A carbon atom can form four separate covalent bonds. In order to minimize steric interactions between the four groups bonded to carbon, carbon adopts a tetrahedral geometry. when the four groups bonded to carbon are all different, there are two distinct ways to arrange the four bonds in three dimensional space. These are shown in Figure 5.5.

Notice that the two molecules are mirror images of each other and that there is no way to superimpose the two isomers. Isomers whose atoms are connected in the same way, but differ in the arrangement of their atoms in space are called stereoisomers. Stereoisomers that are nonsuperimposable mirror images of each other are called enantiomers. The central carbon atom of a stereoisomer - the carbon to which the four different groups are attached - is called a center of asymmetry or an asymmetric carbon, a center of chirality or a stereocenter. Keep in mind that the four groups linked to a carbon atom must be different in order for the carbon atom to be asymmetric. If only three different groups are linked to the carbon, no asymmetry exists.

There are two possible three-dimensional configurations for each asymmetric carbon in a molecule. Thus, a molecule with two asymmetric carbons will have four possible three-dimensional structures, one with three symmetric carbons will have eight possible three-dimensional structures, etc.

All amino acids involved in making proteins, except glycine, have an asymmetric carbon. The two enantiomers are designated D- and L- (see above, for alanine). In theory, therefore, the two possible enantiomers of each amino acid give rise to a huge number of possible three-dimensional structures for a protein, because the three-dimensional structure of a protein depends on the three-dimensional structures of it constituent amino acids. This would wreak havoc in a cell, however, because the exact three-dimensional structure of a protein is essential for its activity. Thus, it turns out that all of the amino acids in proteins have the L-configuration, although there is no obvious reason for all L instead of all D, as shown in Figure 5.5

It is also worth noting that random chemical reactions give a mixture of equal amounts of D and L products. To make a specific isomer, such as D or L, requires a catalytic site on an enzyme that only produces one of the forms. All biologically produced -amino acids for proteins are in the L form.

Researchers looking for signs of extraterrestrial life often examine the structures of amino acids found in meteorites to see if they have a large imbalance of D/L amino acids.