ABSTRACT
Structural glycoproteins are secreted in the extracellular matrix and interact with other macromolecules, but can also be retained on the cell membrane, apparently by reacting with specific cell membrane receptors. Structural glycoproteins, as a matter of fact, have similar amino acid compositions to plasma glycoproteins, are much richer that the other matrix components in aromatic amino acids, polar amino acids, especially dicarboxylic amino acids, and sulfur-containing amino acids. Already in the early 1950s it became evident that besides collagens and acid mucopolysaccharides, as glycosaminoglycans were called at the time, some noncollagenous components are present in connective tissues. Extracellular matrix comprises a great variety of macromolecules which can more or less artificially be divided in four major families: collagens, elastin, proteoglycans, and structural glycoproteins. Some similarities can be seen in the amino acid composition, and mainly striking differences between their amino acid composition and that of other matrix components such as collagens, elastin, or the protein portion of proteoglycans.
