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Why are almost all amino acids used in life found in the L form? Is it possible to construct life forms in which D-amino acids are the dominant amino acids in protein-like structures? Or would life elsewhere also be biased toward L-amino acids? We return to this when we discuss astrochemistry (Chapter 10) and after that chapter you might like to revisit this question. Life on Earth uses D-sugars, but what about a life form that uses L-sugars? What information can you gather to shed light on why life on Earth chose L-amino acids? What range of problems and even advantages could you think of for a lifeform that used a racemic mix of D- and L-amino acids in its biochemical architecture? In Figure 4.8, you can see the four possible ways one might construct a life form. Is there a reason why Gertie is made the way she is or were the three other forms possible?

Bonner, W.A. (1991). The origin and amplification of biomolecular chirality. Origins of Life and Evolution of Biospheres 21: 59–111.

Breslow, R. and Cheng, Z.-L. (2009). On the origin of terrestrial homochirality for nucleosides and amino acids. Proceedings of the National Academy of Sciences of the United States of America 106: 9144–9146.

Figure 4.8 Different chiralities of life? Gertie the aardvark is a rescued aardvark. She is made of L-amino acids and D-sugars. Is it theoretically possible for Gertie to exist in the three other chiral combinations of L- and D-amino acids or L- and D-sugars?

Now an interesting thing about life on Earth is that almost all the amino acids used in life are of the L form. The D form is very rare, although D-amino acids are found in the cell membranes of bacteria. They reappear in the next chapter. The reason why life predominantly uses one form of amino acids is not fully understood, but it might have something to do with molecular recognition. As many biochemical interactions involve one molecule slotting into another to carry out chemical reactions, a system of life that was a mix of L and D forms would cause great complexity because we would need proteins with active sites that could recognize either the L or D form of molecules. It seems plausible to speculate that once a biochemical architecture emerged that had a preponderance to use either the L or D form, then that choice would have been perpetuated and amplified. Life on Earth developed a biochemistry based on the L form. Later in the textbook, we explore the possibility that the preference for the L form in life was caused by a pre-existing enantiomeric excess of L forms of amino acids in the prebiotic compounds from which life originated.