Biological Homochirality: One of Life’s Greatest Mysteries
In every life form on Earth, sugars are always right-handed, and amino acids are always left-handed. What does this mean, and why does it happen? This bizarre phenomenon involves chiral molecules and is known as “homochirality.”
Chiral molecules are molecules of the same formula, but with slightly different structures. Your hand is a chiral object because it is not superimposable on its mirror image; when you place your left hand over your right, your thumbs will point in different directions. In organic molecules, when four different groups surround a central carbon, the molecule will have two different chiral forms called enantiomers. These two molecules are mirror images of each other, as the groups around the carbon are oriented in different directions, like right and left hands. In chemistry, these compounds are called R and S enantiomers (or isomers). However, when amino acids or sugars are chiral, scientists call them D and L isomers (scientists are terrible at naming things consistently). D- means “right-handed” and L- means “left-handed.”
The D and L isomers are normally present in a 50/50 ratio. Chemists call this a racemic mixture. Naturally, you would expect them to both be equally present in our bodies, right? Wrong! In fact, all life forms only use L-amino acids and D-sugars. The presence of only one isomer is called homochirality. From the smallest bacteria to elephants, only L-amino acids make up proteins and only D-sugars make up polysaccharides. In other words, all proteins are right-handed and all polysaccharides are left-handed*. Why is this? It is a mystery that has perplexed biologists for years. Despite the best efforts of physicists, chemists, and evolutionary biologists, we still don’t know for sure. Here, we’ll take a look at some of the leading current theories.
*(It should be noted that some L-sugars can also be broken down by metabolic pathways. Sugars can form rings, and the D-sugar forms a more stable ring than the L-sugar, but L-sugars can still exist. The stability of the ring may be the main contributor to the prevalence of D-sugars. Below, we will focus mainly on amino acids, which strictly adhere to a single chiral formation in biological systems.)
Enzymes and Evolution
Here’s what we do know: the enzymes in cells can only metabolize one isomer, but not the other. Enzymes catalyze reactions to break down and build molecules by binding to them in very specific ways. An enzyme that breaks down D-glucose will not be able to break down L-glucose. If we wanted to use and build both isomers, we would have to have two sets of enzymes. This would require more DNA, more food, and a lot more energy. Naturally, cells want to be as efficient as possible, so having an extra set of enzymes would be taxing.
But if we had those extra enzymes, couldn’t we also get twice as much energy from the environment? We could use the other 50% of isomers if we had the enzymes to break them down instead of wasting them. The answer has to do with evolution and the food pyramid.
When early life was evolving, an organism would have enzymes that selectively react with one isomer. The organisms that ate this first life form would have to have the same selective enzymes in order to digest them. If all of your food only contains D-sugars, so will you! So if the bottom of the food chain only uses L-amino acids, homochirality will spread to every organism in the food chain! Since life had to develop one set of enzymes first, we wound up being stuck with that selectivity. Selective pressure from evolution forced each new organism to use the same amino acid and sugar types as the ones before it.
So once one isomer is picked, every form of life will use it. But why L-amino acids instead of D-amino acids, and why D-sugars instead of L-sugars? This is the real mystery. Something at the very beginning of life had to select one over the other. It is possible that in the early stages of life, some organisms developed one set of enzymes, while different organisms had the other. It could have been a random event that wiped out one line of enzymes, but scientists aren’t satisfied with this answer. There has to be some reason that L-amino acids and D-sugars are better for survival than their counterparts.
Racemic mixtures are 50/50 mixtures of the two isomers. But there is reason to believe that in our star system, there is actually a slight energy preference for one isomer over the other. To understand this preference, we have to journey deep into the Milky Way.
Our galaxy has a chiral spin and a magnetic orientation. This causes cosmic dust to circularly polarize light in one direction. Basically, because light is a 2-dimensional wave, it will propagate in a helix. The cosmic dust cause this helix to rotate preferentially in one direction.
This circularly polarized light degrades D-amino acids more than L-amino acids, so L-amino acids are more stable. This is supported by studies of meteors and comets. Cosmic matter, at least in the Milky Way, has a preference for L-amino acids due to the circular polarization of light.
Another physical explanation for the preference of one isomer over the other has to do with the electroweak force. There are four fundamental forces that govern physics: gravitation, electromagnetism, strong nuclear force, and the electroweak force (a.k.a. weak nuclear force). The first three are achiral: they affect L- and D-isomers the same way. The electroweak force, however, is affected by chirality. The electroweak force governs the radioactive decay of atoms and molecules. In this decay, the parity of the universe is not always preserved. During beta decay, emitted electrons favor one type of spin. The spin of the electrons again degrades D-amino acids more than L-amino acids, causing the balance of molecules to shift in favor of the L-isomer.
Both polarized light and electroweak decay contribute to a slight preference of one isomer over the other in our galaxy. However, in the grand scheme of things, this discrepancy isn’t enough to explain why EVERY life form uses only D-sugars and L-amino acids. There has to be something more. Which leads us to…
Once physics gives a slight edge to one isomer over the other, there is a chemical reason that amplifies the difference. Autocatalysis means that the presence of a chemical stimulates its own production. Around 1950, F.C. Frank suggested that the dominant isomers can activate their own production while suppressing the production of the other isomer.
The above graphic illustrates the point. When starting out with 3 L-isomers and 2 D-isomers, the ratio can be very quickly amplified so the L-isomers greatly outnumber the D-isomers. The L-isomer stimulates its own production, and the L-isomer also binds to the D-isomer in a process called “mutual antagonism.” This prevents the formation of more D-isomers, and the L-isomer is rapidly favored.
This was just a hypothetical framework to explain why we only have L-amino acids and D-sugars. But this concept gained support when the Soai Reaction was discovered. This is one demonstration of how certain isomers can be favored over another in biological processes. Although there is no evidence for ubiquitous autocatlysis causing homochirality, it can certainly be a factor for amplification in some instances.
A Working Model
Today, the most popular theory for biological homochirality includes all of these conclusions. Physical phenomena such as polarized light and electroweak force create a slight imbalance of D- and L-isomers. This small discrepancy is amplified into a more uneven imbalance by autocatalysis. Then, once life was formed, a single isomer was selected for quickly by the food chain and the specificity of enzymes. Thus, today we only see L-amino acids and D-sugars.
This theory still has some holes. There may never be a perfect explanation for an event that began billions of years ago. But we now have at least a potential solution for the mystery of biolgical homochirality.
“Mystery creates wonder, and wonder is the basis of man’s desire to understand.” — Neil Armstrong
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