Reaction catalyzed by polypeptide N-acetylgalactosaminyltransferase (2.4.1.41)
+
=
+
UDP-N-acetyl-alpha-D-galactosamine
+
[protein]-L-threonine
=
UDP
+
[protein]-3-O-(N-acetyl-alpha-D-galactosaminyl)-L-threonine