Sulfonation, sulfurylation, or older, sulfation, in biochemistry is the enzyme-catalyzed conjugation of a sulfuryl group to another molecule. This biotransformation involves a sulfotransferase enzyme catalyzing the transfer of a sulfuryl group from a donor cosubstrate, usually 3'-phosphoadenosine-5'-phosphosulfate (PAPS), to a substrate molecule's hydroxyl or amine. Sulfonation is involved in a variety of biological processes, including detoxification, hormone regulation, molecular recognition, cell signaling, and viral entry into cells. It is among the reactions in phase II drug metabolism, oftentimes effective in rendering a xenobiotic less active from a pharmacological and toxicological standpoint, but sometimes playing a role in the activation of xenobiotics (e.g. aromatic amines, methyl-substituted polycyclic aromatic hydrocarbons). Another example of biological sulfonation is in the synthesis of sulfonated glycosaminoglycans, such as heparin, heparan sulfate, chondroitin sulfate, and dermatan sulfate. Sulfonation is also a possible posttranslational modification of proteins.
Tyrosine sulfonation is a posttranslational modification in which a tyrosine residue of a protein is sulfonated by a tyrosylprotein sulfotransferase (TPST) typically in the Golgi apparatus. Secreted proteins and extracellular parts of membrane proteins that pass through the Golgi apparatus may be sulfonated. Such sulfonation was first discovered by Bettelheim in bovine fibrinopeptide B in 1954 and later found be present in animals and plants but not in prokaryotes or in yeasts. Sulfonation sites are tyrosine residues exposed on the surface of the protein typically surrounded by acidic residues. A detailed description of the characteristics of the sulfonation site is available from PROSITE (PROSITE pattern: PS00003). Two types of tyrosylprotein sulfotransferases (TPST-1 and TPST-2) have been identified.
Sulfonation plays role in strengthening protein-protein interactions. Types of human proteins known to undergo tyrosine sulfonation include adhesion molecules, G-protein-coupled receptors, coagulation factors, serine protease inhibitors, extracellular matrix proteins, and hormones. Tyrosine O-sulfate is a stable molecule and is excreted in urine in animals. No enzymatic mechanism of tyrosine sulfate desulfonation is known to exist. By knock-out of TPST genes in mice, it may be observed that tyrosine sulfonation has effects on the growth of the mice, such as body weight, fecundity, and postnatal viability.
There is very limited evidence that the TPST genes are subject to transcriptional regulation and tyrosine O-sulfate is very stable and cannot be easily degraded by mammalian sulfatases. Tyrosine O-sulfonation is an irreversible process in vivo. An antibody called PSG2 shows high sensitivity and specificity for epitopes containing sulfotyrosine independent of the sequence context.
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