One of the great conundrums in biochemistry is the homochirality of life’s building blocks. Natural sugars exhibit a nearly exclusive right-handedness; DNA demonstrates a supramolecular left-handed chirality, while amino acids exhibit an exclusive left-handedness. Chiral molecules have an asymmetric central atom, surrounded by four different functional groups. This three-dimensional structure makes enantimomers, which are not super imposable mirror images of one another (Figure 1a). These molecules are optically active – that is they rotate the plane of polarized light in opposite directions of one another.
Non-biological or prebiotic methods of amino acid synthesis produce racemic mixtures (about equivalent concentrations L and D forms). This raises the question of how biological processes came to invariably select L forms over D-forms of amino acids? The consequences of this selection are evident in molecular biology today; if both L and D forms were present during synthesis they would be randomly inserted into the amino acid sequence. Enzymatic reactions would not function properly; active sites would be altered not allowing for stereo specific substrate-enzyme selection.
The selection process by which each amino acid has an L chirality is likely not the consequence of a single phenomenon. On the contrary, without identifying a ‘silver bullet’, we must assume a multiple step process in which each step selectively favored L-amino acids. The first step in this selection would be the initial availability or favoring of L-amino acids by either cosmic or terrestrial selection, since prebiotic synthesis methods all seem to produce a 1:1 racemic mixture. Second, an initial excess of L-amino acids had to overcome only slight favorability in the first step, and become dominate through further selective forces. The continued excess of L-amino acid also had to be conserved and protected compared to D-amino acids. Finally, polymerization of these amino acids, had to favor a uniform chirality from the beginning, before specific substrates had formed or were needed. This paper will focus on the discussion of the first two steps in this process.
Prebiotic Synthesis of Amino Acids
Amino acid synthesis is the first step we must look at to consider the reason for homochirality. The first and most well-known of potential terrestrial origins of amino acids comes from Miller and Urey (1953). The experiment synthesized numerous amino acids by applying electric disharges to a mixture CH4, NH3, H2O and H2 – believed at the time to be atmospheric conditions of earth at the time.
Whether organic synthesis of amino acid is terrestrial or extraterrestrial, and the mechanism of synthesis is a topic that remains heavily debated. In part this is due to our limited knowledge about prebiotic conditions on Earth and potential precursors for amino acid synthesis.
Miller-Urey provided evidence that prebiotic conditions could have produced a small number of...