Centre for Excellence in Basic Sciences, Mumbai
Most molecules used by life exist in two forms that are mirror images of each other. One of the great unanswered questions in our evolutionary history is why all life on Earth and their critical biological molecules like amino acids and proteins use only one of these forms. For instance, sugars are exclusively right-handed, amino acids are left-handed, and even DNA coils into right-handed helices. We call this phenomenon of single handedness homochirality, but how and why did our single-handed world emerge?
This is one of the “greatest mysteries” of life and is still an open problem in the field of biochemistry.
Why are only L- Amino acids and D- Sugars involved in living beings?
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A biologically inclined person should answer the biological part in more detail, but a simple answer is that we use these enantiomers because they are the ones that work with our system. It’s a matter of compatibility, if you use D- amino acids, you have to build them all from scratch, because you are not going to get them in the 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 the same pathways. As for carbohydrates, there are so many more stereoisomers, and organisms generally don’t use the monomer they obtain in their diets as building blocks (but rather oxidize them for energy, and build the monomer they need using their own metabolic pathways), that there hasn’t been the same selective pressure for standardization.
This phenomenon is known as Homochirality of Life and from astrochemical/astrobiological perspective, the deeper question is: was there a primordial reason for why life started with a preference for one over the other?
There a number of theories for how say, a very slight overabundance of one enantiomer over another was formed and propagated through to us. I am just discussing two of the intriguing theories, readers can stumble upon a variety of others while searching on the topic
The firs theory suggests:
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- There exists in the interstellar medium intense radiation coming from emission of hydrogen gas that is called “Lymph Alpha” radiation. In certain environments, this light is circularly polarized.
- Experiments have shown that when chiral molecules such as amino acids are exposed to this light and destroyed by it, there can be a slight preference to destroy one enantiomer over another (reasons still not very clear).
- So over the billions of years of the formation and destruction of such molecules, It is possible that when these components are incorporated into planets like ours; one enantiomer is present abundantly and thus life prefers to evolve from the more prevalent enantiomer.
The second theory suggests:
1)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.
2) This circularly polarized light degrades D enantiomers of amino acids more than L enantiomers, and this effect is clear while analyzing the amino acids found o meteors and asteroids. This explains why, at least in the milky way L enantiomers are preferred.
3) Although Gravity, Electromagnetism, and the strong nuclear forces 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 preferentially degrade D- amino acids over L- amino acids.
4) Thus due to the chirality of sunlight and the chirality of nuclear radiation, L- amino acids are more stable enantiomers and therefore favored for abiogenesis.
One more theory regarding the prevalence of L- amino acids suggests that; “Evidence shows that L- amino acids are slightly more soluble than is a racemic mixture of D and L amino acids that tend to form crystals. This small solubility difference could have been amplified over time so that L isomer became dominant in solution and became and got incorporated in early life forms.” Although there is not enough proof for this aspect.
A final interesting note is that: We actually observe an overabundance of some enantiomer in nature, and they tend to prefer L- amino acids. Two articles in the journal Nature by M.H Engel and team are a good starting point, and they discuss the distribution of these enantiomers in extraterrestrial amino acids found in Murchison Meteorite.