Principles of Biochemistry, Volume 1 - A. Lehninger 1985

Biomolecules
Carbohydrates: Structure and Biological Functions
Glycoproteins – Hybrid Molecules

Glycoproteins are Proteins that contain covalently attached CARBOHYDRATES, ranging from single Monosaccharides to relatively short Oligosaccharides. The carbohydrate moiety in a glycoprotein molecule can account for less than 1% or reach 30% or more of its total mass. Some glycoproteins contain one or a few carbohydrate groups, whereas others feature A large number of linear or branched oligosaccharide chains (Fig. 11-22). Nearly all proteins on the external surface of animal Cells are glycoproteins. The vast majority of Cell-secreted proteins, as well as Blood Plasma Proteins, also belong to this Class. In general, most proteins located or functioning outside The Cell are glycoproteins.

Fig. 11-22. Structure of selected glycoproteins and a proteoglycan. A. Three types of glycoproteins differing in the size and composition of their carbohydrate side chains. B. The repeating structural unit of the antifreeze glycoprotein found in certain species of polar fish. This unit is repeated many times. Red squares denote N-acetyl-D-galactosamine, and red circles denote D-galactose.

One of the most fascinating extracellular glycoproteins is the so-called antifreeze protein found in the blood of certain Arctic and Antarctic fish, as well as in flounders and cod along the eastern coast of North America. The structure of antifreeze proteins varies among species; the best-characterized one features a polypeptide backbone made up of a tripeptide (Ala-Ala-Thr) repeated up to 50 times (Fig. 11-22). A D-galactosyl-N-acetyl-D-galactosamine disaccharide is attached to each Threonine residue. Antifreeze proteins lower the freezing point of Water, presumably by inhibiting ice crystal formation. The presence of these proteins in fish blood, combined with very high concentrations of NaCl [which also depresses the freezing point of water (Section 4.4)], enables these fish to survive the extreme sub-zero temperatures of polar seas that would otherwise freeze the blood of terrestrial vertebrates solid.