GLYCOPROTEINS and GLYCOLIPIDS



Glycoproteins:

Glycoproteins are proteins containing covalently attached oligosaccharides

There are two types:

a. O-linked at Ser and Thr
b. N-linked at Asn when it occurs in the sequence: X-Asn-X-Thr, where X is any amino acid

Synthesis of the carbohydrate portion of glycoproteins occurs via a series of specific enzymes that can recognize specific carbohydrate or protein structures. There is no template as in DNA, RNA, protein synthesis.

Since the oligosaccharides are often branched, unlike the amino acids in proteins, they can code for more information with fewer residues.

Functions:

a. cellular zip codes, these carbohydrate structures determine which proteins are exported (secreted), which are incorporated into the cell membrane, and which are destined for the lysosome.

b. involved in cell-cell recognition and adhesion

c. recognition of self vs. other by the immune system


Blood group substances:

The major blood group substances (A, B, O) are glycoproteins they are antigens and trigger an immune response when present in a foreign organism. These are but three of some of 14 genetically recognized blood group systems, others of which are important in tissue transplants. Some secondary blood group antigens are glycolipids.

The ABO blood types are best known because they are important in blood transfusions.

The AB blood type has some of both the A group and the B group antigens; it is called universal recipient blood type. A universal donor has type O blood.


Lectins:

Lectins are proteins that bind to carbohydrates, and they were originally isolated from plants. For example, the castor bean (ricin communis) produces the ricin lectin, which is very toxic to humans.

Lectins also exist in walls of the intestinal bacteria E. coli and help bond it to the glycocalyx of intestinal epithelium cells.

Lectins are also involved in tooth decay because they permit bacteria to adhere to teeth.


Sequencing oligosaccharides:

The early steps of sequencing the carbohydrate portion of glycoproteins is much like those for sequencing proteins:

a. hydrolyze (chemical or enzymatic) the carbohydrate to determine the identity and amounts of various sugars.

b. identify sugars by gas-liquid chromatography, after a per-methylation reaction to make the sugars volatile.

c. determine the sequence and type of linkage by a series of specific enzymatic digestions


Chemists are also developing the use of high-resolution nmr and mass spectrometry to determine the structures of complex carbohydrates.