Fig. 2. General PTMs in proteins.

Specific amino acids such as Ser (S), Thr (T), Lys (K), Arg (R), and Tyr (Y) are reversibly changed by various modifying enzymes. Ser, Thr, and Tyr residues are commonly phosphorylated by kinase and dephosphorylated by phosphatase, and OGT glycosylation of Ser and Thr is induced by OGT but is reversibly removed by O-GlcNAcase (OGA). Both Lys and Arg are methylated by transferring methyl group from S-adenosylmethionine (SAM) and selectively acetylated. While Lys is conjugated with ubiquitin (ub) and SUMO by the action of ligases, Arg is converted to citrulline by PADs. Ac, acetylation; Cit, citrullination; G, glycosylation; Me, mono- & di-methylation; SUMO, SUMOylation; P, phosphorylation; ub, ubiquitination.