Twenty different amino acids go into making up the vast variety of proteins so essential to life. But why does life on Earth use only left-handed versions of amino acids to build them? After all, amino acids can be made in mirror images of each other. Jason Dworkin (NASA GSFC) notes the key issue. Mix left- and right-handed amino acids and “…life turns to something resembling scrambled eggs — it’s a mess. Since life doesn’t work with a mixture of left-handed and right-handed amino acids, the mystery is: how did life decide — what made life choose left-handed amino acids over right-handed ones?”
Image: This artist’s concept uses hands to illustrate the left and right-handed versions of the amino acid isovaline. Credit: NASA/Mary Pat Hrybyk-Keith.
It’s a question with ramifications for life elsewhere in the universe. Suppose the day comes when we finally get a robotic lander to Enceladus. The news flashes around the world: Life discovered on one of Saturn’s moons! But is it truly an extraterrestrial microbe, or the result of contamination from our own lander? Assuming the microbe is based on right-handed amino acids, its extraterrestrial credentials seem assured. But what if it’s left-handed? For that matter, what if left-handedness is fairly common?
We’re beginning to gain evidence for the possibility. In a study of meteorites with an abundance of carbon — carbonaceous chondrites — Dworkin and Daniel Glavin examined their samples for the amino acid isovaline. Chemical reactions in the presence of radiation or temperature can cause amino acids to switch from being left-handed to right and vice versa, but the beauty of using isovaline in these studies is that it preserves its handedness for billions of years. Moreover, there is no question of contamination, since isovaline is rarely used by life.
The result: Three types of carbonaceous meteorites showed more of the left-handed verson of isovaline than the right-handed kind. Is there a bias toward left-handedness in space? If so, that could make the search for extraterrestrial life a bit more tricky. Dworkin amplifies:
“If we find life anywhere else in our solar system, it will probably be microscopic, since microbes can survive in extreme environments. One of the biggest problems in determining if microscopic life is truly extra-terrestrial is making sure the sample wasn’t contaminated by microbes brought from Earth. If we find the life is based on right-handed amino acids, then we know for sure it isn’t from Earth. However, if the bias toward left-handed amino acids began in space, it likely extends across the solar system, so any life we may find on Mars, for example, will also be left-handed.”
So we may find that life based on left-handedness is the norm, a fact that could imply that the crucial choice of ‘handedness’ is readily solved. Dworkin goes on:
“On the other hand, if there is a mechanism to choose handedness before life emerges, it is one less problem prebiotic chemistry has to solve before making life. If it was solved for Earth, it probably has been solved for the other places in our solar system where the recipe for life might exist, such as beneath the surface of Mars, or in potential oceans under the icy crust of Europa and Enceladus, or on Titan.”
The paper is Glavin and Dworkin, “Enrichment of the amino acid l-isovaline by aqueous alteration on CI and CM meteorite parent bodies,” Proceedings of the National Academy of Sciences March 16, 2009 (early edition). Abstract online.
Apparently, prebiotic process induces a large enrichment of at least some L-aminoacids. This enrichment constraints the possibility of primeval proteomes consisting of mixtures of L- and D-aminoacids. So to speak, life forms are forced to be L-aminoacid-oriented. So, the idea of L-aminoacid enrichment as a biomarker seems risky. However, a lot of research is needed in order to understand the abiotic forces in enrichment. In principle, L-aminoacids and D-aminoacids are chemically undistinguishable, or so we were taught in ancient times.
Indeed, it’s curious that abiotic processes would bias amino acid handedness, more study is definitely warranted.
Personally I don’t think this is a serious difficulty in identifying extra-terrestrial life. It certainly would make it far, far easier to identify non-Earth life to be able to recognize dissimilar chirality in AAs. However, even then there’s a 50/50 chance the chirality would be the same regardless, and a 1/4 chance that the chirality of exobiotic sugars would be the same as Earth based life as well.
Realistically, the particular choices of AA bases, nucleotide bases, and sugars will be substantially different for extra-terrestrial life. Indeed, it’s quite likely that the very chemical basis of extra-terrestrial life will be substantially different than that of Earth life. There are a lot of possibilities in the solution space of “copy capable, error correcting genetic media encoding data for the production of bio-functional molecular structures” that we have little knowledge of and may be perfectly suitable as a basis for life. Indeed, finding extra-terrestrial life will likely tell us more about the nature of these processes than studying Earth-life exclusively ever will.
Also, even if extra-terrestrial life uses exactly the same sugars (perhaps likely), nucleotides (perhaps likely), and amino acids (rather unlikely, given the diversity of options on Earth), there is a certainty that the particular encoding of nucleotide sequences to amino acid basis will be different for exobiotic organisms.
Is it just me or is that artwork really bad? I think it’s the evil yellow haze around the rock thingy that gets me.
It’s ugly, sure enough, and the yellow doesn’t help.
I’ll preface my post by saying: alien life hunters, don’t worry! There are a huge number of possible forks in this decision tree.
This property is the biological equivalent of the matter/antimatter asymmetry in cosmology. All biomolecules are chiral, but their handedness is arbitrary. For example, the amino acids are left-handed whereas the sugars that form the nucleic acid backbone are right-handed.
In The Biology of Star Trek I mention one possible explanation for the asymmetry: John Cairns suggested that the complex prebiotic carbon molecules coalesced on silicon templates, which are themselves chiral. Once handedness got established, it propagated by feedback loops.
If alien life has a handedness like ours in one set of biomolecules (say, amino acids) it may still have a different handedness in another. And even if its handedness and building blocks are identical, it may use a different subset of biomolecules: there are more amino acids than the 20 used by terrestrial bioforms.
While the search for alien life is fascinating and exciting, I find a little bit of comfort in knowing that there’s more than a possibility that our biology will be incompatible with biology from other worlds. Alien life that is, say, right-handed or based on different elements will likely be uninterested in, dare I say, eating us. Maybe it’s my background in biology, but being incompatible with an extraterrestrial’s diet is certainly appealing when considering future interactions.
A thermodynamic basis for prebiotic amino acid synthesis and the nature of the first genetic code
Authors: Paul G. Higgs, Ralph E. Pudritz
(Submitted on 2 Apr 2009)
Abstract: Of the twenty amino acids used in proteins, ten were formed in Miller’s atmospheric discharge experiments. The two other major proposed sources of prebiotic amino acid synthesis include formation in hydrothermal vents and delivery to Earth via meteorites.
We combine observational and experimental data of amino acid frequencies formed by these diverse mechanisms and show that, regardless of the source, these ten early amino acids can be ranked in order of decreasing abundance in prebiotic contexts. This order can be predicted by thermodynamics.
The relative abundances of the early amino acids were most likely reflected in the composition of the first proteins at the time the genetic code originated. The remaining amino acids were incorporated into proteins after pathways for their biochemical synthesis evolved.
This is consistent with theories of the evolution of the genetic code by stepwise addition of new amino acids. These are hints that key aspects of early biochemistry may be universal.
Comments: 16 pages, 2 tables, 4 figures. Accepted for publication in Astrobiology
Subjects: Earth and Planetary Astrophysics (astro-ph.EP); Solar and Stellar Astrophysics (astro-ph.SR); Biomolecules (q-bio.BM)
Cite as: arXiv:0904.0402v1 [astro-ph.EP]
Submission history
From: Ralph Pudritz [view email]
[v1] Thu, 2 Apr 2009 15:00:17 GMT (213kb)
http://arxiv.org/abs/0904.0402