The Asymmetry of Life

This book is aimed at catching this fingerprint and discovering its evolutionary origin. We will not review all the general aspects of the origin of life,2 but we will focus on one particularly fascinating and crucial period therein, namely the emergence of chirality. More scientifically spoken it is the emergence of "biomolecular homochirality". I will present cutting edge research on the elucidation of the ultimate cause of the "homochirality" phenomena on Earth and the possible correlation with physicochemical parameters. This first chapter will serve as a general, unifying introduction to all these phenomena, letting us develop an overview of the manifold aspects of "biomolecular homochirality". The upcoming chapters will systematically go through all these phenomena in more detail.

So, what are we talking about when we say "biomolecular homochirality"? Life is not symmetric; the phenomenon of asymmetry can well be observed in living organisms commencing with microorganisms, to plants, animals, and even human beings. Just to give two examples from everyday life: soccer players prefer to use their right or left food to shoot, and people favour to write distinguishingly either with their right or left hand. In scientific terms, biochirality describes the phenomenon that organisms display laterality. Here, symmetry is obviously broken. If we imagine living beings in a mirror-world their mirror-world properties would be non-identical to their real world properties.3

But does this well-known macroscopic phenomenon also exist on the microscopic molecular level? "Yes"! A recent advertisement for "right-handed yogurt" has introduced the general public to the concept that many biomolecules exist in distinguishable "right-" and "left-handed" versions. Molecules or parts of molecules can be structured as their image or mirror image. Image and mirror image molecules are often not identical and can possess different biological properties. In the frame of this book we will hence reflect on mirrors and their effects on molecules, we will discuss properties of mirror image molecules and speculate on the existence of mirror-image worlds including our difficulty in accessing them.

Back to the real world's molecular level: the basic building blocks of life are mostly found in exclusively one out of two principally possible mirror image

2 A selection of recent literature on general aspects on the origin of life contains Brack (1998), Smith and Szathmary (1999), Ward and Brownlee (2000), Schopf (2002), Adams (2002), Davies (2003), RauchfuB (2005), and Luisi (2006).

3 This phenomenon is witnessed, for example, by the high rate of accidents of central European drivers on left-running highways on the British Islands.

forms, designated as d or l.4 Natural amino acids, the components of proteins (enzymes), are found in the L-form such as L-alanine, L-valine, L-leucine, L-aspartic acid, and others. Sugar molecules are - in contrast to this - biosynthesised in the right-form for implementation into nucleic acids. D-ribose and D-deoxyribose are used in ribonucleic acid RNA and deoxyribonucleic acid DNA respectively, and D-glucose in glycogen, starch, and cellulose. The mirror image structures of amino acids (called D-enantiomers) are not tolerated for the molecular architecture of proteins; in a similar selective way, mirror image sugars (L-carbohydrates) do not contribute to the molecular construction of the nucleic acids RNA and DNA. In natural sciences, this widely distributed phenomenon is called biomolecular asymmetry.

In other words, living organisms are based on biopolymers that are strongly selective towards the chirality of their monomer subunits. Monomers of biopolymers are almost exclusively homochiral. Homochirality is a property of matter, which is made up of only one "hand" (that is what is literally meant by "cheir" in the ancient Greek language) out of the potentially two "hands" available within confined boundaries of abundance. The boundary thus defined by this homochirality versus symmetry could even be interpreted as the borderline between living organisms here and non-living matter there. As we will see later, the intrinsic symmetry aspect of a chiral molecular structure will be used by future space missions, for example the upcoming ExoMars mission, as an elegant way of finding life traces somewhere else in the universe.

