Straight up, with a twist: New model derives homochirality from basic life requirements

October 13, 2015 by Claudia Lutz, University of Illinois at Urbana-Champaign
This is a computer simulation of the emergence of homochirality. The colors represent the degree of chirality, with red being (e.g.) right and blue being (e.g.) left. At the beginning the mixture has equal numbers of right and left-handed molecules, and during the time evolution, the red and blue phases compete over the spatial domain, resulting in the eventual dominance of the blue chiral phase. Credit: Nigel Goldenfeld Lab, University of Illinois

Life is quirky. Although the molecules that make up all living things obey physical and chemical laws, they do so with a puzzling twist. How did the distinctive molecular features of life emerge, and what can they tell us about life on Earth and elsewhere in the universe?

University of Illinois Swanlund Professor of Physics Nigel Goldenfeld, graduate student Farshid Jafarpour, and postdoctoral researcher Tommaso Biancalani have made a breakthrough in one of the most central chemical quirks of life as we know it: homochirality, the uniform "handedness" of biological molecules. Their new model addressing the emergence of this feature, published in Physical Review Letters and highlighted by Physics suggests that homochirality can be used as a universal signature of life.

All three researchers are members of the Biocomplexity research theme within the Carl R. Woese Institute for Genomic Biology (IGB), and performed their work with funding from the NASA Astrobiology Institute for Universal Biology at UIUC, which Goldenfeld directs.

Many chemicals, organic or otherwise, are chiral; that is, if the structure of each was reflected in a mirror, its "looking-glass" copy could not be turned or flipped to match the original. Like a pair of gloves, the left-handed and right-handed versions of a chiral molecule are functionally equivalent, but their fundamental asymmetry makes them distinct.

Inorganic reactions produce and consume both versions of chiral molecules at equal rates. This is what makes the chirality of biological molecules, such as sugars produced by microbes and plants or the that make up proteins, so shocking. In every living thing on Earth, all amino acids are left-handed, and all sugars are right-handed. Goldenfeld highlighted the central mystery of this phenomenon.

"Imagine you've got a coin, and it's perfectly made, so it's not biased at all, and you start flipping the coin. Each time you flip it, it keeps coming up heads," he said. "So then you say, something must be operating that's causing this to happen . . . you get the same puzzle with these , and that's the problem of homochirality."

Many scientists have proposed hypotheses for how this remarkable asymmetry became dominant. Perhaps the most prominent, put forward by noted physicist Sir Charles Frank in 1953, argued that homochirality could be produced by one of the fundamental properties of life—autocatalysis, the ability to self-replicate. He argued that in a system where one left-handed or right-handed molecule begets more like itself, and each type inhibits the self-replication of the other, an initial unevenness in the ratio, appearing by chance, would ultimately allow one handedness to completely outcompete the other.

Frank's work was ground-breaking, but it left unanswered questions that no subsequent work has adequately addressed. His idea appeared to rely on the inhibition of self-replication of each chirality by the other, a mechanism that might not have existed early on in life's history.

The Illinois team wanted to develop a simpler model, one based on only the most basic properties of life: self-replication and disequilibrium. They showed that with only these minimal requirements, homochirality appears when self-replication is efficient enough.

"There are other models, and they may be correct for the origin of homochirality on earth, if you can prove that those prerequisites existed during the emergence of life," said Jafarpour. "But whether those foundations exist or not, for life that emerged anywhere in the universe, you'd expect that it would have self-replication, and our model says that's enough to get homochirality."

The model relies on mathematical and computational techniques that were not available in Frank's time. It takes into account the chance events involving individual molecules—which chiral self-replicator happens to find its next substrate first. The detailed statistics built into the model reveal that if self-replication is occurring efficiently enough, this incidental advantage can grow into dominance of one chirality over the other. The forerunner of this mathematical mechanism came from Biancalani's previous work on how chance events influence the foraging patterns of ant colonies.

Goldenfeld attributes part of their success to the interdisciplinary environment of the IGB and of the Institute for Universal Biology (IUB), a member of the NASA Astrobiology Institute. "If we hadn't been in this environment, we wouldn't have been so prepared to think about this problem; we might have just stuck with ants, and never made the jump to realizing that we can apply this to this origin of life problem," he said.

The work leads to a key conclusion: since homochirality depends only on the basic principles of life, it is expected to appear wherever life emerges, regardless of the surrounding conditions.

"For me, the most exciting thing is that this mechanism shows that homochirality is really a biosignature of life, a 100% signature, and should be expected anywhere life emerges," said Goldenfeld. "So for example, we just learned that there is a global ocean of liquid water under the ice of Enceladus ... I think that looking for homochirality in the organic molecules that have been detected there would be a fantastic way to look for there."

