78 Steps Health Journal

Lectins Are Proteins That Read the Sugar Code and Mediate Many Biological Processes

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Lectins, found in all organisms, are proteins that bind carbohydrates with high affinity and specificity (Table 7-3). Lectins serve in a wide variety of cell-cell recognition, signaling, and adhesion processes and in intra-

cellular targeting of newly synthesized proteins. In the laboratory, purified lectins are useful reagents for detecting and separating glycoproteins with different oligosaccharide moieties. Here we discuss just a few examples of the roles of lectins in cells.

Some peptide hormones that circulate in the blood have oligosaccharide moieties that strongly influence their circulatory half-life. Luteinizing hormone and thy-rotropin (polypeptide hormones produced in the adrenal cortex) have ^-linked oligosaccharides that end with the disaccharide GalNAc4S(31n4)GlcNAc, which is recognized by a lectin (receptor) of hepatocytes. (GalNAc4S is ^-acetylgalactosamine sulfated on the —OH group of C-4.) Receptor-hormone interaction mediates the uptake and destruction of luteinizing hormone and thyrotropin, reducing their concentration in the blood. Thus the blood levels of these hormones undergo a periodic rise (due to secretion by the adrenal cortex) and fall (due to destruction by hepatocytes).

TThe importance of the oligosaccharide moiety of these hormones is apparent from studies of individuals with a defective enzyme in the pathway that produces this oligosaccharide. Females with this congenital defect often fail to undergo the sexual changes of puberty (although males with the same defect develop normally). ■

The residues of Neu5Ac (a sialic acid) situated at the ends of the oligosaccharide chains of many plasma glycoproteins (Fig. 7-31) protect those proteins from uptake and degradation in the liver. For example, ceru-loplasmin, a copper-containing serum glycoprotein, has several oligosaccharide chains ending in Neu5Ac. Re

TABLE 7-3 Some Lectins and the Oligosaccharide Ligands They Bind
Lectin source and lectin	Abbreviation	Ligand(s)
Plant

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