Structural and Antigenic Diversity in Mucin Carbohydrate Chains

Abstract

There is considerable information on the structure and genetics of the peripheral regions of the complex carbohydrate chains of mucins which vary depending on blood group isotype specificity (1). However, much less is known of the detailed structures and biosynthetic control within the backbone and core regions of these chains. The backbones consist of either Type 1,

$$\begin{array}{*{20}c} {{\rm{Ga1\beta 1}} \to {\rm{3G1cNAc\beta 1}} \to, } & {{\rm{or Type 2,}}} & {{\rm{Ga1\beta 1}} \to {\rm{4G1cNAc\beta 1}} \to {\rm{,}}} \\\end{array}$$

sequences (1) and they vary in length and branching. Type 2 sequences in the liner repeating structure

$${\rm{Ga1\beta 1}} \to 4{\rm{G1cNAc\beta 1}} \to {\rm{3Ga1\beta 1}} \to {\rm{4G1cNA\beta 1}} \to $$

are recognized by anti-blood group i antibodies (2). Addition of a ${\rm{Ga1\beta 1}} \to 4{\rm{G1cNAc\beta 1}} \to 6$ branch onto the internal galactose of this structure confers blood group I activity (3). Type 1 sequences are apparently not involved in the Ii antigenic system. Species and individual differences in blood group I and i activities may therefore reflect differences in branching patterns and the proportions of Type 1 and Type 2 chain sequences. Ii activities may also be masked by further glycosylation, for example by the addition of sugar residues confering blood group ABH activities.