Monday, March 16, 2009

Why are all earthly lifeforms lefties?

ResearchBlogging.orgWhat, you think you are not left-handed? Just because you favor your right hand to write/eat/pitch that baseball, etc.? Actually, in case you didn't already know this: deep down, at the amino acid level, we are all lefties! Southpaws, each and every one of us! That's just another one of those wonderfully weird arbitrary fact about life on earth! Amino acids, the building blocks of proteins, are fundamental to the structure and function of life as we know it on our planet (which is pretty much how we know life at all, because we haven't found any other kind - yet!) - and the atoms making up each molecule can be put together in different ways to produce configurations which are mirror-images of each other (chirality, as the chemists call it); we know this because we can make amino acids in the lab. Yet, all life-forms we know of only contain one version of the amino acids - the left-handed one from the mirror-image pair! So this is another deep molecular homology involving amino acids (like the language of the genetic code we discuss in Evolution class), which unites all life-forms on this planet with a shared common ancestor. But there is no inherent reason why that ancestor had to be a southpaw in its use of amino acids. Was it just a random accident that that is how we turned out? A new study of the famous Murchison meteorite takes on the question...



Did lefty molecules seed life? (as reported in The Scientist today): "Amino acids come in left-handed and right-handed forms, which, like a pair of human hands, are mirror images that cannot be superimposed onto each other. Yet living organisms use only the left-handed version, which presents a conundrum: There's no biochemical reason why one mirror image should be better than the other, so scientists have long debated whether life's left-handed leaning arose because of random processes or whether rocks from outer space seeded a southpaw solar system."


...and suggests an explanation: that there was a left-bias in the amino acids that rained onto early earth from our solar system!


The current study argues for the latter possibility by showing that some extraterrestrial meteorites contain an abundance of left-handed molecules. "The implications are that all life in our solar system could be the same handedness as life on Earth," Jeffrey Bada, a geochemist at the Scripps Institution of Oceanography in La Jolla, Calif., who was not involved in the research, told The Scientist.


Daniel Glavin and Jason Dworkin, astrobiologists at NASA's Goddard Space Flight Center in Greenbelt, MD, compared the ratio of left- and right-handed 5-carbon amino acids found in six primitive, carbon-rich meteorites that have an elemental composition similar to that presumably found in the early solar system. Three of these rocks were heavily left-skewed, while the remaining three showed equal handedness, or chirality, the researchers found. Of the lefty rocks, the meteorite that fell on Murchison, Australia, in 1969 -- arguably the most widely studied carbonaceous meteorite in the world -- contained the largest imbalance ever observed: a 18.5% excess of the left-handed form of the amino acid isovaline.



Or, in their own words from the abstract of the PNAS paper:


The large asymmetry in isovaline and other α-dialkyl amino acids found in altered CI and CM meteorites suggests that amino acids delivered by asteroids, comets, and their fragments would have biased the Earth's prebiotic organic inventory with left-handed molecules before the origin of life.


Amino acids found in these and other meteorites landing on earth tend to exhibit a bias favoring left chirality, suggesting our entire solar system has the same left-bias, setting life as we know it on its lefty path. And Glavin & Dworkin think they know what favored left chirality in the meteorites:


The mechanism that Glavin and Dworkin propose to explain the observed left-handed excess is that polarized light -- which is twisted and can rotate molecules -- probably set the imbalance in motion. Then, once the balance was slightly askew, water within the meteorites further drove an enrichment of left-handed amino acids in the liquid phase and relegated right-handed molecules to the solid phase. "The whole amplification is due to this process of aqueous alteration," said Dworkin.


Not everyone is convinced, yet, but the search for answers to these puzzles about life's origin continues. (Or we could simply say goddidit, and stop asking any further questions, couldn't we? but where's the fun in that?)


