Origins of a Homochiral Microbial World

Introduction

How did life begin? Where do we come from? What were the first organisms on earth? Questions like these have long puzzled even the brightest of scientists, and answers are only just beginning to surface. Historically, questions about the origins of life and evolution have evaded conversation. In societies heavily reliant on religion and faith, scientific hypothesis about the origins of life is usually disapproved and waved to the side. In 1925, John Scopes, a biology teacher, was even arrested for teaching evolution in a classroom. Those days are gone, however, and today, society has started to use a more scientific analysis of the origins of life on Earth.

Microbes are the oldest life forms that have been identified today. Their simplicity, size, and metabolic diversity makes them ideal candidates for life in a world without oxygen, in a world under constant attack from meteorites, and in a world where no other life forms exist. Stromatolites dating back 3. 4 billion years traces depict communities of cyanobacteria in a tidal pool in the Bahamas. Microfossils depict early microbial cells decayed by time. Biosignatures found in sedimentary rock show signs of organic molecules formed only by microbes. The question now sought after asks how these microbes even came to be. Though microbial cells are simpler than multi-cellular, complex eukaryotic cells, life is by no means “easy.” Enzymes, arguably the most important feature of life, are specific and highly regulated. They have evolved to be exact, even perfect. How can a world of nothing evolve to perfection?

In 1848, Louis Pasteur demonstrated a novel way that molecules and crystals can be even more specific without differing in molecular weight and chemical make-up. Pasteur identified two types of crystals of a solution and determined that they were the same in every way except that they were mirror images of each other. He then separated these two crystals and made solutions of each, deeming one “+” and one “-“. By shining polarized light through each solution, Pasteur then discovered that the two solutions had equal but opposite optical activity. In other words, the direction of polarized light passing through these molecules was in opposite directions, one spinning left (L, levo), and one spinning right (D, dextro). When the two solutions were combined, what is now known as a racemic mixture, the solution demonstrated no optical activity. The two types of crystals, mirror images of each other, are today known as enantiomers.

Since Pasteur’s discovery of enantiomers, there has been much research in the specific orientation of atoms around organic carbon. Today it is generally understood that all living organisms contain sugars and amino acids of only one enantiomeric form. Thus, organisms are called “homochiral.” Homochirality is one of the most remarkable features of living organisms. In fact, it is critical to the formation of life. Enzymes act on only one enantiomer of a sugar molecule. Proteins themselves are homochiral; amino acids in living organisms are (L)-enantiomers. The positioning of atoms in non-random, specific orientations, however, is actually quite difficult to do in a laboratory, especially without the aid of chiral enzymes. Kinetically, molecules prefer to isomerize in water, driving a homochiral system toward a racemic system. The prebiotic environment, if not covered by ice, would have been characterized by large waves of water that would have diluted prebiotic chiral molecules, resulting in a racemic composition of amino acids and sugars. How then did the prebiotic world select for a homochiral world? How do (L)-amino acids arise? From where did these homochiral molecules originally come? How did they survive the harsh prebiotic soup that the earth was billions of years ago? How did the world select only one enantiomer biochemically and develop an excess of that enantiomer? Answering these questions will give researchers more insight into the evolution of proteins, enzymes, and microbes on Earth.

The question of homochirality on Earth is an active area of research today. Three specific hypotheses incorporating all branches of the natural sciences—physics, chemistry, and biology—will provide the missing links that will help us find more finite solutions. These hypotheses include: photolysis of one enantiomer of a racemic amino acid with circularly polarized ultraviolet light, transfer of chirality from enantioenriched amino acids to proteinogenic amino acids, and aqueous amplification of one enantiomer of an amino acid residue. br>

Circularly Polarized Ultraviolet Light

Include some current research in each topic, with at least one figure showing data.

Transfering Chirality to Proteinogenic Amino Acids

Include some current research in each topic, with at least one figure showing data.

Aqueous Amplification of Enantiomers

Include some current research in each topic, with at least one figure showing data.

Conclusion

Overall paper length should be 3,000 words, with at least 3 figures.

References

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Bonner, William A. “The Origin and Amplification of Biomolecular Chirality.” Department of Chemistry. Stanford University. 1991.

Flores, Jose J., William A. Bonner, and Gail A. Massey. “Asymmetric Photolysis of (RS)-Leucin with Circularly Polarized Ultraviolet Light.” The Journal of the American Chemical Society. 1977. Volumer 99, No 11. p. 3622-3624.

Glavin, Daniel P. and Jason P. Dworkin. “Enrichment of the Amino Acid L-Isovaline by Aqueous Alteration of Cl and CM Meteorite Parent Bodies.” PNAS. 2009.

Levine, Mindy, Craig Scott Kenesky, Daniel Mazori, and Ronald Breslow. “Enantioselective Synthesis and Enantiomeric Amplification of Amino Acids under Prebiotic Conditions. Organic Letters. 2008. Volumer 10, No 12. p. 2432-2436.

Toxvaerd, Soren. “Origin of Homochirality in Biosystems.” International Journal of Molecular Sciences. 2009. Volume 10. p. 1290-1299.

Edited by student of Joan Slonczewski for BIOL 238 Microbiology, 2009, Kenyon College.