Evidence for the oxidant-mediated amino acid conversion, a naturally occurring protein engineering process, in human cells

Reactive oxygen species (ROS) play an important role in the development of various pathological conditions as well as aging. ROS oxidize DNA, proteins, lipids, and small molecules. Carbonylation is one mode of protein oxidation that occurs in response to the iron-catalyzed, hydrogen peroxide-dependent oxidation of amino acid side chains. Although carbonylated proteins are generally believed to be eliminated through proteasome-dependent degradation, we previously discovered the protein de-carbonylation mechanism, in which the formed carbonyl groups are chemically eliminated without proteins being degraded. Major amino acid residues that are susceptible to carbonylation include proline and arginine, both of which are oxidized to become glutamyl semialdehyde, which contains a carbonyl group. The further oxidation of glutamyl semialdehyde produces glutamic acid. Thus, we hypothesize that through the ROS-mediated formation of glutamyl semialdehyde, the proline, arginine, and glutamic acid residues within the protein structure are interchangeable. In support of this hypothesis, mass spectrometry demonstrated that proline 45 (a well-conserved residue within the catalytic sequence) of the peroxiredoxin 6 molecule can be converted into glutamic acid in cultured human cells, establishing a revolutionizing concept that biological oxidation elicits the naturally occurring protein engineering process.