Homochirality in Biology

 by Saanvi Ganesh

Homochirality in biological molecules has fascinated scientists for decades. The fact that life seems to prefer one enantiomer over the other across all organisms, from humans to amoeba to fungi, is almost contradictory to the random nature of chemical reactions that started and sustain life. Chirality is the property of molecules to form two molecules that are non-superimposable, called enantiomers. In other words, the two molecules are mirror images of each other, similar to our hands. We each have two hands that are identical in components, but are opposites because one hand is the mirror image of the other. This is why chirality if often described as 'handedness'. Homochirality is the uniformity of chirality (an enantiopure substance), so you would have two left hands or two right hands.

 

Biological molecules have the potential to be chiral because they are organic compounds; they contain carbon chains or centres. Carbon has the ability to form four bonds to four substituents, forming a tetrahedral shape. If each substituent is different, the central carbon atom is said to be chiral. Having said that, carbon is not the only element that could form a chiral centre; other elements like nitrogen can also form chiral centres. Nitrogen is also used in organic compounds, for example to form amino acids for proteins.

 

Biopolymers and the molecules from which they are constructed have the potential to exist in either enantiomer form. Abiotic experimental data has shown that in the absence of a chiral directing force, a racemic (equal) mixture of both enantiomers is produced. This gives rise to the theory that molecules exist in racemic mixtures in the primordial soup that existed before the life. Today however, almost all chiral bio-molecules are found as one enantiomer: sugars are exclusively right-handed (D), amino acids left-handed (L) and DNA coils into right-handed helices.

 

So how did our homochiral living world emerge from an equal mixture of both enantiomers and how was it sustained? The answer to the first part of this question concerns symmetry breaking. Symmetry breaking in chemistry, is a term used to describe the spontaneous generation of an excess of one enantiomer over the other without an external chiral source (like an enzyme). The answer to the second part concerns chiral amplification, where the chiral imbalance is sustained and amplified to increase the enantiomeric excess.

 

One study in 1953 suggested a model to create enantiomeric excess based on a chiral molecule that catalyses its own self-production and suppresses synthesis of its enantiomer at the same time. In 1995, a laboratory demonstration by K Soai used autocatalytic alkylation of pyrimidyl aldehydes with diisopropylzinc. The rate of the reaction was not only accelerated by addition of catalytic amounts of its own product, but this autocatalytic product may ultimately be formed in very “high” enantiomeric excess starting from a very “low” enantiomeric excess in the original catalyst. Hence, providing a theory for both symmetry breaking and chiral amplification.

 

There are many other theories for the origins of homochirality and  whether homochirality provides advantages or was formed by chance. Understanding this would not only give us an insight into the beginnings of life on Earth, but also aid us in producing pharmaceuticals that work more effectively within the body or produce fewer side effects. It may also aid us in finding cures for some diseases that do not yet have one.

 

Sources:

 

https://www.chemistryworld.com/features/the-origin-of-homochirality/9073.article

 

https://en.wikipedia.org/wiki/Homochirality

 

https://cshperspectives.cshlp.org/content/11/3/a032540.full