I think the prevalence of L-amino acids in biological systems is a quirk of our evolutionary history; there's no reason D-amino acids wouldn't work chemically, but once early life forms started building proteins out of L-amino acids, L-amino acids became abundant an it became selectively advantageous to use L-amino acids because then you could use the amino acids you obtained by eating someone else. It's a matter of compatibility; if you use D-amino acids, you have to build them all from scratch, because you're not going to find them in your environment. As long as you use L-amino acids, you don't even need to maintain the de novo synthetic pathways; you can just eat something that has those pathways (hence the 'essential' dietary amino acids).

As for carbohydrates, there are so many more stereoisomers, and organisms generally don't use the monomers they obtain in their diets as building blocks (but rather oxidize them for energy, and build the monomers they need using their own metabolic pathways), that there hasn't been the same selective pressure for standardization.

Cheers

I think the prevalence of L-amino acids in biological systems is a quirk of our evolutionary history; there's no reason D-amino acids wouldn't work chemically, but once early life forms started building proteins out of L-amino acids, L-amino acids became abundant an it became selectively advantageous to use L-amino acids because then you could use the amino acids you obtained by eating someone else. It's a matter of compatibility; if you use D-amino acids, you have to build them all from scratch, because you're not going to find them in your environment. As long as you use L-amino acids, you don't even need to maintain the de novo synthetic pathways; you can just eat something that has those pathways (hence the 'essential' dietary amino acids).

As for carbohydrates, there are so many more stereoisomers, and organisms generally don't use the monomers they obtain in their diets as building blocks (but rather oxidize them for energy, and build the monomers they need using their own metabolic pathways), that there hasn't been the same selective pressure for standardization.

Cheers

I'd like to see that link; I've also heard that the poly-D-alanyl in gram positive bacteria is something they evolved as a defence mechanism, and that part of the reason it works is that most proteases are flummoxed by D-amino acids.

This question raises the more general issue of handedness. There are numerous examples of handedness in nature in spite of the obvious chemical equivalence of mirror image counterparts. It goes to the question: How does such asymmetry in preference develop, and why is it maintained?

L and d are notations used by chemists based on their use by E. Fischer. They have little to do with why it should be L (in case of amino acids ) and D in case of sugars per se. But the living system prefers chiral blocks. Look up research by Stephen F mason for more answers.

L amino acids are found because we(eukaryotes) have enzymes which can only recognize L confomation and that is also true for D carbohydrates which can be recognized by specific enzyme during metabolism..

Here is a reference to a nice article that gives several hypotheses for how asymmetrical biological molecules could have evolved. For example, if biological molecules arose from synthesis on asymmetrical inorganic catalysts, then there may have been a preponderance of one enantiomer over the other in the primoridal ooze. Alternatively, the L form of amino acids may have been slightly more stable when irradiated with polarized light during the prebiotic period on earth. These ideas and more are discussed in the article referenced.

Morozov, L. 1979. Mirror symmetry breaking in biochemical evolution. Origins Life Evol Biosphere 9: 187-217.

David Oppenheimer, thank you very much. Pls if you have the article I will appreciate if you can send it to me at [email protected]

Brijesh Dabhi, if you read my question very well you will observe that I have already made a comment on the issue of stereospecificity and selectivity of receptors. The same applies to enzymes. That cannot be the reason.

Michael Fenn, I will be highly obliged if you can provide me with the link. Thank you

Actually all organisms are having selective stereo types of amino acids and sugar even different selective enzymes.that is based on nature selectivity.

I guess it has to do with enzymatic specificity

This should give some insight on how it all works:

Science 5 June 1992:

Vol. 256 no. 5062 pp. 1445-1448

DOI: 10.1126/science.1604320

Abstract: http://www.sciencemag.org/content/256/5062/1445.short

Having L instead of D probably happened by chance as did many things in evolution.

There had to be a stereospecific natural selection process that facilitated the use of L-amino acids and D-sugars.

To expand on David Oppenheimer's reference on asymmetric inorganic catalysts:

Bondy SC, Harrington ME (1979) L-Amino acids and D-glucose bind stereospecifically to a colloidal clay. Science 203:1243–1244.

