# Roles of D-Amino Acids on the Bioactivity of Host Defense Peptides
## 2 Natural d-Amino Acids (d-AA)-Containing HDPs  
Biosynthesized d-AA-containing proteins was first discovered in 1927 [24] and were once thought to be an exception because the presence of stereoisomers contradicts the classical central dogma stating that ribosomes cannot utilize anything else other than canonical l-AAs. However, subsequent research had revealed that not only are d-AAs widely prevalent but also that most organisms are capable of generating diastereomeric proteins [25]. Natural diastereomers can be produced by a variety of mechanisms, for instance, by spontaneous racemization of l-AA residues. This type of diastereomers is primarily located in metabolically inert proteins of aging tissues such as the elastin, myelin and crystallin of elderly people [26] where the original l-enantiomeric proteins had sufficient time for amino acids to form and accumulate [27]. For a long time, this process was thought to be the only manner in which diastereomeric proteins could be created in animals and that the racemization had no functional meaning. However, subsequent research had revealed the importance of D-residues for biological functions. Before the discovery of diastereomer synthesis in animals, it was known that bacterial cell wall peptidoglycans possessed d-AAs. These form a class of non-ribosomal peptides/proteins where racemase catalyzes peptide epimerization prior to amino acid activation or during chain elongation [28]. Dermorphin from the skin secretions of the tree frog _Phyllomedusa suavagii_ was the first animal discovered to have a ribosomally synthesized diastereomeric peptide [29], which surprised the scientific community at the time because all known evidence indicated that ribosomes cannot utilize d-AAs. Since then, a considerable number of animal ribosomal proteins containing d-AAs were discovered. In all cases, however, the genetic codon encoding the d-AA is the classical one [25]; hence, the genetic material by itself does not provide information about which residue should be a d-AA. Furthermore, there is no evidence that ribosomes are able to directly incorporate d-AAs into a peptide chain; thus, the only way for ribosomally synthesized proteins to actively acquire d-AAs is via modifications of peptides/proteins that contain all l-AAs [30; 31; 32]. Notably, certain members of the lantibiotic bacteriocin antimicrobial peptide (AMP) family are natural diastereomers that gain their diastereomeric property via post-translational enzymatic modification. Particularly, the AMP nisin belongs to this family and is used as a food preservative [33]. As of now, all available evidences indicated that ribosomal diastereomers are first synthesized as all-l precursors, whereas isomerization occurs through post-translational modification events. Neither direct incorporation of d-AAs into a precursor nor excision of L-residues followed by reinsertion of D-residues has ever been experimentally confirmed [25]. The biological function of natural diastereomers is diverse, where in some cases the function of d-AAs may be unclear while in others (i.e., the stochastic racemization of aging metabolically inert proteins) they may not have any biological significance at all. In the development of synthetic therapeutic HDPs, it is well known that the introduction of d-AAs tends to enhance the activity and stability of the peptide. Similarly, the natural incorporation of D-residues can have the same function. The presence of d-AAs in certain venom peptides of spiders and platypus have been shown to increase both the venom effective lifetime in the prey as well as its potency [34]. The presence of d-AAs can also be crucial for receptor recognition. For example, in the case of dermorphin, the corresponding opioid receptors can bind dermorphin only if the Ala residue at the C-terminal is in the d-form. Synthetic all-l dermorphin does not exhibit any biological activity [35].