mackdaddy

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I've read plenty of textbooks which state that biological systems in nature only use L amino acids over D.

But I can't find a reason as to why. Why are only L amino acids found in proteins in nature?
 

muhali3

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I think this is a mystery that has not been answered. But D-amino acids are sometimes found in nature, like in bacterial cell walls.
 

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I think this is a mystery that has not been answered. But D-amino acids are sometimes found in nature, like in bacterial cell walls.

its to do with evolution, I read it somewhere before. Perhaps its benificial to have them all the same becuase less enzymes are needed. The same enzyme can break down many bonds. The reason why they are L? Who knows maybe they are easier to break..
 

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its to do with evolution, I read it somewhere before. Perhaps its benificial to have them all the same becuase less enzymes are needed. The same enzyme can break down many bonds. The reason why they are L? Who knows maybe they are easier to break..

Yeah, I would agree with that.

But L and D amino acids are enantiomers of each other, so there is really no functional difference of one over the other.

I really think it comes down to random chance when the first amino acids started forming in the primordial soup.
 
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Yeah, I would agree with that.

But L and D amino acids are enantiomers of each other, so there is really no functional difference of one over the other.

I really think it comes down to random chance when the first amino acids started forming in the primordial soup.

Enatiomers can have different functions, a D-amino acid is basically trash unless it can be converted to a L-amino acid, their functions in the human body are different. As for why its L or D, it just is, thats what the enzymes can work with, how or why it became that way doesnt really matter for the scope of anything past curiosity.
 

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Enatiomers can have different functions, a D-amino acid is basically trash unless it can be converted to a L-amino acid, their functions in the human body are different. As for why its L or D, it just is, thats what the enzymes can work with, how or why it became that way doesnt really matter for the scope of anything past curiosity.

In the body they can because enzymes are stereospecific.

But the question was that why are they stereospecific for L, why not for D? Why has nature chosen L over D?

From an organic chemistry perspective, there should be no reason for a preference of one over the other.

It was probably just random chance that made L predominate over D.
 

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I really think it comes down to random chance when the first amino acids started forming in the primordial soup.

As good an explanation as any other...essentially a "founder effect." There's really no physical (thermodynamic) reason why L-aa enantiomers would be preferred over D-enantiomers, but of course once they were established in living things all of the stereo-specific enzymes would follow. Some have suggested that some physical assymmetry in the environment (such as light polarization) may have favored the L-enantiomer (but then we have D-glucose stereisomers to account for...).

The evolution of organismal assymetry is a really interesting topic. Why, for example, do vertebrate nervous systems show left-right asymmetries? A "first level" answer is that such assymetries have "evolved." That's clearly true, but it is an answer that explains nothing. More interesting are hypotheses that try to explain why these asymmetries exist.:)
 

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Stereospecific enzymes.

that just pushes the question one step back.

The question the OP is asking, framed in your answer, is "why are enzymes stereospecific for L-amino acids and not D-amino acids." Theres a wrong way to answer this question (because life uses L-aa, which is a tautology) and the "right" way to answer the question, which is no one knows. The hypothesis given in this thread (random luck) is about as good a guess as any.
 

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"Why" questions aren't important. What's important is for you to know the function of amino acids, how they exist, and how they usually are/act. Then the question will give you a scenario with them, in which you apply your knowledge and logic to understand how they will act.

If you wanted to know why you'd have gone for the PhD. ;)

There's a similar question in the clinical forum: Why does tapping veins make them bulge? *wave hands and say big scientific words about cytokines and mediation and dynamic stasis distention*
 
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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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muhali3

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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.

...amazing.
 
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Morsetlis

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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.

Black magic.
 

Hexon

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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.
that's a really cook take on things:thumbup:

where did u get all that?
 
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that's a really cook take on things:thumbup:

where did u get all that?

lol. wikipedia of course.

In the article "abiogenesis" look for section "Models to explain homochirality".
In the article "circular polarization" look for the section "starlight".
In the article "Parity (physics)" look for the section "Parity violation".

I also followed several footnotes to read the original journal articles.
 

Hexon

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lol. wikipedia of course.

In the article "abiogenesis" look for section "Models to explain homochirality".
In the article "circular polarization" look for the section "starlight".
In the article "Parity (physics)" look for the section "Parity violation".

I also followed several footnotes to read the original journal articles.

awesome, thanks a lot!
 

Danlee07

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This is just logical. Stable, repeating substructures of protein require one series of either L or D. Active enzyme sites are asymmetric so stereospecific catalysis will be reacted; cells synthesize accordingly.

i have no clue where some of these other answers are coming from...
 

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i did some reading up on this and it appears the creationists are trying to use homochirality as some sort of proof of Intelligent design
 

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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.

