Biosynthesis of Glycoconjugates

Covalent binding of carbohydrate to protein or lipid brings about large changes in the physical properties of these substances that allow them to serve specialized biochemical functions. Sulfated polysaccharides in glycoproteins, for example, are effective biological lubricants and linking carbohydrates to lipids allows them to be inserted into membranes.

Mammalian glycoproteins are classified as O-linked or N-linked. N-linked glycoproteins contain an N-acetylglucosamine residue linked to the amide nitrogen of an asparagine residue in the protein (see here). The most common O-linkage involves a terminal N-acetylgalactosamine residue in the oligosaccharide linked to a serine or threonine residue of the protein (see here).

O-Linkage - A synthesis pathway for O-linked glycoproteins is shown in Figure 16.15. Note that oligosaccharide assembly occurs on the polypeptide chain. The A,B, and O blood group substances on erythrocyte surfaces are the best-known O-linked glycoproteins. Note that both the A and B antigens are made by building onto the O substance backbone. The AB blood type arises from individuals who are heterozygous for A and B antigens. They have both enzymes and carry both antigens in their blood.

N-Linkage - Oligosaccharide assembly occurs not on the polypeptide chain, but on a lipid-linked intermediate. A precursor oligosaccharide is then transferred to a polypeptide chain, which is itself still in the midst of being synthesized. Finally, the transferred oligosaccharide is subject to further processing as it passes from the rough and smooth endoplasmic reticulum through the Golgi complex. Figure 16.16 lists the three common structures found on asparagine-linked (N-linked) oligosaccharides. All of the known N-linked oligosaccharides have a common core pentasaccharide structure (boxed in Figure 16.16). The core pentasaccharide is assembled as part of a larger oligosaccharide intermediate linked to the isoprenoid lipid compound, dolichol phosphate.

Figure 16.17 illustrates biosynthesis of the dolichol-phosphate-linked oligosaccharide intermediate. A short summary is as follows:

> The entire process occurs in the endoplasmic reticulum (ER).

> The first seven sugars are transferred to dolichol phosphate from nucleoside diphosphate sugars, UDP-N-acetylglucosamine and GDP-mannose. Each reaction is catalyzed by a separate glycosyltransferase. The antibiotic, tunicamycin, inhibits synthesis of all N-linked glycoproteins by inhibiting the first enzyme in the process.

> The next seven reactions utilize dolichol-linked sugars as substrates. These are dol-P-mannose and dol-P-glucose, which are, in turn, synthesized from dol-P plus GDP-mannose and UDP-glucose, respectively. During this stage, the lipid-linked intermediate (the third intermediate in Figure 16.17) is translocated from the exterior surface of the ER-membrane to the luminal, or interior side.

> The oligosaccharide unit is transferred to a polypeptide acceptor in a reaction catalyzed by a specific oligosaccharyltransferase. The acceptor site is an asparagine residue in the sequence Asn-X-Ser/Thr, where X is any amino acid. The acceptor site must be accessible in a loop or a bend in the polypeptide chain.

> Processing of the oligosaccharide-linked polypeptides begins in the lumen of the rough endoplasmic reticulum and continues as the nascent glycoprotein moves into the smooth ER and ultimately through the Golgi apparatus. In virtually all cases, processing begins with removal of the three glycosyl residues in the rough ER, followed by removal of some of the mannosyl residues in the Golgi apparatus. Complex glycoproteins are further processed by addition of N-acetylglucosamine, followed by further trimming of mannosyl residues. Fucose, galactose, and Sialic acid residues are added from appropriate nucleotide-linked sugars by specific glycosyl-transferases.

Oligosaccharide chains help direct glycoproteins to appropriate intracellular destinations.

See also: Biosynthesis of Amino Sugars, Oligosaccharides and Polysacchardies as Cell Markers (from Chapter 9), Dolichol Phosphate