Also flavour and fragrance ingredients serve as typical examples of molecular asymmetry. Taste, scent, and other physiological properties can differ between right-and left-handed molecules. One can easily use one's nose and establish that enan-tiomers can smell different. Right-handed carvone for example shows the odour of caraway, whereas its mirror image, the left-handed carvone, disposes an odour of spearmint. Right- and left-handed limonene (orange versus lemon odour) and right-and left-handed menthol are other examples of differences in organoleptic properties of enantiomers. This is not surprising, because - as we have learned above - proteins of the olfactory system are composed of homochiral L-amino acids linked together via peptide bonds. The highly asymmetric arrangement of the odorant receptor protein is fine-tuned to distinguish between odorants in general and right- and left-handed odorants in particular. In addition to odour-reception, our taste is concerned by this homochirality phenomenon in a way that S-asparagine tastes bitter whereas the mirror image molecule called R-asparagine tastes sweet (Quack 1993). The fact that two enantiomers can have such different functions illustrates that stereochemistry controls whether, and how, molecule and receptor recognize one another and "shake hands" (Siegel 2001).

A dramatic example of the importance of the homochirality phenomena is the chemical thalidomide, in Europe better known under the trade name contergan. After medicinal application of the mixture of the R- and the 5-form, it became dramatically apparent that the S-form of this drug is able to cause severe teratogenic effects, whereas its mirror image, the R-contergan, showed tranquillising properties.

4 The prefix D is derived from the Latin dexter, right; the prefix l is deduced from laevus, left. Chap. 2 will introduce stereochemical terms systematically.

Today, the useful medicinal properties of ^-contergan are employed against leprosy, HIV, and several types of cancers.5 In fact, differences between mirror image forms concern numerous medicinal drugs. S-penicilliamine serves as an antiarthritic drug whereas ^-penicilliamine is known to be highly toxic (Quack 1989).

We can conclude from the above paragraphs that the use of homochiral monomers is considered vital for the construction of biopolymers and must therefore be considered a fundamental step in chemical evolution towards the origin of life. The absolute homochirality of "simple", non-complex organic molecules is a prerequisite for an efficient self-replication of the first living systems. As a consequence, numerous examples of evolving structures and systems, which develop asymmetrically in timescales that can be observed in the laboratory, have been studied in great detail. However, despite a manifold of research activities, the underlying question is not yet fully understood: how did life select the left-handed form of amino acids for the construction of its proteins? Why did living organisms not opt for the right-handed form of amino acids? Was this a random or a determinate process at life's origin? In spite of this well known violation of molecular symmetry it is astonishing that so far there is no one convincing theory yet explaining the cause of the driving force behind the long process of evolution of such a biosphere as we observe today on Earth - astonishing the more while searching through the literature, where a large number of papers of more theoretical or experimental nature are devoted to this very essential problem (Thiemann 1998).

Would we get something a bit different if we wind back the tape of life again? Will there again be a mix of plants, animals, and humans? Certainly, we are unlikely to get a creature using heterochiral biocatalytic (enzymatic) and genetic systems. But focusing on homochiral systems only, we may ask whether life would have again selected L-amino acids for the construction of enzymes or whether a mirror world on the molecular level would have been possible as well. This will be one of the great, interesting questions discussed in this book.

Our aim will be to spread out the different attempts in detail to uncover the secrecy behind the phenomenon, why we see living nature in this one-handed form only today. We will collect and present information on the origin of biomolecu-lar asymmetry, a mysterious phenomenon still that obviously continues to receive tremendous attention in science, as it is manifested by a constant high number of letters published in the high-impact journals Nature and Science. Strong and ongoing scientific debates on the international top-level will be outlined involving contributions from various scientific disciplines.

Furthermore, as scientists we are interested in questions like: what are the next experiments? Are the chemical reactions that determined life's handedness reproducible in the lab? Are fundamental stages assumed to have occurred in atmospheric, hydrothermal, or even extraterrestrial and interstellar environments?

5 The medicinal application of a specific thalidomide enantiomer is more complicated than described above. The use of the pure ^-thalidomide form that shows the useful effects is hampered by its slow interconversion (racemization) in the human body to harmful ¿"-thalidomide. Consequently, thalidomide is not given to pregnant women today, in order to avoid the teratogenic potential of its ¿"-enantiomer.

Are concentrations, temperature, pressure, pH values, and other physico-chemical parameters known to reproduce this crucial event? This book aims to contribute to the better understanding specifically of these questions by giving the answers in the appropriate chapters.

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