Abstract
The biosynthesis of proteins containing unnatural amino acids, alloproteins, is a promising way of expanding the structural and chemical diversity in proteins. Thus, chemical probes, photo-crosslinkers, heavy atoms, and the unique groups for chemical conjugate have been incorporated into proteins. Unnatural amino acids can be attached to the adaptor tRNAs by the method of chemical aminoacylation, or by the use of the wild-type and engineered aminoacyl-tRNA synthetases. These tRNAs recognize the amber codon, four-base codons, the artificial codons with unnatural bases, and sense codons. The first three types of these codons can be used for the site-specific incorporation of unnatural amino acids into proteins. Such “site-specific alloproteins” have been synthesized in cell-free protein synthesis systems, Xenopus oocytes, the E. coil cells, yeasts, and cultured mammalian cells. The availability of the various aminoacylation methods, the various codons for unnatural amino acids, and the various translation systems adds to the usefulness of unnatural amino acids in proteins.
