Principles of Biochemistry Volume 1 - A. Lehninger 1985

Biomolecules
Water
Optically active amino acids in proteins are L-stereoisomers

Almost all naturally occurring biological compounds containing a chiral center exist exclusively in a single stereoisomeric form, either D or L. With the exception of Glycine, which lacks an asymmetric carbon atom, all Amino Acids that make up protein molecules are L-stereoisomers. This Conclusion is based on numerous meticulous chemical studies comparing the Optical Properties of amino acids with their behavior in Chemical Reactions. As we will see below, certain D-amino acids also occur in living nature, but they are never incorporated into Proteins.

The exclusive presence of L-stereoisomers of amino acids in proteins is quite remarkable, given that conventional Chemical reactions used to synthesize compounds with an asymmetric carbon atom always yield optically inactive products. This occurs because standard chemical reactions generate both D- and L-stereoisomers at equal rates. The result is a racemic mixture, or racemate—an equimolar mixture of D- and L-isomers that is optically inactive. A racemic mixture can be separated into its D- and L-isomers only through highly labor-intensive Methods based on differences in the Physical Properties of the stereoisomers. Over time, separated D- and L-isomers spontaneously revert to a racemic mixture (see Supplement 5-2).

Supplement 5-2. How to Determine Human Age Using Amino Acid Chemistry

Optical isomers of amino acids undergo very slow, spontaneous non-enzymatic racemization, meaning that over a sufficiently long period, a pure L- or D-isomer can convert into an equimolar mixture of D- and L-isomers. The rate of racemization for each L-amino acid is constant at a given Temperature. This phenomenon can be used to determine the age of humans and animals, as well as fossilized organic remains. For example, in dentin, a protein found in hard tooth enamel, L-aspartate undergoes spontaneous racemization at human body temperature at a rate of 0.10% per year. In children, while Teeth are still forming, dentin contains exclusively L-aspartate. It is possible to isolate dentin from a single tooth and measure its D-aspartate content. Such analyses have been performed on the dentin of residents in mountain villages of Ecuador, many of whom exaggerated their age. Since this raised doubts in several cases, the racemization test was applied to verify their age and proved to be remarkably accurate. For instance, for a 97-year-old woman whose age was documented by records, the test yielded an estimated age of 99 years.

Tests performed on fossilized remains of prehistoric animals—such as elephants, dolphins, and bears—have demonstrated that data obtained by this method correlate closely with the results of radiometric dating based on radioactive isotope decay rates.

Living Cells possess a unique ability to synthesize L-amino acids using stereospecific Enzymes. The stereospecificity of these enzymes is due to the asymmetric nature of their active sites. As we will see below, the characteristic three-dimensional Structure of proteins, which enables their diverse biological activities, arises only when all constituent amino acids belong to the same stereochemical series.