Heparan/chondroitin sulfate biosynthesis - Structure and mechanism of human glucuronyltransferase I

Human beta1,3-glucuronyltransferase I (GlcAT-I) is a central enzyme in the initial steps of proteoglycan synthesis. GlcAT-I transfers a glucuronic aci d moiety from the uridine diphosphate-glucuronic acid (UDP-GlcUA) to the co mmon linkage region trisaccharide Gal beta1-3Gal beta1-4Xyl covalently boun d to a Ser residue at the glycosaminylglycan attachment site of proteoglyca ns, We have now determined the crystal structure of GlcAT-I at 2.3 Angstrom in the presence of the donor substrate product UDP, the catalytic Mn2+ ion , and the acceptor substrate analog Gal beta1-3Gal beta1-4Xyl, The enzyme i s a alpha/beta protein with two subdomains that constitute the donor and ac ceptor substrate binding site. The active site residues lie in a cleft exte nding across both subdomains in which the trisaccharide molecule is oriente d perpendicular to the UDP. Residues Glu(227), Asp(252), and Glu(281) dicta te the binding orientation of the terminal Gal-2 moiety. Residue Glu(281) i s in position to function as a catalytic base by deprotonating the incoming 3-hydroxyl group of the acceptor. The conserved DXD motif (Asp(191), Asp(1 95), Asp(196)) has direct interaction with the ribose of the UDP molecule a s well as with the Mn2+ ion. The key residues involved in substrate binding and catalysis are conserved in the glucuronyltransferase family as well as of her glycosyltransferases.