D-Amino Acid
The mirror-image form of the standard L-amino acids found in human proteins. Incorporating D-amino acids into synthetic peptides is a common strategy to improve resistance to enzymatic degradation, since most human proteases only recognise and cleave L-amino acid sequences.
Technical Context
Natural proteins are composed exclusively of L-amino acids (left-handed stereochemistry). D-amino acids (right-handed) are their mirror images. Since human proteases have evolved to recognise and cleave L-amino acid sequences, D-amino acid incorporation at key positions creates peptide bonds that resist enzymatic degradation. D-amino acids can also alter the peptide's secondary structure and receptor binding properties. Certain D-amino acid substitutions are tolerated at non-critical binding positions while dramatically improving metabolic stability. Some naturally occurring antimicrobial peptides from bacteria (e.g. gramicidin) contain D-amino acids as a natural resistance mechanism against host proteases.