I've been lurking for some time now, and decided to join in with this question.
Life as we know it is dependent on homochirality. I was wondering: Why aren't there life forms based on opposing homochirality (D-AAs)(of course they would have to have their own evolutionary path)? Chance for getting both homochiralities(HC) is not that much higher than getting just one. How would life based on one HC outcompete life based on the other HC? I mean, they aren't even competing for the same nutrients.
P.S. I'm not sure my question is clear, because English is not my native language, so if it turns out it's not clear, I'll try to clarify it.
That's a great question and I'll be interested to see if the majority of the answers come from Biologists, Chemists or those from the Physics arena. There do see to be indications that the weak force is handed and that left handed molecules might have a slight advantage over right handed ones, but I don't know how far we've come in that area of study.
There was an article I came across once from someone at Cambridge or Cornell that discussed some findings in that area. I'll see if I can find a reference to it.
My thoughts on why we're all made of L-amino acids and D-sugars is that it was "luck" - the very early almost-life forms with L-amino acids got a small head start and "ate" the D-forms. Polypeptides (chains of amino acids) with both chiralities would be much more disordered - by which I mean only that they would have a much tougher time forming the neat little coils and pleats we see in homochiral proteins - and would probably lose out in any self-catalytic games that would lead to more of themselves.
If I muster the energy, I'll try to go look up how sugars and amino acids are biosynthesized wrt their handedness. I'll bet a nickel that sugars are D because amino acids are L, or perhaps vice versa.
Added by edit: Hello, New Person! We're glad you delurked!
This message has been edited by Coragyps, 04-16-2005 06:46 PM
quote:My thoughts on why we're all made of L-amino acids and D-sugars is that it was "luck" - the very early almost-life forms with L-amino acids got a small head start and "ate" the D-forms. Polypeptides (chains of amino acids) with both chiralities would be much more disordered - by which I mean only that they would have a much tougher time forming the neat little coils and pleats we see in homochiral proteins - and would probably lose out in any self-catalytic games that would lead to more of themselves
You didn't quite understand my question. I was not asking why isn't our biochemistry based on both L- and D-AAs, but why are there no life forms based on D-AAs. Also, why and how would L-based early life forms "eat" D-forms? Are racemases evolutionary old enzymes?
quote:I'll bet a nickel that sugars are D because amino acids are L, or perhaps vice versa.
I hope they are, because I hate the idea of a frozen accident, but I hate the idea of two frozen accidents twice as much
I'm guessing here, but perhaps early life or proto-life was non-specific enough in its reactions that it was able to assimilate D-amino acids or L-sugars for their carbon and nitrogen - catabolizing them for raw materials instead of racemizing or using them directly. I don't know what modern life forms do with wrong-handed nutrients, asides from an old science fiction story where a guy got "wormholed" into his mirror-image and slowly starved.....
And the more I think on it, the more sure I am that D-sugars are a direct consequence of L-aa's getting an upper hand early on. My biochem was too far in the past and too sketchy to say just how.
quote: Now I would like to say about the model of chirality formation. A model of arising of asymmetry in the bioworld is proposed. It is supposed that chemical evolution on Earth may have started from the uniform distribution of enantomorphous forms of substances. It is postulated and substantiated the role of Earth rotation in the selection of asymmetry signs of living and inorganic macro-structures (higher levels hierarchical structures) could lead to the selection of asymmetric substructures (i.e. microstructures forming macro-structures) including D- and L- forms of molecules.
It was established that the evolution is described both by thermodynamic and kinetic factors and by the trajectory factor (symmetry or chirality) which should be accounted for in rotating coordinate systems. The Coriolis forces can support spiral bio-structures to a different degree depending on the habitation area of organism.
I will explain my own understanding in this if this is not enough. The true challenge is to conceive the error in Nelson and Plantnick's lack of "informative pathway" in the habitation areas by perhaps a spatial synthesis of Croizat and Gladyshev via Wright equilibria.
"Modern workers have sought in spatial representation a means for resolving natural groups, rather than merely portraying the affinites of groups already recognized by other means. Spatial analysis has been extended to many dimensions (Sneath and Sokal 1973), and various graphic devices have been developed in order to portray spatial affinites (figure 2.46) Yet the overall goal - the resolution of natural groups - has proved elusive through spatial analysis and representation, inasmuch as an informative pathway seems unspecifiable by these considerations alone."
SYSTEMATICS AND BIOGEOGRAPHY by Gareth Nelson and Norman Platnick Columbia Press 1981 page 105
This message has been edited by Brad McFall, 04-17-2005 06:14 PM