The Technology of Incorporating Unnatural Amino Acids into Proteins, Expanding the Genetic Code, and Applications

An investigator interested in modifying a protein’s characteristics or in designing a protein de novo has a variety of methods at his or her disposal. These techniques include conventional site-directed mutagenesis, where one can designate the replacement of specific amino acid residues in a target polypeptide chain with any of the other 19 naturally occurring amino acids (Hendrickson et al. 2004), 21 if we consider Pyl and Sec (Sackmann-Sala, 2009). Another technique for protein engineering is that of solid-phase peptide synthesis, which allows for many modifications to protein structure but is currently limited to the production of smaller size proteins (Wang et al. 2006). Techniques allowing the incorporation of novel or “unnatural ” amino acids into proteins hold great promise for expanding molecular knowledge. Hypotheses about the structure-function relationship of proteins might best be tested using a molecular tool that could site-specifically insert any chemical structure into a protein. Insertion of amino acid residues beyond the 20 “natural ” amino acids can allow the inclusion of groups having side chains with finely tuned chemical characteristics (for example see Magliery et al. 2005). Some characteristics no