The genetic code consists of 64 triplets of nucleotides called codons. With the exception of three codons, each codon encodes for at least one of the 20 canonical amino acids and most of the amino acids are encoded by more than one codon.
The genetic code may be seen as a universal language shared by all living organisms. It is imperative for the interpretation of genes and the production of proteins.
Moreover, it is a precise guide for the construction of the genome along fundamental and biochemical constraints. This played a key role in allowing the code to be conserved over the course of 3 billion years and enables it to shape how mutations affect the evolution of the genome.
In order to add new building blocks to the existing 20-amino acid repertoire, unique aminoacyl-tRNA synthetase and tRNA pairs are needed together with a source of the amino acid and a specific amino acid codon.
To date, over 70 unnatural amino acids (UAAs) have been added into E. coli, yeast and mammalian cells. These UAAs possess features, such as special labels and unique functional groups. These special properties allow for the endless possibilities for the usage of UAAs with regards to probing protein function and structure both in vitro and in vivo.
Our canonical 20 amino acids are clearly more than sufficient to support our existence. However, our ability to manipulate the biological and physicochemical properties of proteins may confer several advantages, especially from an evolutionary standpoint.
UAAs can be rationally exploited to investigate and perhaps even remedy biological problems that involve proteins. To this end, UAAs open the door to engineering an extensive range of chemical, electrical and structural properties that may prove otherwise very challenging and/or non-existent in the common 20 amino acids.
Adding chemically reactive groups to UAAs can allow them to function as bio-orthogonal handles for specific sites in vitro. In addition to this, they may be able to function as intracellular protein modifiers and can be used to directly introduce novel or enhanced catalytic properties to proteins.
Selective chemistries that can be used to engineer such novelties include oxime condensation reactions and click chemistry. Among others, UAAs may also be used as heavy atoms for determining the structure of X-rays, redox-active reagents and probes of hydrogen bonding as well as interactions in proteins.
UAAs may be widely used for therapeutic purposes - they are particularly useful in cases where large quantities of a modified protein are required for production. For instance, immunogenic amino acids may be used to generate vaccines against self-proteins by the breakdown of immunological tolerance in conditions, such as inflammation or cancer.
UAAs can also be used to generate vaccines against the conserved epitopes of diseases like HIV and malaria, which are fairly difficult to target with our traditional vaccines.