For many years, enzymes were thought to be basically the only biological catalyst molecules out there. Then things like ribozymes were discovered, and it was appreciated that the nucleic acid polymers could also bind substrates and catalyze their reactions. That brought up a natural question: what other sorts of complex polymers might be able to do the same thing? Proteins can clearly do a terrific job, but is that because they’re clearly the best choice, or just the one that evolutionary biochemistry landed on?
That’s a hard question to answer, but it covers a lot of important ground. Can enzyme-like catalysts be made from chemically more robust scaffolds? These could be of great industrial use – proteins don’t stand up to very much rough handling. And since catalysis would seem essential for living systems, how many alternatives are there for extraterrestrial chemical-based life? Those may not be too far off of what we know, but still be strange to us: we find amino acids, nucleotides, and simple carbohydrates in carbonaceous solar system debris, so they would appear to be produced by the sorts of processes (heat, pressure, irradiation) involved in the formation of planetary systems. And they may well form the most likely building blocks for life in general, assuming that we’re not particularly special. But did we have to end up with ribose, 2-deoxyribose and the four nucleotides we have (or our twenty-ish amino acids?)
Maybe not, according to this new paper. The authors describe several new “synthetic generic polymers“, with new and completely unnatural carbohydrate backbones, and show that these, too, can fold into catalytic species. Random pools of these polymers were selected out to find ligase and endonuclease activity:
We have shown the discovery of catalysts (RNA endonucleases) in four such XNA sequence spaces (ANA, FANA, HNA, CeNA) and the elaboration of three different catalytic activities (RNA endonuclease, RNA ligase and XNA ligase) in one (FANA). These results indicate that properties such as catalysis (as well as heredity and evolution) are generalizable to a range of nucleic acid scaffolds and are likely to be emergent properties of many synthetic genetic polymers. This argues against a strong functional imperative for the chemistry of life’s genetic systems.
Indeed it does. A few billion years of selection pressure on any of these, one imagines, might well produce biochemistries just as robust as ours. And how many more are possible? We might eventually find that life in the universe might be, in general, sort of like us (DNA-ish genetics to make protein-ish molecules, decorated with carbohydrate-y surfaces), but very different in the particulars. I hope we get a chance to find out.
As an aside, this sort of thing brings up a thought that has occurred to me many times about extraterrestrial life: if it does tend to be broadly similar to us, biochemically, might it not also be the source of wildly potent
allergans allergens? A few science-fiction writers have speculated about this, but most popular treatments tend to ignore this possibility, for obvious dramatic reasons: imagine Kirk and Spock beaming down to the surface of a new planet, greeting the natives, and then swelling up while collapsing in sneezing fits. . .