Reference:


Glavin, D. P., & Dworkin, J. P. (2009). Enrichment of the amino acid l-isovaline by aqueous alteration on CI and CM meteorite parent bodies Proceedings of the National Academy of Sciences DOI: 10.1073/pnas.0811618106

4 comments:

"the Dude" March 16, 2009 at 7:29 PM  

Just something from left field:

med ~ madhu ~ hunnig ~ honey
Russian Malay German English

Anonymous August 27, 2009 at 12:36 PM  

(Old posting, slightly modified)

Teleology Is Universal
And Explains Life's Amino Acids Chirality


A. Amino Acids Chirality In Life
http://scienceandreason.blogspot.com/2009/04/amino-acid-chirality.html


B. Again, the probable reason for life's amino acids chirality:

Darwinian evolution started at life's day one, with the genesis of Earth's primal organisms, the replicating oligomers, the pre-archaea genes. It started under yet-unknown energetic conditions, by a serendipitous occurrence, most probably with oligomeric RNA conformations, in sunlight-bathed soup containing all their essential molecular progenitors. These conformations happened to maintain a balance of energy in the direction of their polymerization to lengths precipitated out of their soup as determined by the nature and conditions of the soup.

The sugars and their appended nucleotide bases, that together with the phosphates are the structural components of genes, of life's primal organisms, are chiral. There probably is an energetic advantage in homochirality and chiral homogeneity for the self-replication of these biopolymers.

This serendipitous occurrence set up a matrix-field of energy with a potential extended between its source, on-Earth incoming energy, and the precipitating organisms, the genes. This was the genesis of the ongoing formation and maintenance of Earth's biosphere.

And since the biosphere had thus started it could only evolve in the directions of more favorable energy balances-packages and towards stabler energy-packages conformations, in the direction of survival. Earth's biosphere embarked on the course of its evolution.


C. Some definitions, with "end" replacing dictionary's 'cause' or 'design' or 'purpose'

- tele-, telos end

- teleology = a doctrine explaining phenomena by final end; the fact or character attributed to nature or natural processes of being directed toward a specific end; the referral to the end of universe as an explanation of natural phenomena.

teleonomy = the quality of survival of structure or function in living organisms due to evolutionary adaptation.


D. Teleology exist in all nature's systems, both living and nonliving. It is universal.

The universe, with all its sub-systems, including life, is a continuously evolving fractal system, with energy as the base element of everything. Cosmic evolution is evolution of energy. At the beginning of the present cosmic cycle was the energy singularity. At its end there will be a small amount of mass and an infinite dispersion of the beginning energy. In-between, the universe undergoes continuous evolution, consisting of myriad energy-to-energy and energy-to-mass-to-energy transformations.

Evolution ensues from and consists of systems modifications, "mutations", inherently ever more of them as more new options arise for the systems, be they non-living or living.

Life systems, like other mass systems, are temporary packages of constrained energy. On Earth they are components of the grand, temporarily constrained, biosphere energy store. Their temporary survival 'end' is to enhence and maintain their bio matrix, the biosphere.

Modifications of genomes functional capabilities, of genes expressions, can be explained by the
feedbacks they receive from their (3rd stratum) cellular organisms, feedbacks from their culture-life-experiences. The route-modification selection of a replicating gene, when it is at its alternative-splicing-steps junctions, is biased by the feedback received by the genome. THIS IS HOW LIFE EVOLUTION COMES ABOUT.

For non-life systems there is no mediating culture. The interaction between environments and systems are direct, in the direction towards the 'end' of the present phase of the universe.


Suggesting,

Dov Henis
(Comments From The 22nd Century)
Updated Life's Manifest May 2009
http://www.physforum.com/index.php?showtopic=14988&st=480&#entry412704
http://www.the-scientist.com/community/posts/list/140/122.page#2321

ABK January 19, 2011 at 7:22 PM  

Beautifully written. Thank you.

Raphite1 August 1, 2011 at 2:07 PM  

I'm not a scientist, so please forgive a probably-stupid question:

How much of the protein that composes our bodies is exclusively left-handed? Just the genetic material, or also the tissues that get built based on instructions from left-handed genetic material?

I'm assuming that it's just the former, and not the latter. If it were the latter, then I assume Earth organisms would only be able to productively digest left-handed protein, and that consuming the bodies of right-handed alien organisms would provide no usable proteins/amino acids.

So, safe to say that only our genetic material is exclusively left-handed, and not tissues as well?

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