Specialty Chemistry Forums > Biochemistry and Chemical Biology Forum
presence of only L-amino acids in natural proteins
2810713:
Why r only L-AA present in the natural proteins?
Why not a mixed composition i.e. both Ls and Ds.
or just Ds???
I have tried making something with the molecular mekano, i feel that mixed one is quite unstable or envolving non-uniform stability... But why not just D???
or its just by chance that only Ls are selected and not only Ds???
help me out, please.
hrushikesh
Albert:
Maybe it's because of a supreme design :angel1:
There isn't any known reason for it...for the moment.
Yggdrasil:
Basically, the reason is that the biosynthetic enzymes which produce amino acids evolved to produce only the L form. It is an advantage to organisms to produce only one stereoisomer since utilizing both forms of the amino acid would require two enzymes, one recognizing the L-form and one recognizing the D-form, for each biochemical process involving the amino acid. Plus, by producing enzymes which are composed of only L-amino acids, these enzymes are able to catalyze reactions with very high stereoselectivity and stereospecificity, which is an essential feature of many biological reactions.
However, as to why all L as opposed to all D, biologists are still debating. Some claim that asymmetric crystals in clay may have lead to an overabundace of L-amino acids in the "primordial soup". Others believe that polarized light may have had an effect on the relative ratios of D- and L- amino acids. There is also the possibility life forms based on D- and L- amino acids evolving separately, then the L-amino acid-based life forms outcompeting the D-amino acid-based lifeforms (by chance or by genetic drift). Still, there is only the possibility of random chance. Maybe the first lifeform arbitrarily incorporated L-amino acids only, so life evolved using only L-amino acids. So the ultimate origin of chirality in biological systems still remains unknown.
2810713:
I think followed much of what u said Yggdrasil, and am really thankful for that.
I couldn't understand the mechanisms behind these 2 possibilities -
Some claim that asymmetric crystals in clay may have lead to an overabundace of L-amino acids in the "primordial soup". Others believe that polarized light may have had an effect on the relative ratios of D- and L- amino acids.
By what mechanism did the assymetric crystals would lead to over abundance of Ls and not Ds, same thing. For that polarized light possibility, it says that light polarity will affect molecule stability , am i right???
Please explain a bit.
Albert, good way of telling that no scientific explanation exsists today.
hrushikesh
Borek:
It will be enough for the assymetric crystals to catalyze some organic reaction faster for L-particles than for D-particles, to create initial abundance of some L-particles. Once there is some inequity in amounts it can easily propagate in a self-amplifying process. This explanation assumes assymetry was external so one may ask why the crystals were assymetric in the first place and we are back in the same place.
However, there are many mechanisms that - while not necesarilly have any chemical meaning - can be used to explain how it may happened without any initial assymetry.
Assume racemic 50/50 mixture of D and L. Imagine some process requiring three particles. If they are randomly selected they can be all the same (DDD or LLL) or one can be different (DDL and DLL). Now imagine the process is such that if initial particles are DDD or LLL nothing happens, but DDL gets converted to DDD and DLL gets converted to LLL. Let it go for a long time. At first nothing happens, but the equilibrium is unstable, so sooner or later all particles get converted to D (or L) - and it is not possible to predict which way the system will move.
It is pretty easy to program such simulation and play with it. While it not necesarilly gives an answer, it shows that the natural process of conversion is possible.
Navigation
[0] Message Index
[#] Next page
Go to full version