Explore further: Discovery demystifies origin of life chirality phenomenon

More information: Farshid Jafarpour et al. Noise-Induced Mechanism for Biological Homochirality of Early Life Self-Replicators, Physical Review Letters (2015). DOI: 10.1103/PhysRevLett.115.158101

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2 / 5 (4) Oct 14, 2015
This work removes the constraints of having auto-catalysis mechanisms of chiral amplification (pre-RNA) or chiral inhibition (RNA). But they rely on a model where there is no chiral interaction to show that autocatalysis is enough by itself in some conditions. [Their eqs 2 & 6; ]

The authors seem to have missed that not only is cross-chiral cooperation possible, it is a priori more likely as it produce shorter RNA-polymerase ribozymes. ["A cross-chiral RNA polymerase ribozyme", Jonathan T. Sczepanski & Gerald F. Joyce, Nature, 2014; http://www.nature...900.html ]

2.4 / 5 (5) Oct 14, 2015

Indeed, since the eventually evolved ribosome has a frozen in two-tier chiral selection while modern cell metabolism preferentially is chiral, it seems chirality evolved when it was first needed: to enable the triplet nucleotide code of protein translation elaborating on the older quaternary nucleotide code of ribozyme base pairing. [ http://www.ncbi.n...2926754/ ]
not rated yet Oct 14, 2015
It seems like there are a lot of theories out there about what became homochiral and when. This article suggest that you don't need a lot to get homochirality. Autocatalysis is enough; this is something that has gone unnoticed for however long we've been thinking about homochirality.
1 / 5 (1) Oct 15, 2015
Kudos to them for at least coming up with a theoretical model that hypothetically might work under certain conditions. This of course does not solve the problem. They start with a reproducing chiral molecule which is able to make accurate copies of itself using whatever it has to work with. That is a mighty big jump from ground zero and they show no evidence that such a molecule could emerge on it's own from random chemicals. How does an accurate and efficient copying process arise from blind lifeless purposeless chemicals? If it cannot make accurate copies, it's game over before it even begins and whatever gains might have accumulated prior to the errors would be lost. If it is not accurate from the get go, it seems hopeless to survive the flood of errors that would ensue. You can't just assume miracles and then claim to have solved the problem!
Oct 15, 2015
This comment has been removed by a moderator.
1 / 5 (2) Oct 18, 2015
Re: "...all amino acids are left-handed, and all sugars are right-handed. Goldenfeld highlighted the central mystery of this phenomenon."

Isn't glycine an achiral amino acid?

If so, doesn't that make everyone who has commented so far, except tj10, appear to be a biologically uninformed science idiot?
1 / 5 (2) Oct 18, 2015

Excerpt: "On the basis of mutational studies of active site residues, the authors suggest an RNA-assisted catalytic mechanism in which the RNA 2′-OH activates a water molecule."

But wait, didn't the biologically uninformed science idiot Torbjorn_Larsson_OM tell us that "...chirality evolved when it was first needed..."

Does automagically evolved chirality link the speed of light on contact with water to the energy spectrum of life in the context of Schrodinger's claims about the need for an anti-entropic force?
1 / 5 (2) Oct 19, 2015
No comments from Torbjorn_Larsson_OM on achiral glycine or his ridiculous claim that "...chirality evolved when it was first needed..."

No comments from anyone else. They know nothing about the single amino acid substitution of glycine in the GnRH molecule that links the epigenetic effects of food odors and pheromones to hormones that affect behavior in all vertebrates.

If they wanted to learn, the information is easy to find.
3 / 5 (2) Oct 19, 2015
No comments from Torbjorn_Larsson_OM on achiral glycine or his ridiculous claim that "...chirality evolved when it was first needed..."

No comments from anyone else. They know nothing about the single amino acid substitution of glycine in the GnRH molecule that links the epigenetic effects of food odors and pheromones to hormones that affect behavior in all vertebrates.

If they wanted to learn, the information is easy to find.

1 / 5 (2) Oct 19, 2015
Thanks for reporting the pattern of ignorance displayed here.

See also: http://www.resear...lic_Acid

Torbjorn_Larsson_OM and others aren't likely to comment on that, either. But, if no one calls their bluff, they get away with pretending to be experts, not just antagonists like you, Vietvet.

See also:
1 / 5 (2) Oct 19, 2015
See also: Fingerprint of dissolved glycine in the Terahertz range explained

Excerpt: "THz analysis may be used to represent both the motion inside the glycine molecule and the motion of the glycine molecule together with its bound water molecules. The bands in the Terahertz spectrum, moreover, reflected the glycine's opening and closing motion."

The link from glycine to "Light-Induced Opening and Closing of the Intramolecular Hydrogen Bond in Glyoxylic Acid" is not clear to me because I am not a biophysicist. If it is not clear to people who comment as if they had expertise in biophysics, I wonder why they aren't commenting about the issue of chirality in achiral glycine.

Captain Stumpy
3 / 5 (2) Oct 20, 2015
1 / 5 (2) Oct 20, 2015
The added stability of organized vertebrate genomes attributed to glycine in GnRH is not creationist dogma. It was included in this representation of evolution across hundreds of millions of years.

Evolution of Constrained Gonadotropin-releasing Hormone Ligand Conformation and Receptor Selectivity

See also: http://sandwalk.b...ife.html The simplest amino acid is glycine where the R group is just a hydrogen atom. Thus, glycine is not a chiral compound and there's no such thing as L-glycine or D-glycine. All other natural amino acids are chiral. Glycine might have formed spontaneously from acetate or glycerol. According to this scenario, the exclusive presence of L-amino acids instead of D-amino acids is just an accident.

Of course, all that was claimed before the claim about re-evolution of the bacterial flagellum over-the-weekend.

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