NASA is reporting on research concerning the chirality of amino acids and how they may have impacted the development of life here on Earth, and perhaps, elsewhere.
The mystery of why life uses molecules with specific orientations has deepened with a NASA-funded discovery that RNA — a key molecule thought to have potentially held the instructions for life before DNA emerged — can favor making the building blocks of proteins in either the left-hand or the right-hand orientation. Resolving this mystery could provide clues to the origin of life. The findings appear in research recently published in Nature Communications.
Proteins are the workhorse molecules of life, used in everything from structures like hair to enzymes (catalysts that speed up or regulate chemical reactions). Just as the 26 letters of the alphabet are arranged in limitless combinations to make words, life uses 20 different amino acid building blocks in a huge variety of arrangements to make millions of different proteins. Some amino acid molecules can be built in two ways, such that mirror-image versions exist, like your hands, and life uses the left-handed variety of these amino acids. Although life based on right-handed amino acids would presumably work fine, the two mirror images are rarely mixed in biology, a characteristic of life called homochirality. It is a mystery to scientists why life chose the left-handed variety over the right-handed one.
DNA (deoxyribonucleic acid) is the molecule that holds the instructions for building and running a living organism. However, DNA is complex and specialized; it "subcontracts" the work of reading the instructions to RNA (ribonucleic acid) molecules and building proteins to ribosome molecules. DNA's specialization and complexity lead scientists to think that something simpler should have preceded it billions of years ago during the early evolution of life. A leading candidate for this is RNA, which can both store genetic information and build proteins. The hypothesis that RNA may have preceded DNA is called the "RNA world" hypothesis.
If the RNA world proposition is correct, then perhaps something about RNA caused it to favor building left-handed proteins over right-handed ones. However, the new work did not support this idea, deepening the mystery of why life went with left-handed proteins.
In the experiment, the researchers simulated what could have been early-Earth conditions of the RNA world. They incubated a solution containing ribozymes and amino acid precursors to see the relative percentages of the right-handed and left-handed amino acid, phenylalanine, that it would help produce.
[...]
"The findings suggest that life's eventual homochirality might not be a result of chemical determinism but could have emerged through later evolutionary pressures," said co-author Alberto Vázquez-Salazar, a UCLA postdoctoral scholar and member of Chen's research group.
[...]
"Understanding the chemical properties of life helps us know what to look for in our search for life across the solar system," said co-author Jason Dworkin, senior scientist for astrobiology at NASA's Goddard Space Flight Center in Greenbelt, Maryland, and director of Goddard's Astrobiology Analytical Laboratory.Dworkin is the project scientist on NASA's OSIRIS-REx mission, which extracted samples from the asteroid Bennu and delivered them to Earth last year for further study.
"We are analyzing OSIRIS-REx samples for the chirality (handedness) of individual amino acids, and in the future, samples from Mars will also be tested in laboratories for evidence of life including ribozymes and proteins," said Dworkin.
Journal Reference: Kenchel, J., Vázquez-Salazar, A., Wells, R. et al. Prebiotic chiral transfer from self-aminoacylating ribozymes may favor either handedness. Nat Commun 15, 7980 (2024). https://doi.org/10.1038/s41467-024-52362-x
(Score: 3, Informative) by NotSanguine on Wednesday November 27, @12:44AM (2 children)
Left out the link to the underlying Nature Communications paper.
Here it is:
Prebiotic chiral transfer from self-aminoacylating ribozymes may favor either handedness [nature.com]
Sorry about that.
No, no, you're not thinking; you're just being logical. --Niels Bohr
(Score: 4, Informative) by hubie on Wednesday November 27, @03:47AM (1 child)
Thank you. I added it at the bottom.
(Score: 2) by NotSanguine on Wednesday November 27, @03:56AM
No, no, no. Thank YOU! :)
No, no, you're not thinking; you're just being logical. --Niels Bohr
(Score: 3, Interesting) by HiThere on Wednesday November 27, @01:23AM (7 children)
Why the resistance to "random chance". A mix of handedness would seem to have problems, so whichever got started most quickly would "soon" be the only one around.
