Structures of the I and i antigens and their biosynthetic pathways.

The structure of the i antigen determinant is a linearN-acetyllactosamine unit repeated, and that of the I antigen determinant is a branched poly-N-acetyllactosamine. Conversion of the i structure to the branched I structure is catalyzed by I-branching β1-6-N-acetylglucosaminyltransferase (IGnT). Centrally acting IGnT is illustrated in panel A, whereas peridistally acting IGnT (C2GnT-2) presented in panel B catalyzes the I-branching reaction using an acceptor substrate with the terminal structure GlcNAcβ1-3Galβ1-4GlcNAcβ1-R instead of Galβ1-4GlcNAcβ1-3Galβ1-4GlcNAcβ1-R.