Modification of Tyrosine Residues
Tyrosine represents approximately 3%–4% of the amino composition of an average protein.1-3 There are differences in composition relating to protein size and cellular location; however, these are not signicant for tyrosine.4 Some amino acids such as cysteine/cystine, arginine, and lysine are markedly increased in small proteins, while aspartic acid and glutamic acid are decreased in small proteins.5 Pace and coworkers6 have summarized the pKa values for the various ionizable groups in proteins. They reported that 67% of the tyrosine residues were buried with an average pKa value of 10.3 with a low value of 6.1 and a high value of 12.1. The pKa for the tyrosyl residue in the pentapeptide, AlaAlaTryAlaAla, was found to be 9.8. Tyrosine is considered to be a hydrophobic amino acid7,8 and is found at protein interfaces.8-15 As one example of this function, Ploug and coworkers16 used TNM to identify tyrosine residues at the interface between urokinase-type plasminogen activator and a glycolipid-anchored receptor.