Javascript is what you use to allow unknown third parties to run software you have no idea about on your computer.
(Score: 3, Insightful) by NotSanguine on Wednesday November 27, @01:58AM (6 children)
A reasonable hypothesis, and one that's not lost on the researchers methinks.
But there isn't evidence (yet) to support it. Hence the research.
No, no, you're not thinking; you're just being logical. --Niels Bohr
(Score: 2, Interesting) by khallow on Wednesday November 27, @05:31AM (3 children)
Aside from the dominance of a single handedness. Note that this isn't the only place where handedness shows up in evolution. We see it in humans today. Dyslexia seems to be associated with a reduction in handedness, called "mixed handedness" where the person has behaviors/tasks that they prefer with different hands rather than a strong preference for one hand.
So the same hypothesis: an even mixture of handedness probably results in an increase in evolutionary disfavorable traits (here, mental issues like dyslexia). For example, if you have a chiral molecule that interacts with a second chiral molecule, then there's two such interactions possible (pair one of the first with one of the second, and then swap the chirality of the second molecule to get the other interaction). That's a substantial increase in reactions that would have to be evolved against, but which can be eliminated by only having one of each chirality present. Viewing as an intentional reaction, you would have 50% of your molecules paired in an expected way versus 100% paired in the second case with restricted chirality.
(Score: 1, Insightful) by Anonymous Coward on Wednesday November 27, @06:03AM (1 child)
This isn't a topic I know much about, but I believe amino acid chirality is linked with protein folding. We know that protein folding is linked with prion diseases, causing very real and severe harm. Could there have been issues with protein folding in life that had mixed chirality of amino acids causing disease, which might lead to evolution favoring a single chirality, whether it's right-handed or left-handed?
(Score: 2) by ben_white on Wednesday November 27, @09:56PM
I agree completely. There would be strong evolutionary pressure to use only one chirality if proteins fold less predictably when made up of AA's of mixed chirality. May be just chance that the tree of life on earth chose L. You could have organisms using either L or R chirality, but once one of these evolutionary branches gained any advantage I suspect it created a positive feedback loop in selection for its chirality, leaving to the extinction of the other branch.
(Score: 2) by NotSanguine on Thursday November 28, @05:30PM
Perhaps.
There are many hypotheses around homochirality [wikipedia.org], including certain types of radiation favoring one chirality over another, as well as many others.
The fascinating part WRT this paper has to do with analysis of the samples returned from Bennu by OSIRIS-Rex.
No, no, you're not thinking; you're just being logical. --Niels Bohr
(Score: 1) by shrewdsheep on Wednesday November 27, @08:51AM (1 child)
Well, it has Occam on its side. There is full symmetry between opposite chiral worlds, so the default assumption has to be chance. I think the deeper question looked at here is whether chirality of DNA/RNA translates through to chirality of amino acids. Given that there is a level of abstraction in between (tRNA), such an expectation is rather peculiar.
Even if they would have found a connection, that would certainly not answer questions about the origin of life. Valid basic research, but over-hyped in terms of publication prominence (a lower ranked journal would have been more appropriate) and journalistic presentation, IMHO.
(Score: 3, Insightful) by NotSanguine on Thursday November 28, @05:37PM
A fair point.
And while random chance could certainly play a role, it seems unlikely that's the whole story.
Why? Because chirality isn't an *exact* mirror image [wikipedia.org] in biological contexts. If that were the case you wouldn't see stuff like:
It's a mystery. And one that makes the wonderful and endlessly fascinating complexity of biology even more so!
No, no, you're not thinking; you're just being logical. --Niels Bohr
(Score: 2) by jman on Friday November 29, @01:15PM
Wonder if there's a correlation between that and actual left-handedness?
The ubiquituous "they" say something like ten percent of mammals are southpaws. So, it's been in the genes for awhile.
Maybe something like two gears, where the next one runs in reverse direction? So this mysterious second gear is responsible for most folks being righties.
(Of course that analogy breaks down a little because it would imply three gears, not two, to get the handedness going back in the other direction.)