The universal genetic code allows for 20 amino acids to be used in proteins. They can be smaller, bigger, acidic, basic, polar, nonpolar, linear, or with rings—but there are only 20. Of course there are plenty of other amino acids that can be synthesized, but DNA cannot instruct cells to put them into proteins.
Or at least it couldn’t until 2001, when researchers at The Scripps Institute in La Jolla published a paper, entitled "Expanding the genetic code of Escherishia coli," in Science. They described the creation of a unique tRNA/aminoacyl-tRNA synthetase enzyme pair that incorporated a synthetic amino acid into proteins in bacteria. Similar feats have since been accomplished in eukaryotic and mammalian cells, and a new report in PNAS uses the technique to make a modified protein with clinical relevance: human growth hormone (hGH).
Humans who don’t make enough of their own growth hormone can be treated with recombinant hGH, but because it is so small, it is cleared very rapidly by the kidneys. Thus, successful treatment consists of daily injections.
A common way to increase the circulating half-life of peptide drugs in the body, which enables lower doses and/or less-frequent administration, is to link them to polyethylene glycol (PEG). PEG consists of a repeating chain of ethylene oxide moieties. Once they are linked to a peptide, each ethylene glycol subunit becomes tightly associated with two or three water molecules. This has the dual function of rendering the peptide more soluble in water and making it larger. Since the kidneys filter substances by size, the addition of PEG’s molecular weight cuts down on the speed with which they pass through the kidneys. PEG’s globular structure also acts as a shield to protect the peptide from proteolytic degradation, and can reduce the immunogenicity of foreign peptides by reducing uptake by dendritic cells.
Figuring out exactly how and where to best link PEG to a peptide can entail some complicated chemistry. Cho et al. used an expanded genetic code to insert an unnatural amino acid, p-acetylphenylalanine (pAcF), into specific positions in hGH. pAcF is similar to the natural amino acid phenylalanine, but it has a key difference: it bonds well with PEG. The researchers were able to easily link PEG to their modified hGH.
When injected weekly into rats, this PEGylated hGH caused the same amount of weight gain as daily injections of the unmodified hGH. Promising results were also obtained in hGH-deficient humans. The modified hGH was well tolerated, causing only transient and minor reactions at the injection sites.
The authors note that "the twenty canonical amino acids derived from strict evolutionary pressure have evolved protein sequences for physiological, but not therapeutic, purposes." The marriage of synthetic chemistry with recombinant DNA technology such as the approach used here can expand the genetic code and allows the insertion of unnatural amino acids into peptides and proteins. The authors suggest that this technology should be exploited to develop and optimize a new class of therapeutic options.