another interesting effect when you change a certain physical parameter. For now you will just have to believe me when I say that the possibility of biochemistry in perchlorate brines may not be as bleak as has been previously thought.
