NNAs

NNAs is an acronym for non-natural amino acids. Non-natural amino acids are amino acids that are not among the twenty canonical proteinogenic amino acids used to build most natural proteins. They can have modified side chains, unusual stereochemistry, or additional functional groups, and they are used to expand the chemical repertoire of proteins for research, biotechnology, and medicine.

Incorporation into proteins is typically achieved through genetic code expansion. This involves orthogonal tRNA and aminoacyl-tRNA synthetase pairs that specifically recognize the chosen NNA and a defined codon, often the amber stop codon (UAG), to insert the NNA at a predetermined site. Alternative approaches include reassignment of sense codons or the use of expanded or recoded translation systems, in cells or in cell-free translation setups. These methods enable site-specific incorporation with varying efficiency and fidelity across host organisms.

Applications of NNAs include site-specific labeling for imaging, introduction of photo-crosslinkers to map interactions, attachment of affinity tags or reactive handles for conjugation, and incorporation of chemical functionalities that mimic posttranslational modifications or enhance protein properties such as stability or catalytic activity. NNAs are also used in peptide therapeutics and synthetic biology to create proteins with novel functions.

Challenges include limited incorporation efficiency and fidelity, potential toxicity or metabolic burden on host cells, and constraints on uptake or biosynthesis in certain organisms. Regulatory considerations can also arise for therapeutics that contain NNAs. Notable examples of widely used NNAs are p-azidophenylalanine and p-benzoyl-L-phenylalanine, among others.