Recent Data on the Structure of the Carbohydrate Moiety of Glycoproteins. Metabolic and Biological Implications

The author reviews the information acquired in the last four years on the structure of isoglycans (branched glycans) N-glycosidically linked to asparagine. Developments of the structural methodology of conjugated carbohydrates are analysed with reference to the following procedures: sugar determination, chemical (partial acid hydrolysis, acetolysis, hydrazinolysis—nitrous deamination), and enzymatic (exoand endoglycosidases) cleavage of polysaccharide chains, and methylation. Several structures of glycans are described. It appears that all possess the common pentasccharidic core of the following mannotriosido-di-N-acetylchitobiose on which are conjugated either neuraminyl (or fucosyl)-N-acetyllactosamine or oligomannoside residues. It is proposed to term these glycan structures, respectively, N-acetyllactosaminic type and oligomannosidic type. The author demonstrates that these structures are in a good agreement with recent data concerning, on the one hand, the biosynthesis and pathological catabolism of glycoproteins and, on the other hand, the biological role of carbohydrate moieties considered as cell recognition signals. INTRODUCTION Four years ago, in a lecture given at the Fifth International Symposium on Carbohydrate Chemistry, I reviewed the' present state of knowledge concerning the structure of the carbohydrate moiety of the glycoproteins. The conclusion was that about one century after the discovery of these compounds, no structure was known, except those of some linear glycans (n-glycans). Our ignorance was due, partly, to the microheterogeneity of glycoproteins and glycans and partly to the lack of precise methods for structural investigation of the branched glycans (iso-glycans). The review82 concluded by emphasizing the fact that 'an enormous methodological work 431