What Do Mirror Images And Chirality Have To Do With The Biochemistry In Our Body?

Mirror images aren’t necessarily the real deal when we look at biochemistry.

Whenever we look into a mirror, we see a mirror image of ourselves. This mirror image is a reflection of ourselves in the mirror, as the light rays all bounce about and create an image of shape and colour.

Even our hands appear to be mirror images — our right hand looks roughly similar to our left hand, but they are not superimposable.

Left-handed people would be facing much difficulty trying to use a right-handed scissors with their left master hands. I was born left handed and I never knew that it was a problem — I just thought I was clumsy as a kid and couldn’t use the scissors properly. Even as of now, my right hand isn’t the most skilled at cutting out an intricate tracing from a piece of paper.

Amazing, isn’t it? That feeling when I realised that I wasn’t dumb.

But this is something that we can actually see. There are smaller things that we can’t see with the naked eye. Especially with molecules and things that happen on the molecular level — these molecules can come in the exact same chemical structure and formulae, but their orientations can be changed up.

Which brings us to the concept of chirality, which is so important to understand because the biochemical receptors in our bodies have specific 3D configurations.

Much like how a left hand cannot use a right-handed scissors well, nor a right hand cannot use a left-handed scissors well. Guitars are also designed, by default, to suit right-handed people. I’ve been trained to play a right-handed guitar, but I am completely clueless as to how to play a left-handed guitar because I do need to re-orientate my brain to remember how the 6 strings in a left-handed guitar are placed.

In the same way, we have molecules and drugs that are chiral.

For example, the drug thalidomide was used for treating nausea in pregnant women in the 1950s-1960s. Of course, what the medical professionals didn’t know at that time was that thalidomide would also be responsible for causing birth defects for the babies being borne by the pregnant women who were taking that drug for morning sickness. It’s tragic, really.

And we can look at the chirality of thalidomide to see what happens.

Thalidomide comes in the form of R- and S-stereoisomers. The R-form functions as a sedative, while the S-form is the teratogenic culprit that contributed to those birth defects. Biologically, the human body is capable of converting R-thalidomide to S-thalidomide and S-thalidomide to R-thalidomide.

What’s the difference between R-thalidomide and S-thalidomide chemically? It’s just the orientation of the various chemical groups in the molecule!

The problem being that the orientation of these chemical groups gives the stereoisomers unique 3D shapes.

Much like how a right handed pair of scissors can only be operated by the right hand, even though the left hand is also a hand.

These unique 3D shapes will be able to fit into different biological receptors to effect or dampen biochemical reactions and signals within the body.

For example, in an allergic reaction, the body’s mast cells release large loads of histamines, which will bind to histamine receptors in the nose to trigger the onset of a runny nose. Common antihistamine drugs function by binding to histamine receptor sites as antagonists to prevent actual histamines from binding to those sites, which then dampens the symptom of the runny nose.

The problem with an intensification or a dampening of these biochemical signals is that they do open new cans of worms (also known as side effects).

Some keys are able to open multiple locks, and hence the 3D shapes of certain stereoisomers can fit into different biological receptors to either dampen or intensify other biochemical processes, which would bring about some undesirable side effects.

Statins, for instance, prevent the HmG-CoA reductase enzyme from synthesising mevalonate, which is a key intermediate in the synthesis of fresh cholesterol:

However, the mevalonate pathway is necessary for the body to also be able to synthesise Coenzyme Q10:

The side effect of reducing cholesterol synthesis in our bodies with a statin drug is that we’d also be producing less Coenzyme Q10, which would result in the symptom of one feeling much more fatigued and lethargic after having done less strenuous work than someone else.

And in the same way, a pregnant woman could have consumed pure R-thalidomide for morning sickness, but the more tragic side effect would be her giving birth to a child with birth defects just because her body was able to convert the R-thalidomide that she consumed into its teratogenic S-form.

Similarly, S-ibuprofen retains a majority of the pharmacological activity for pain relief purposes, while R-ibuprofen is not exactly the best for pain relief. An overconsumption of ibuprofen can also lead to gastric discomfort and ulcerations, and the administration of pure S-ibuprofen can reduce the gastric damage - hence if ibuprofen is to be prescribed, a person is better off consuming pure S-ibuprofen instead of the racemic (equal proportions of the R- and S-stereoisomers) mixtures that are more commonly found in pharmacies.

So we have this major consideration of how the chirality of different molecules can affect the biochemistry in our body that significantly.

The arrangement of specific molecules in specific orientations can play significantly different roles in the complex human body that we have!

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