Reaction catalyzed by galactosylgalactosylxylosylprotein 3-beta-glucuronosyltransferase (2.4.1.135)
+
=
+
UDPglucuronate
+
3-beta-D-galactosyl-D-galactose
=
UDP
+
3-beta-D-glucuronosyl-3-beta-D-galactosyl-D-galactose