Glycoproteins from protein

Kirkpatrick RB, Matico RE, McNulty DE, Strickler JE, Rosenberg M 1995 An abundantly secreted glycoprotein from Drosophila melanogaster is related to mammalian secretory proteins produced in rheumatoid tissues and by activated macrophages. Gene 153 147-154 Leconte I, Carpentier JL, Clauser E 1994 The functions of the human insulin receptor are affected in different ways by mutation of each of the four N-glycosylation sites in the fl subunit. J Biol Chem 269 18062-18071... 

An exotic function of glycoproteins is to act as antifreezes. Specifically, a number of Antarctic fish live in water cooled to about -1.9°C, a temperature below the freezing point of water and below that where the blood, mostly water, of these fish is expected to freeze. Clearly, this would be a disaster for these fish. They are saved from this fate by antifreeze glycoproteins. These proteins contain about 50 repeats of the tripeptide Ala-Ala-Thr. To each of these threonine residues is hooked a specific disaccharide. 

Secretion of nonglycosylated macromolecules in the presence of tunicamycin has also been investigated in a number of other cells. Rat-liver cell-secretion ol albumin (a carbohydrate-free protein), transferrin, and a-acid glycoprotein was not inhibited, and, in chick-liver cells, only a decrease by 10-25% in the secretion of transferrin and the apoprotein B chain of very-low-density lipoprotein was noted.463,464 The secretion of ovalbumin (a glycoprotein) from hen oviduct was not blocked by tunicamycin.465... 

The tannins are special phenolic compounds characterized by their ability to combine with proteins and other polymers such as polysaccharides. This characteristic explains their tanning properties as arising from a tannin-collagen matrix and then astringency caused by precipitation of the proteins and glycoproteins from saliva. Also tannins are used in fining wines because they combine with proteins. Finally, they inhibit enzymes by combining with their protein fraction. 

Fish living in Arctic and Antarctic waters may encounter temperatures as low as -1.9°C. The freezing point depression provided by dissolved salts and proteins in the blood is insufficient to protect the fish from freezing. As winter approaches, they synthesize and accumulate in their blood serum a series of eight or more special antifreeze proteins.a d One type of antifreeze glycoprotein from winter flounder contains the following unit repeated 17-50 times. 

From J. T. Segrest and L. D. Kohn, Protein-lipid interactions of the membrane penetrating MN-glycoprotein from the human erythrocyte, Protides of the Biological Fluids, 21st colloquium, ed. by J. Peeters, Pergamon Press, New York, 1973. 

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