Collagen protein-carbohydrate link

A further relevance of glycoproteins to collagen is the nature of the protein-carbohydrate link found. Blumenfeld and Gallop (1962b) have shown that aspartic acid is involved in the ester-like links of collagen which may be binding hexose molecules. Aspartic acid also constitutes the point of attachment of carbohydrate in most glycoproteins studied to date. 

Figure 1.10 Various Other Types of Protein-Saccharide Link. Many other types of protein-carbohydrate link are known. Structure (a) is common to many collagens, including those of mammals (b) is found in earthworm cuticular collagens and (c) is known from the collagen of Nereis sp. and related structures occur in earthworms.

Basement membrane thickening plays a key role in the pathogenesis of diabetic nephropathy. Extra renal sites such as the retina, peripheral nerves, and skeletal muscle capillaries may be involved. The normal basement membrane is made up of collagen-like glycoproteins with the carbohydrate subunits linked to hydro-xylysine and asparagine residues. The increased blood glucose in diabetics perhaps results in increased enzymatic incorporation of carbohydrates into the basement membrane. A reduction in cross-linked cystine residues may render these basement membranes leakier than normal. It is known that diabetic capillaries leak plasma proteins excessively and the size of the leak is dependent on both the duration and success of diabetic control. 

The fibronectins consist of isohomodimers of two nearly identical cysteine-linked 225 kDa subunits. They are adhesive proteins and can bind to a range of molecules including denatured collagen, fibrin and DNA. They are also important for cell adhesion. The carbohydrate content of fibronectins varies considerably depending on the source and is thought to be important for certain physical properties [58]. 

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