Human Biochemistry, Volume 2 - Murray R. 1993

Special Topics
Glycoproteins and Proteoglycans
Glycoproteins - Exoglycosidases and Endoglycosidases

A series of glycosidases with defined Specificity is used to study the Structure and function of Glycoproteins (Table 54.4). These Enzymes act either on outer (exoglycosidases) or inner (Endoglycosidases) positions of oligosaccharide chains. Examples of exoglycosidases include neuraminidases and galactosidases; their sequential application leads to the removal of terminal NeuAc residues and pre-terminal Gal residues from most glycoproteins. Endoglycosidases F and H serve as examples of the second group of glycosidases; these enzymes cleave oligosaccharide chains at sites harboring specific GlcNAc residues adjacent to the polypeptide backbone (i.e., at internal positions) and can therefore be used to obtain large oligosaccharide chains required for structural analysis. Treatment of a glycoprotein with one or more glycosidases makes it possible to assess The Effect of removing specific sugars on the biological properties of glycoproteins.

Experiments conducted by Ashwell in the early 1970s demonstrated that the "exposure" of Gal residues through neuraminidase treatment leads to the rapid disappearance of ceruloplasmin from plasma. Subsequent research showed that Liver Cells contain a receptor that recognizes the galactosyl component of many desialylated (i.e., lacking NeuAc) Plasma Proteins and mediates their endocytosis. These findings indicate that an individual sugar can play a crucial role in regulating at least one of the biological properties of certain glycoproteins—their circulatory half-life.

Class="center">Table 54.4. Selected glycosidases used to study the structure and function of glycoproteins. These Enzymes can be derived from various sources and often exhibit specificity toward particular types of glycosidic bonds, including anomeric configurations. Fig. 54.4 shows the sites of action for endoglycosidases F and H. Form F acts on both high-mannose and complex oligosaccharide chains, whereas endoglycosidase H attacks only type 1 chains.