Membranebound glycoproteins are

involved in a multitude of cellular phenomena (A). These include cell-surface recognition by other cells, hormones, or viruses, as well as antigenic characteristics (e. g., blood groups). Inside the extra-cellular matrix and in the mucins of the gastrointestinal and urogenital tracts, glycoproteins serve as protective, biologically active lubricants.

Nearly all of the globular plasma proteins as well as the secreted enzymes, are actually glycoproteins—albumin is one of the few exceptions.

The carbohydrates are either bonded to the O atom of the serine or threonine R-groups with O-glycosidic bonds or to the N atom in the asparagine R-group with N-glycosidic bonds. The basic structure of the sugars consists mainly of N-acetyl-glucosamine (GlcNAc), fu-cose, and mannose. At their ends they have galactose (gal), mannose, and/or sialic acid (sia) attached. Since these carbohydrates can be linked in different ways, the resulting multitude of combinations can have different information contents. For instance, the life span of the entire sialoglycoprotein (B) molecule depends on the sialic acid residue at its end. Many immunoglobulins and pep-

tide hormones that circulate in the bloodstream are equipped with this residue. The point at which the sialic acid is enzymatically removed is encoded in these proteins. The asialoglyco-protein receptor in the plasma membrane of liver cells then recognizes the galactose released during the removal reaction and endocytoses the entire protein. This mechanism allows proteins to be so equipped that their life span, depending on specific physiological requirements, may range from a few hours to several weeks.

When aldoses (e. g., glucose) bond with the amino groups of proteins, instable glycosylamines form. These can rearrange themselves into stable products in so-called Amadori reactions (C). However, the alkaline nitrogen introduced thereby now favors enolization within the molecule. The resulting radical anion can react with oxygen and other glycoproteins and lead to the formation of hydroxyl radicals. These radicals may cause polypeptide chain breaks, ultimately resulting in the loss of functional units. Such processes are being discussed in the literature pertaining to late consequences of diabetic hyperglycemia.

Amadori products also form between collagen polypeptide chains. They cross-link the chains and may thereby enhance the aging of connective tissue.

Glycoproteins 77

- A. Glycoprotein Functions -

Cell surface recognition y.

Cell membrane h

Cell surface antigens

Cell surface antigens

Extracellular matrix

B. Lectins

Sia Does not bond

GlcNAc

Glycoprotein carbohydrates ■ I ■

Bonds to liver cells receptors and is endocytosed

Bonds to liver cells receptors and is endocytosed

GlcNAc

Glycoprotein carbohydrates ■ I ■

Sialoglycoprotein

Protein Asialoglycoprotein

C. Amadori Product

Hydroxyaldehyde R

O Amadori product

NCCR

C OH

Endiol

, Keto OH O amino methylol

H2O2

Ketoaldehyde

CO i

Endiol radical anion

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