Genetic codes expanded

V.W. Cornish, D. Mendel, P.G. Schultz, Probing protein stmcture and function with an expanded genetic code, Angew. Chem. Int. Ed. 34(6) (1995) 621-633. 

J.C. Anderson, N. Wu, S.W. Santoro, V. Lakshman, D.S. King, P.G. Schultz, An expanded genetic code with a functional quadmplet codon, Proc. Natl. Acad. Sci. USA 101(20) (2004) 7566-7571. 

Chung, H.-H., Benson, D. R., and Schultz, P. G. (1993). Probing the structure and mechanism of ras protein with an expanded genetic code. Science 259, 806-809. 

Xie J, Schultz PG. A chemical toolkit for proteins—an expanded genetic code. Nat. Rev. Mol. Cell Biol. 2006 7 775-782. 

DNA-strand exchange between a ssDNA and a duplex in which all G and C residues have been replaced by 2 -deoxyisoguanosine (iG) and 2 -deoxy-5-methyl-isocytidine MiC) by the E. coli RecA protein in vitro occurred at a similar rate and efficiency to unmodified DNA. This provides further potential for the role of iG and MiC in an expanded genetic code. Using ODNs rich in isoguanosine residues, Chaput and Switzer have shown that iG quintet structures may be formed from a metal-assisted hydrogen bond-mediated self-assembly process. The structures were stabilised particularly in the presence of Cs+ ions. 

Schultz, An expanded genetic code with a functional quadruplet codon, Proc. Natl. Acad. Sci. U.S.A. 2004,... 

Cropp A T, Schultz P G (2004). An expanding genetic code. Trends Genet. 20 625-630. 

Nevertheless, current knowledge of biochemical systems and synthetic techniques may allow us to explore them from a slightly different perspective. Namely, what would life look like with an expanded genetic code—that is, with additional amino acids added into the proteins of life. Indeed, over the past few years chemists have been able to utilize native biochemical systems as well as evolved tRNA molecules (which we will discuss in Chapter 25) to load many unique amino acids into proteins of interest at any specific point desired in a number of different cells, including those of yeast, some mammals, and bacteria like E. coli. Some of the unnatural amino acids are shown below. They include ones with unique metals (like selenium), reactive functional groups (such as a ketone and an azide) that can be used for additional chemistry, and a boronic acid that can be used to bind certain sugars covalently. These synthetic amino acids are all derivatives of phenylalanine, but many other amino acid parent structures can be used as well. 

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