The incorporation of unnatural amino acids (UAAs) into proteins has transformed chemical biology, enabling site-specific labeling, bioorthogonal chemistry, and the synthesis of proteins with novel functions. Several complementary strategies are now available, each with distinct advantages depending on the scale, fidelity, and chemical functionality required.
Amber suppression (stop codon suppression) is the most widely used approach for genetic code expansion. An orthogonal tRNA/aminoacyl-tRNA synthetase (aaRS) pair is engineered to recognize the UAG stop codon and charge it with the desired unnatural amino acid. This method enables site-specific incorporation at any position defined by amber codon placement in the gene.
Quadruplet codon suppression uses engineered ribosomes and tRNAs that recognize four-nucleotide codons, freeing up codon space for UAA incorporation without competing with natural stop codon readthrough. This approach allows simultaneous incorporation of multiple distinct UAAs in a single protein.
Cell-free protein synthesis (CFPS) systems offer an open platform where all translation components can be manipulated directly. By supplementing purified CFPS reactions with orthogonal aaRS/tRNA pairs and the desired UAA, researchers achieve incorporation efficiencies often exceeding those possible in live cells.
For simpler applications where site-specificity is not required, chemical modification of natural amino acid side chains (e.g., NHS ester acylation of lysines, maleimide conjugation to cysteines) remains a practical alternative. However, this approach lacks the site-selectivity achieved with genetic code expansion.
SynthAxis supplies a comprehensive range of UAA building blocks including azidohomoalanine, propargylglycine, bicyclo[1.1.1]pentane amino acids, and various Fmoc-protected derivatives for direct SPPS incorporation — supporting both cell-based genetic code expansion experiments and purely synthetic peptide programs.