Question

Which stereoisomer (D or L) of amino acids is biologically active? Why is this important when considering molecular interactions within cells?

Answer 1

L-amino acids are biologically active and are components of proteins in the cell. A few proteins in bacteria do contain D-amino acids. But, they are incorporated/modified post-translationally.

The biological machinary that is responsible for protein synthesis is highly stereo-selective. It has evolved to recognize only L-amino acids. Almost all the metabolic reactions in the cell are dependent on proteins/enzymes. These proteins can recognize only L-amino acids.

D-amino acids are mirror images of L-amino acids. If a protein has to recognize a D-amino acid, its active site must be a mirror image of the enzyme that recognizes the L-amino acid.