Collagen ester links

VII. The Intekrelatio.n ship between Carbohydrate and Ester Links and the Cross-Links of Collagen... 

The structure must somehow accommodate the various unusual covalent bonds (e.g., hydroxylamine-sensitive ester-type bonds, 7-car-boxylglutamyl and e-aminolysyl peptide linkages, see Section V) which have been reported in collagen and gelatin and which may possibly be involved in interchain cross-linking. 

B. Determination of Ester-Like Links in Collagen and Gelatin... 

To avoid any possible a — 7 conversion promoted by the anhydrous conditions during esterification Franzblau (1962) and Franzblau el al. (1963) developed a method by which the hydroxamic acids could be formed directly from the unmodified proteins in an aqueous system. This was achieved using a water-soluble carbodiimide, 1-cyclohexyl-3[2-morpholinyl-(4)-ethyl]-carbodiimide metho-p-toluene sulfonate (Fig. 5). The reaction conditions (pH 4, 25°C, in aqueous medium) eliminated any possibility of a —> 8 conversion. These conditions also precduded the possibility of hydroxyaminolysis of the natural ester linkages of collagen which will be discussed in Section V. This method, in common with all the others used to study 7-glutamyl links, was not quantitative. After subsecpient dini-... 

The structures of some of these linkages are given in h ig. 8. Kelatively few proteins, or even peptides, have been shown to have ester-like linkages incorporated into their structure. The evidence for such links in collagen will now be reviewed. 

Involvement of esters in the intermolecular cross-links accounts for the dissolution of mature collagen by hydroxylamine, hydrazine or alkali in the presence of hydrogen-bond breakers. Similarly, the participation of hexoses accounts for the dissolution of collagen by periodic acid and a hydrogen-bond breaker. Recently, it has been shown that collagen can be completely dissolved by some proteolytic enzymes (see Section II). These enzymes also cleave the intramolecular cross-links (Kiihn et al, 1963a Rubin et al., 1963). It appears therefore that either the crosslinks include a peptide chain or the chain close to the cross-links contains pepsin- and trypsin-sensitive bonds. 

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. 

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