Think of a pair of gloves. If you take two right-hand gloves, or two left-hand gloves, you’ll find that you can exactly superimpose one on top of the other. It is impossible to superimpose a left hand on to a right hand glove. This holds for an asymmetrical object and its mirror image – the left-hand glove is a mirror image of the right-hand glove. This general phenomenon was discovered by Louis Pasteur (1822-1895).
Life is based on the element carbon (C). The C atom is very flexible – it can form chemical bonds with other C atoms and with a variety of other elements. An individual C atom can form bonds with four other atoms or atom groupings. If these four other atoms or atom groups are different from each other, they can connect to the C atom in 3D space in one or other of two ways. These two ways are non-superimposable mirror images of each other. Remembering the glove analogy, I will call them left-hand (L) and right-hand (R) forms. Although L and R variants have the same chemical composition, they have slightly different physical characteristics and they are not interchangeable in chemical reactions.
Life is composed of four classes of biochemicals: proteins, carbohydrates, fats and nucleic acids. These biochemicals are polymers, ie their molecules are large and made up of repeating units of simple molecules (monomers) linked together – these are amino acids in the case of proteins, sugars in carbohydrates, fatty acids in fats, and nucleotides in nucleic acids.
The monomers could occur in the L or R forms but, since these forms are not interchangeable in chemical reactions, the living cell uses only L or R forms of the monomers exclusively. To use a mixture of both would make the chemistry of life impossibly complicated. Hence all amino acids in the cell are L forms and all the sugars are R forms. The only exception to this general rule occurs in bacteria where some amino acids in the walls that surround each cell are R forms.
At least, this was the generally accepted picture up until recently. However, an increasing number of instances have gradually come to light where animals use R amino acids for specific purposes. Thus a number of animal venoms have been shown to contain R amino acids. It is guessed that the evolutionary logic behind this is that such venom is more effective because the prey into whom the venom is injected finds it more difficult to break down these “strange” amino acids.
R amino acids in cell coats of bacteria play an important signalling role in inducing formation of bacterial films (biofilms), where bacteria stick together to form 2D films. Research is underway to understand how R amino acids work in this regard with a view to producing treatments to break up biofilms that develop in cystic fibrosis, cover our teeth and build up in medical equipment.
Any of the monomers that occur naturally in the biological cell can be synthesised in the laboratory by an organic chemist. When this is done, the product, say an amino acid, is a 50-50 mixture of L and R types. How life got started on Earth 3.8 billion years ago based purely on L amino acids is not known. The primordial soup in which life began would surely have contained both L and R types. One possibility is that polarised light emanating from a nearby star destroyed all the R amino acids in the primordial soup, so that life automatically started up with L amino acids.
Amino acids have been detected in outer space and in meteorites that have landed on Earth.
Scientific American
, May 2013, carries an interesting article on Mirror Molecules by Sarah Everts.
William Reville is an emeritus professor of biochemistry and public awareness of science officer at UCC. http://understandingscience.ucc.ie