Stereochemistry

Introduction to Protein Structure

Stereochemistry of the Amino Acids

The a carbon of all of the amino acids, except glycine, is a chiral center. Thus, the amino acids are optically active, and each amino acid exists in two enantiomeric forms. These forms are designated as "D-" and "L-" by comparison to the stereoisomers of glutaraldhyde.

The L-forms of the amino acids are most common. Some small peptides contain both D- and L-amino acids; these compounds are synthesized non-ribosomally. Polypeptides which are synthesized using ribosomes are made exclusively from L-amino acids.

The fact that proteins are made of only L-amino acids, and are therfore chiral, is important in understanding their function. Many proteins are exquisitely stereospecific. Some enzymes bind one stereoisomer of a compound with a thousand times higher affinity than the related molecules.

Rotate these models to convince yourself that these two molecules are, in fact, enantiomers (i.e non-superimposable mirror images).


D-alanine	L-alanine

If terms like "chiral center" and "enantiomer" sound only vaguely familiar, a refresher course on stereochemistry can be found at "Isomers of Organic Compounds"!