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Carbohydrate sulfotransferases are sulfotransferase enzymes that transfer sulfate to carbohydrate groups in glycoproteins and glycolipids. Carbohydrates are used by cells for a wide range of functions from structural purposes to extracellular communication. Carbohydrates are suitable for such a wide variety of functions due to the diversity in structure generated from monosaccharide composition, glycosidic linkage positions, chain branching, and covalent modification. Possible covalent modifications include acetylation, methylation, phosphorylation, and sulfation. Sulfation, performed by carbohydrate sulfotransferases, generates carbohydrate sulfate esters. These sulfate esters are only located extracellularly, whether through excretion into the extracellular matrix (ECM) or by presentation on the cell surface. As extracellular compounds, sulfated carbohydrates are mediators of intercellular communication, cellular adhesion, and ECM maintenance. ==Enzyme mechanism== Sulfotransferases catalyze the transfer of a sulfonyl group from an activated sulfate donor onto a hydroxyl group (or an amino group, although this is less common) of an acceptor molecule.〔 In eukaryotic cells the activated sulfate donor is 3'-phosphoadenosine-5'-phosphosulfate (PAPS) (Figure 1). PAPS is synthesized in the cytosol from ATP and sulfate through the sequential action of ATP sulfurylase and APS kinase. ATP sulfurylase first generates adenosine-5'-phosphosulfate (APS) and then APS kinase transfers a phosphate from ATP to APS to create PAPS. The importance of PAPS and sulfation has been discerned in previous studies by using chlorate, an analogue of sulfate, as a competitive inhibitor of ATP sulfurylase. PAPS is a cosubstrate and source of activated sulfate for both cytosolic sulfotransferases and carbohydrate sulfotransferases, which are located in the Golgi. PAPS moves between the cytosol and the Golgi lumen via PAPS/PAP (3’-phosphoadenosine-5’-phosphate) translocase, a transmembrane antiporter. The exact mechanism used by sulfotransferases is still being elucidated, but studies have indicated that sulfotransferases use an in-line sulfonyl-transfer mechanism that is analogous to the phosphoryl transfer mechanism used by many kinases, which is logical given the great level of structural and functional similarities between kinases and sulfotransferases (Figure 2). In carbohydrate sulfotransferases a conserved lysine has been identified in the active PAPS binding site, which is analogous to a conserved lysine in the active ATP binding site of kinases. Protein sequence alignment studies indicate that this lysine is conserved in cytosolic sulfotransferases as well.〔 In addition to the conserved lysine, sulfotransferases have a highly conserved histidine in the active site. Based on the conservation of these residues, theoretical models, and experimental measurements a theoretical transition state for catalyzed sulfation has been proposed (Figure 3).〔 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「carbohydrate sulfotransferase」の詳細全文を読む スポンサード リンク
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