Life is not symmetric. L-Mirror image isomers of amino acids play a crucial role in living organisms. They predominate in molecules produced via biological pathways such as protein sequences, whereas their optical antipodes, the D-enantiomers, are quantitatively of negligible occurrence in biological systems. In the case of life's genetic material, homochiral D-ribofuranose enantiomers are used for the molecular architecture of deoxyribonucleic acid (DNA) and ribonucleic acid (RNA) backbones. Here also, the L-configuration of the sugar enantiomers is largely underrepresented.
Based on the molecular asymmetry of today's life, one might assume that evolutionary processes at life's beginnings exclusively incorporated L-amino acids in an enantioselective manner for polycondensation and formation of peptides and proteins on the primitive Earth. Inappropriate D-amino acids and L-sugars might have been eliminated preliminarily due to physico-chemical processes of chemical evolution and excluded entirely from biological reactions. This assumption turns out not to be true.
Despite the dominance of L-amino acids and D-sugar molecules in the living world - the ratio of d- to L-glucose on Earth is estimated to be at least 1015 to 1 - a few deviant D-amino acids and L-sugars have found their way into the molecules of life where they occupy specific roles in various living higher organisms (MacDermott and Tranter 1989). In most of these roles homochirality is also maintained in their specific domain. New studies point to the assumption that some of these deviant amino acids and sugar molecules are not negligible, instead they are of considerable importance for upholding biochemical reactions for individual organisms.
In this chapter, a minority report on life's chiral molecules of opposite handedness will be given. Life's "desired" amino acids of the unusual D-configuration have been detected in plants (and food), bacteria, and also higher organisms such as frogs, snails, and spiders, but also rats, chickens, and even humans. Biochemical properties and the value of these D-amino acids in specific peptides will be given. We will then widen our view towards toxic and "undesired" amino acids in the living world, as some of these D-enantiomers are discussed to be involved in Alzheimer's disease, processes of eye-lens opacification, and also ageing phenomena. After the death of an organism, amino acids start to interconvert by racemization in a way that
U. Meierhenrich, Amino Acids and the Asymmetry of Life. Advances in Astrobiology 47
and Biogeophysics, © Springer-Verlag Berlin Heidelberg 2008
the ratio between D- and L-amino acids in archaeological and geochemical samples increases according to laws of chemical kinetics. This increase has been successfully measured and could be applied as a dating method called "the amino acid clock". The use of this method is complementary to the well-known 14C-dating method and will be described briefly.
In view of the different theories on the origin of life, such a systematic collection of information on the occurrence of D-amino acids in living organisms is important. Some scientists argue that D-amino acids occurring in biological organisms today might be interpreted as molecular relicts of an ancient life form that was based on these "wrong" enantiomers. According to this theory, left (amino acid)-life and their mirror images right-life forms existed contemporaneously in early times and the selection of left-life occurred during biological evolution long after the first appearance of life on Earth. This model - called the biotic theory - will be discussed at the end of this chapter when we are aware of the occurrence of D-amino acids in living organisms.
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