This is actually a chemistry question, but as all chirality arises from natural sources, organisms, I am putting it under biology.

I have a hypothesis about early cells and have posted it to Cali to see if it is new. Anyone else a biochemistry expert?

Wiki has articles:
http://www.dailymail.co.uk/news/article ... tates.html

and RNA world
http://en.wikipedia.org/wiki/RNA_world_hypothesis

The moleculres in question are amino acids and sugars. The heterocycle in RNA or DNA has no chirality, only the ribose or deoxyribose.


The issue is really ribose, as DNA is made by ribocucleotide reductase taking off the ribose OH.

More theories? Post links.

I'm not sure what the question is that you are asking. More theories for what? The origin of homochirality?

Tero wrote:This is actually a chemistry question, but as all chirality arises from natural sources, organisms, I am putting it under biology.

I have a hypothesis about early cells and have posted it to Cali to see if it is new. Anyone else a biochemistry expert?

Wiki has articles:
http://www.dailymail.co.uk/news/article ... tates.html

and RNA world
http://en.wikipedia.org/wiki/RNA_world_hypothesis

The moleculres in question are amino acids and sugars. The heterocycle in RNA or DNA has no chirality, only the ribose or deoxyribose.


The issue is really ribose, as DNA is made by ribocucleotide reductase taking off the ribose OH.

More theories? Post links.

they have, but not there. you have to go back a bit in the sugars structure

so heres a diagram of the sugars in their open chain form, and the isomers that result and thats just the variation in the D- oses.
and there are equivalent L- oses as well.



expanding on ribose for example.
L-ribose
D-ribose

on top of that theres a further level of complexity in the cyclic forms of the sugar. 6 carbon sugars can form 4 or 5 carbon rings furanose and pyranose respectively. and on top of this you have different orientations (epimers) of the hydrogen/hydroxides on the chain.


for making DNA/RNA the enzyme responsible for attaching the nucleobase will select the correct isomer/epimer of ribose,

like Rumraket, I'm not 100% what your asking, but hopefully something in there has covered it. also one of your links is for the Daily Heil, on a completely unrelated subject

Yes, the second post, theories of the origin of homochirality.

Need not be biochemical.

Tero wrote:Yes, the second post, theories of the origin of homochirality.

Need not be biochemical.

theres a quick and easy answer to that, energy.

take amino acids as an example. all amino acids used in eukaryotes are in the L-form (there are a few exceptions, but I'll come back to it), ribosomes have a cleft that fits these L-form amino acids, but an D-form wont. now if you were to use both you'd either need two sets of ribosomes, or some major structural changes to the ribosome (which potentially would allow water into the active site, which would make them rather useless (peptide bonds are metastable, ie in the presence of water they spontaneously break, but at a very slow rate (guess whos been revising....))). on top of that you'd need to specify which form of the amino acid is required for the protein - mixing and matching D and L forms would have interesting steric clashes and the protein wouldn't fold properly and therefore protein is useless.
there are a few exceptions, glycine is achiral, so therefore not an L-form by default. then you have proline, which is isomerised post translationally - that is its incorporated as L-proline, but is transformed by PPI into cis/trans isomers (it does it randomly until it reaches lowest free energy state. this also applies to things like the D-amino acids found in platypus venom - its modified post-translation. some bacteria also use D-forms in their peptidoglycan wall though AFAIK this is synthesized from L-form and assembled by enzymes (therefore designed to fit the specific substrates, opposed to more generalized protein assembly in the ribosome), and high levels of D-form can inhibit cell wall growth.

on the same basis, if every organism is using the same set of amino acids/sugars etc. then these can be scavenged instead of synthesized de-novo. remembering that all life has a common ancestor, we've therefore inherited these chiral preferences

That much is obvious or perhaps just assumed.

But if the amino acids and sugars were ALL the enantiomers of the current ones, we would still have a fully functional set.

Tero wrote:That much is obvious or perhaps just assumed.

But if the amino acids and sugars were ALL the enantiomers of the current ones, we would still have a fully functional set.

yes, but as it happened one set was chosen, for whatever reason - energetically wise theres no difference. however, obviously the most successful organism happened to utilize L-amino and D-sugars.

AFAIK theres nothing pushing for selection either way, just once that selection is made then it sort of sticks.

note the major use of D-amino acids in proteins is to restrict protease activity, mainly because the standard proteases are designed to fit around normal L-amino acids, and being in the alternate isomer, it stops them fitting into the active site correctly.

OK, now that we got this far, I will post what I had written before opening the topic.

(runs downstairs...)


Homochirality

Biological molecules that have chirality have evolved through the system that makes them, cells. There is a set of sugars and amino acids that are now in use. There is a relationship of one to the other, as they are all used in peptides, DNA and RNA.

