sentences:  PAPS enters one site of TPST and the sulfonate group is transferred to a Histidine residue in the enzyme and PAP is release.Then, the target protein and tyrosine bind TPST and the histidine transfers the sulfonate group to the target tyrosine.Based on crystal structure of TPST-2 with C4 complement and PAP, an SN2-like in-line displacement mechanism has been proposed.