Here is also a good review: Darwin’s warm little pond revisited: from molecules

to the origin of life

http://schools.bvsd.org/p12/FairviewHS/fhsstaff/faculty/strode/IB%20Biology%20Course%20Documents/Scientific%20Papers/Darwin's%20Warm%20Little%20Pond%20Origin%20of%20Life%20Hartmut%20and%20Brownson%202009.pdf

David G. Oppenheimer, thank you for the journal article you sent to me. I will study it very well and give a feedback

Skomantas Serapinas, could you send me a copy of the article by email to: [email protected] Thanks

Dear Steingrimur Stefansson, Thank you for your useful contribuiton. Kindly send a copy of the article to me at [email protected]

Hi I think because active site in enzyme is special for isomer

and for amino acid L isomer give a left handed helix

Here are a couple of papers that may be of interest:

1990.01.01.sci.am.262(1).108

2004.chem.record.4(5).267

The gross solution to the problem is rooted on two facts, or rather principles.

ONE : The living process and metabolism always follow or is based on the economic principle of minimum resource and maximum utilization for optimum outcome or performance.

TWO : Evolution is the ultimate and final selector and evaluator for selecting and retaining the perfect set of molecules necessary or rather capable of following the principle of economy in living systems whenever a set of molecule is presented to a living system.

There was equal chances for formation of D and L forms of most of the biomolecules associated with living systems during the period of genesis of primordial living systems in the universe, and the evolution has retained only the best matched pairs of substrates and enzymes as components of the metabolic machinery, rejecting the mismatched sets just for the interest of maintaining economy.

It all happened by chance, but accepted, retained and continued by the supreme quality control system of evolution.

But the great problem is that, there is no such a thing as "chance" in physics and without physics biology is incomplete.

I do not know much physic and although Einstein explained that, I do not have the required capacity to understand that.

Muhammad Ramzan Manwar, thank you for the very useful article.

Danilo Boskovic, kindly send those papers to my email address: [email protected] Thank you.

This is very interesting question. Could normal cells consume the D aa and then covert it to L aa for building proteins and cell use. On the other hand, could cancer cells consume D aa and not be able to convert it to L aa.?

If normal cell consume D aa our body recognise them as foreign and are processed. 

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There are a couple of reasons for biological homochirality.

One, our galaxy has a chiral spin and a magnetic orientation, which causes cosmic dust particles to polarize starlight as circularly polarized in one direction only. This circularly polarized light degrades D enantiomers of amino acids more than L enantiomers, and this effect is clear when analyzing the amino acids found on comets and meteors. This explains why, at least in the milky way, L enantiomers are preferred.

Two, although gravity, electromagnetism, and the strong nuclear force are achiral, the weak nuclear force (radioactive decay) is chiral. During beta decay, the emitted electrons preferentially favor one kind of spin. That's right, the parity of the universe is not conserved in nuclear decay. These chiral electrons once again preferrentially degrade D amino acids vs. L amino acids.

Thus due to the chirality of sunlight and the chirality of nuclear radiation, L amino acids are the more stable enantiomers and therefore are favored for abiogenesis.

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I found this explanation elsewhere.  Original thread is here:

http://forums.studentdoctor.net/threads/why-are-only-l-amino-acids-found-in-nature.854644/

So, Dear Mira Kaspar what;s your opinion about D hydrocarbons?

If all of the bio-molecules selected the same enantiomers, this reason seems to be wonderful. but we know that is not true. May be if we had D-aa instead of L form, the question changed to " why D form instead of L aa?" therefore I think that  it can be attributed to natural selection and there is no a clear explanation for it.

The lock and key enzymes are themselves chiral. Consequently, there is a large driving force for them to recognize a specific enantiomer of sugar. This derives from a classic feedback loop: even if at some time in the ancestral history of life on earth, if at some time D hydrocarbons became slightly more prevalent or the enzymes that operated on them became slightly more preferential towards D hydrocarbons, this would result in a runaway transition to all-D glucose preference.

In actuality, for small molecules, many enzymes are not 100% specific, but the lack of available L-glucose makes this a relatively rare event.

Of course, this leads to the thought experiment of "what happens if you make an enantiomeric organism?". This would be tremendously expensive, though, and will probably never be undertaken.

Our biological system is based on the L chital in amino acids and D chital in Sugar. Which is possibly developed due to the available environment when a first biological activity started to develop and growth to final form of biological system. Otherwise there are no other specific reasons known at present. It is the same type answer as for why all biological system are fully based carbon atoms not on the base of silicon etc. There is possibility that life in planet of some other solar system may be based on Si element or D amino acid & D glucose.

https://forums.studentdoctor.net/threads/why-are-only-l-amino-acids-found-in-nature.854644/