Possible Functions of Membrane Carbohydrate (With Diagram)

It has already been noted that carbohydrate is pres­ent in the plasma membrane as short, sometimes branched chains of sugars attached either to exterior peripheral proteins (forming glycoproteins) or to the polar ends of phospholipid molecules in the outer lipid layer (forming glycolipids).

No membrane carbohy­drate is located at the interior surface.

The oligosaccharide chains of membrane glycoproteins and glycolipids are formed by various combina­tions of six principal sugars D-galactose, D-mannose, L-fucose, N-acetylneuraminic acid (also called sialic acid), N-acetyl-D-glucosamine, and N-acetyl-D- galactosamine. All of these may be derived from glucose.

Possible Functions of Membrane Carbohydrate:

Several roles have been suggested for the carbohy­drate present on the outer surface of the plasma mem­brane. One possibility is that because they are highly hydrophilic, the sugars help to orient the glycopro­teins (and glycolipids) in the membrane so that they are kept in contact with the external aqueous environ­ment and are unlikely to rotate toward the mem­brane’s hydrophobic interior.

Certain plasma trans­port proteins, hormones, and enzymes are glycoproteins, and in these molecules, carbohydrate is important to physiological activity. It would therefore not be inappropriate to expect that in certain glyco­proteins of the plasma membrane the carbohydrate moiety is basic to either enzymatic or some other ac­tivity.

Carbohydrate chains of surface glycoproteins are clearly responsible for the various human blood types (e.g., ABO types, MN types, etc.) and other tissue types. That is, the sugar sequence and the arrange­ment of the sugar chains in the membranes of blood cells of an individual with type A blood differ from those of an individual with type B blood, and so on.

The carbohydrate is responsible for cell type specific­ity and is therefore fundamental to the specific anti­genic properties of cell membranes. These antigenic properties are linked in some manner to the body’s im­mune system and the capability of that system to dis­tinguish between cells that should be present in the organism (native cells) and foreign cells. Foreign cells (such as bacteria or other microorganisms, trans­planted tissue, or transfused blood) may be recog­nized as foreign because their membrane glycopro­teins contain different carbohydrate markers than those present in the individual’s own tissues.

Such a situation triggers the immune response. In contrast, an individual’s own plasma membrane carbohydrate organization is recognized as being native (referred to as “recognition of self”) and does not normally trig­ger an immunological response. Of course, neither does blood transfusion nor tissue transplantation if the carbohydrate organization in the membranes of the “donor’s” and “recipient’s” cells is the same. Cell- specific membrane carbohydrate organization is con­sidered further in connection with the actions of lec­tins and antibodies.

Oppenheimer, Roseman, Roth, and others have clearly implicated surface carbohydrate in the adhe­sion of a cell to its neighbors in a tissue; presumably, the carbohydrate acts as an adhesive maintaining the integrity of the tissue by linking neighboring cells to­gether.

Density-dependent inhibition, the phenome­non in which cells grown in culture stop dividing when their numbers attain a certain level, may be attribut­able to a mechanism triggered by interaction of carbo­hydrates on neighboring cells. Carbohydrate is also present at the surface of cells as glycolipid, especially glycosphingolipids. Although the role of this glycolipid is uncertain, it is believed to function in cell-to-cell recognition.