Biosynthesis of heparin. Concerted action of late polymer-modification reactions.
The substrate specificity of O-sulfotransferases involved in the biosynthesis of heparin was studied by incubating exogenous polysaccharide acceptors with mouse mastocytoma microsomal fraction in the presence of phosphoadenylyl[35S]sulfate. Characterization of the labeled products showed that O-sulfation occurs preferentially in the vicinity of N-sulfate groups; that 2-O-sulfation of L-iduronic acid residues occurs preferentially or exclusively in the absence of a 6-O-sulfate group on adjacent D-glucosamine units; and that 6-O-sulfation of D-glucosamine residues occurs readily in the presence of 2-O-sulfate groups on adjacent L-iduronic acid units. Furthermore, structural analysis of microsomal heparin-precursor polysaccharides showed a distinct intermediate species that contained 2-O-sulfated L-iduronic acid units but essentially no 6-O-sulfate groups on the (N-sulfated) D-glucosamine residues. The results suggest that 2-O-sulfation of L-iduronic acid units is tightly coupled to the formation of these units (by 5-epimerization of D-glucuronic acid residues) and, furthermore, that both processes are completed before 6-O-sulfation of the polysaccharide molecule is initiated. D-Glucuronosyl 5-epimerization not accompanied by 2-O-sulfation occurs at a still earlier stage of polymer modification; the resulting L-iduronic acid units appear to remain nonsulfated throughout the subsequent modification reactions.
D000818:Animals; D002236:Carbohydrate Conformation; D002240:Carbohydrate Sequence; D004187:Disaccharides / Q000032:analysis; D006493:Heparin / Q000096:biosynthesis*; D012515:Mast-Cell Sarcoma / Q000201:enzymology; D051379:Mice; D008861:Microsomes / Q000201:enzymology*; D009374:Neoplasms, Experimental / Q000201:enzymology; D013379:Substrate Specificity; D013462:Sulfur Radioisotopes; D013466:Sulfurtransferases / Q000378:metabolism*
D004187:Disaccharides; D013462:Sulfur Radioisotopes; D006493:Heparin; D013466:Sulfurtransferases