:thumbup:
 
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has to do with conformations of enzymes... off the tangent of thermodynamics and stability.
even though they are enantiomers. different orientation of the amino acid would interfere with proper binding to the enzymes and they wouldn't be recognized as well
 

Erythropoietin

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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.

had to bump this cool explanation again :D
 

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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.

I've a crap ton of respect for you MT, but I'm calling BS on this one. Can't say how or why, but i am :)
 
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I explained my sources in post #18. It took quite a bit of sleuthing in wikipedia to construct my answer. I'm including some of the original articles here:


article on circular polarized light selectively degrading D amino acids (2005)
http://www.newscientist.com/article/dn7895-space-radiation-may-select-amino-acids-for-life.html

article showing beta decay violates parity (1957)
http://prola.aps.org/abstract/PR/v105/i4/p1413_1

article on achiral beta decay selectively degrading D amino acids (1976)
http://www.springerlink.com/content/wu801259101p3725/

For $100 you can read the whole shebang here in the Uwe's book (2008)
http://www.springer.com/chemistry/organic+chemistry/book/978-3-540-76885-2


I hope it isn't all bullcrap, because this is the subject of my biochemistry final project / presentation!!!
 

Pisiform

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I explained my sources in post #18. It took quite a bit of sleuthing in wikipedia to construct my answer. I'm including some of the original articles here:


article on circular polarized light selectively degrading D amino acids (2005)
http://www.newscientist.com/article/dn7895-space-radiation-may-select-amino-acids-for-life.html

article showing beta decay violates parity (1957)
http://prola.aps.org/abstract/PR/v105/i4/p1413_1

article on achiral beta decay selectively degrading D amino acids (1976)
http://www.springerlink.com/content/wu801259101p3725/

For $100 you can read the whole shebang here in the Uwe's book (2008)
http://www.springer.com/chemistry/organic+chemistry/book/978-3-540-76885-2


I hope it isn't all bullcrap, because this is the subject of my biochemistry final project / presentation!!!

I think its pretty legitimate.
 

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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.
Nature prefers stability and evolution is also based on stability.Now L-amino acids may be more stable in nature than D amino acids and that's the reason they are more abundant.
I think this is a simple language for what MT had tried to explain.
 

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Nature prefers stability and evolution is also based on stability.Now L-amino acids may be more stable in nature than D amino acids and that's the reason they are more abundant.
I think this is a simple language for what MT had tried to explain.

Oh. Put it that way. Yeah, that makes more sense.

No, I assumed you all were having this discussion and then MT comes in out of the blue. The immediacy of it and galactic chirality tour de force made me suspicious, but if it's something you've studied then my hat's off.
 

mackdaddy

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I'm getting back on this thread really late but thank you everyone for your input. I asked the question out of mere curiosity. At the time I was reading a few review and textbooks, none of which offered an explanation.

My biochemistry professor covered this subject a few weeks later in class and he actually gave the same explanation that MT Headed gave. There is definitely creditably in his answer. My professor said though, it can go back to just pure chance/luck whatever you wanna call it. Either way I'm not that curious as to pursue a PhD in this matter, lol.
 
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This was actually the subject of my senior thesis in biochemistry.

I'd like to say that after additional research (but not the kind of Research that can be put on an application) the effect of chiral beta decay is likely too small to have an effect on the homochirality of amino acids.

The current thinking is that the strong chirality of circular polarized light in our galaxy (but not on earth, due to scattering) due to its magnetic orientation causes L amino acids to form on comets and meteors. These building blocks were deposited on earth when it was bombarded billions of years ago.

Then a series of chemical reactions amplified this small (5% ish) enantiomeric excess into a larger one, and the large availabilty of only one kind of amino acid set the stage for abiogenesis.

Uwe's 2008 book seems to be the definitive layman's book on the subject.

It turns out this small question is still an area of active research, and millions of dollars have been spent to analyze chirality in extraterrestrial material, including a European spacecraft that will land on a comet in 2014. This spacecraft carries not one, but two chiral analysis modules! Similar analysis is being done on mars.

Such a simple question... who knew how big it really was?
 
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I've read plenty of textbooks which state that biological systems in nature only use L amino acids over D.

But I can't find a reason as to why. Why are only L amino acids found in proteins in nature?
That's bec most receptors and enzymes biological systems are steriospesific towards the L conformation.. Also the DNA "stamps" for synthesis of amino acid are spesifically L stamps so they produce L amino acids ,receptors and enzymes.
 

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That's bec most receptors and enzymes biological systems are steriospesific towards the L conformation.. Also the DNA "stamps" for synthesis of amino acid are spesifically L stamps so they produce L amino acids ,receptors and enzymes.

No need to start up an old thread but your response uses circular reasoning. Enzymes and receptors are all peptides themselves, which also use L amino acids. They're stereospecific for L amino acids but they're stereospecific because they themselves are one stereoisomer. The reason for biological chirality has not been definitively solved and I doubt it will be anytime soon: The Origin of Biological Homochirality
 
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