Among all PTMs, phosphorylation is one of the most abundant and important modifications. The phosphate group is a highly negatively charged molecule with a tetrahedral structure around the phosphorus atom. Phosphorylated peptides mainly refer to the modification of the side chain hydroxyl groups of Ser, Tyr, and Thr residues in the peptide chain into acid phosphate peptides. 

Many hormones regulate the activity of specific enzymes by increasing the phosphorylation status of serine (Ser) or threonine (Thr) residues. Phosphorylation is highly correlated with human diseases. For example, the excessive/multiple phosphorylation of tau, alpha synuclein, and huntingtin is highly correlated with human diseases, and aggregation phosphorylation is also one of the main causes of neurodegenerative diseases such as Alzheimer's disease, Parkinson's disease, and Huntington's disease. At present, the main research on phosphorylated amino acids is focused on hydroxyl phosphate monoester phosphorylated amino acids (O-phosphorylated amino acids), namely phosphorylated serine, phosphorylated threonine, and phosphorylated tyrosine.