According to Wikipedia:

In 2011, Albert Erives of Dartmouth College showed how proto-anti-codon RNAs or pacRNAs would require complementary homochiralities ofamino acids (L-amino acids) and nucleotides (D-ribose sugar-based) in the ancestor of living organisms.[17] The pacRNA model posits that homochirality of amino acids and homochirality of nucleotides were required in order for pacRNAs to auto-aminoacylate themselves. The pacRNA model explains almost all of the features of the genetic code, including the origin for the 20 proteinogenic L-amino acids. This marked the first time that the genetic code was linked to the homochirality of life's molecules, and possibly also the first time that the homochirality of amino acids was linked to homochirality of nucleotides.

However, this does not explain why we have the set that we have. The mirror image set of ”our” set would equally wll function. It would make cells, photosynthesize etc.

My proposal is that evolution itself acted very early. There was perhaps a replicating cell-like unit. I do not know what the cell walls would have been made of, but certainly there had to be something. Then you have these cells of molecules of both chirality evolving. ”Our” chirality (L-amino acids, D sugars) and the other (D amino acids, L sugars). At some point our chirality invented something. I do not know what this was, but probably much before photosynthesis. Perhaps some other energy source it was able ro efficiently use.

The second step is that ”our” life started cranking out amino acids and sugars out of the starting materials available. The ”other” cells missed a major step in development and we deleted all the carbon sources to our pipe line, or at least a major part. The ”wrong chiral” life then went extinct as there were no amino acids floating around it could use or simple precursors. ”Our” life could recycle much of the building blocks to our uses.

You might find this an interesting read Tero.

J E Hein, E Tse and D Blackmond, Nat. Chem., 2011, DOI: 10.1038/nchem.1108

From the Chemistry World summary;

Given a tiny push one way or the other, simple racemic precursors can lead to the chiral building blocks of RNA using a combination of chemical and physical factors. The work brings together a possible explanation of how life began on Earth with its preference for L-amino acids and D-sugars, say US researchers.

It's a new example of the original case in the Wiki entry I gave.

"...Starting with a slight excess of L-proline..." You had to get that small excess somewhere in nature.

circular polarized light is another claim that was made:
http://microbewiki.kenyon.edu/index.php ... bial_World

pdf of a paper
http://www.acadeuro.org/fileadmin/user_ ... /Cline.pdf

Differential chirality of amino acids seems common in abiotic systems, too. Early life on Earth may have become homochiral simply because L-amino acids were over-represented when life arose.

ScientistLive wrote:Over the last four years, a team of researchers carefully analysed samples of meteorites with an abundance of carbon, called carbonaceous chondrites. They looked for the amino acid isovaline and discovered that three types of carbonaceous meteorites had more of the left-handed version than the right-handed variety - as much as a record 18 percent more in the often-studied Murchison meteorite. "Finding more left-handed isovaline in a variety of meteorites supports the theory that amino acids brought to the early Earth by asteroids and comets contributed to the origin of only left-handed based protein life on Earth," said Dr. Daniel Glavin of NASA's Goddard Space Flight Center in Greenbelt, Md. Their findings validate and extend the research first reported a decade ago by Drs. John Cronin and Sandra Pizzarello of Arizona State University, who were first to discover excess isovaline in the Murchison meteorite, believed to be a piece of an asteroid.

http://www.scientistlive.com/European-Science-News/Opinion/Clues_for_space-based_chirality/22115/

Astrobiology Magazine wrote:One possibility is radiation. Space is filled with objects like massive stars, neutron stars, and black holes, just to name a few, that produce many kinds of radiation. It’s possible that the radiation encountered by our solar system in its youth made left-handed amino acids slightly more likely to be created, or right-handed amino acids a bit more likely to be destroyed, according to Glavin.

http://www.astrobio.net/pressrelease/3748/meteorites-may-answer-lifes-chirality-%20question

physicsworld.com wrote:The latest work, published in Astrophysical Journal Letters, provides further backing for the alternative view, that the asymmetry existed before life got going. A group of astrophysicists, physicists and chemists in France, led by Louis le Sergeant d'Hendecourt of the University of Paris South, irradiated molecules of water, ammonia and methanol at low temperatures using circularly polarized ultraviolet light at SOLEIL. The idea was to recreate the conditions found in star-forming regions, where partially circularly polarized light has been observed, and to test the hypothesis that this polarization could induce an imbalance in the creation of left- and right-handed versions of certain amino acids. Other researchers have previously shown experimentally that chiral organic molecules can be created in space-like conditions, and that organic matter might therefore have its origins in space, but could not induce any asymmetry because they lacked a suitable source of radiation.

http://physicsworld.com/cws/article/news/44795

Edited to add:
I see much of this has already been gone over in the thread.

Homochirality is necessary for the replication of nucleic acid sequences.