Stereochemistry of the a-Amino Acids

When a carbon atom has four different substituents attached to it, it is said to be chiral, or a stereocenter.

All a-amino acids have a stereocenter at the a-carbon.

The sole exception is glycine, whose R-group is hydrogen, therefore is achiral.

a-Amino acids’ stereochemistry is designated as D- or L-, which is best visualized from its Fischer projection

There is a preference for L-amino acids in proteins

Compare this to the preference for D-configured carbohydrates.

The preference for L-amino acids in natural proteins has two important consequences:

The surface of any given protein, which is where the interesting biochemistry occurs, is asymmetric. This asymmetry is the basis for the highly specific molecular recognition of binding targets by proteins.

The stereochemistry of the amino acids plays an important role in the formation of so-called “secondary structure” (i.e., a-helices and b-strands) and thereby the overall structure of proteins.