Textbook - BIOLOGICAL CHEMISTRY - Gubsky Yu.I. - 2000

Chapter III. METABOLISM OF MAJOR CLASSES OF BIOMOLECULES

CHAPTER 13. CARBOHYDRATE METABOLISM. III. METABOLISM OF GLYCOGEN AND GLYCOCONJUGATES

13.4. METABOLISM OF CARBOHYDRATE COMPONENTS OF GLYCOCONJUGATES

Glycosylation of non-carbohydrate molecules represents an important Class of biochemical reactions leading to The formation of hybrid molecules known as Glycoconjugates. As a result of glycosylation reactions, oligosaccharide and polysaccharide chains are attached via covalent bonds to Polypeptides or Lipids, yielding physiologically vital Glycoproteins, Glycolipids, and Proteoglycans.

Biosynthesis of Glycoconjugates

The glycosylation of Proteins and lipids takes place in the Endoplasmic reticulum and the Golgi apparatus, catalyzed by specific Glycosyltransferases. The biochemical reactions involved in the synthesis of oligosaccharide fragments of glycoproteins and glycolipids, as well as heteropolysaccharide fragments of proteoglycans (glycosaminoglycans), are generally similar and will be examined using the better-studied pathway of carbohydrate moiety formation in glycoproteins. Nevertheless, significant differences exist in the molecular mechanisms governing The biosynthesis of O- and N-linked glycoproteins.

1. Synthesis of O-Linked Glycoproteins

Oligosaccharide chains of O-glycosidic glycoproteins are constructed through the stepwise addition of monosaccharide residues to the OH groups of Serine or Threonine residues within the polypeptide backbone. The Donors of carbohydrate residues in these reactions are nucleotide sugars, specifically UDP-N-acetylgalactosamine (UDP-GalNAc), UDP-galactose (UDP-Gal), and CMP-N-acetylneuraminic (sialic) acid (CMP-NeuAc).

The Enzymes that catalyze The transfer of a monosaccharide residue from nucleotide sugars to the OH group of Ser(Thr) are membrane-bound glycoprotein glycosyltransferases.

The first sugar in the oligosaccharide chain of a glycoprotein, which attaches directly to the OH group of the serine (threonine) residue in the polypeptide component, is typically N-acetylgalactosamine (GalNAc), followed by galactose (Gal) or N-acetylneuraminic acid (NeuAc) as the second:

Such a sequence of glycosylation reactions leads to the formation of Biomolecules such as the O-linked Polysaccharides of salivary glycoproteins (mucins) (a) and human erythrocyte membrane sialoglycoproteins (b):

The initiation of glycosylation—that is, the attachment of the initial monosaccharide residues to the peptide chain—occurs in The endoplasmic reticulum concurrently with polypeptide synthesis (Translation), whereas The addition of terminal Monosaccharides takes place within the Golgi complex.

2. Synthesis of N-Linked Glycoproteins

A distinctive feature of N-linked glycoprotein synthesis is the involvement of a high-molecular-weight isoprenoid alcohol, dolichol phosphate, which serves as an intermediate carrier of oligosaccharide fragments.

Chemically, dolichol is a high-molecular-weight alcohol that, much like rubber, possesses the longest chain among natural Hydrocarbons. The Hydrophobic core of dolichol is embedded within Cell/29.html">The Lipid Bilayer of Introduction/36.html">Biological Membranes, while its OH group, facing the aqueous phase, is capable of accepting monosaccharide fragments through the successive action of specific glycosyltransferases.

The incorporation of dolichol (Dol) into glycosylation reactions is preceded by its phosphorylation, mediated by the enzyme dolichol kinase with ATP acting as the phosphate donor:

Phosphorylated dolichol interacts with an activated molecule of N-acetylglucosamine in the form of UDP-GlcNAc in a reaction that constitutes The First stage of dolichol glycosylation:

Through the sequential action of specific glycosyltransferases and the participation of corresponding nucleotide sugars, oligosaccharide-pyrophosphoryldolichol is formed—a molecular structure that serves as the donor of oligosaccharide fragments during N-glycosylation of proteins. Subsequently, the Oligosaccharides linked to dolichol pyrophosphate molecules are transferred en bloc to the amide groups of one or more asparagine residues on the acceptor protein. This reaction is catalyzed by a membrane-bound oligosaccharyltransferase and takes place in the endoplasmic reticulum and the Golgi apparatus.

The N-linked glycoproteins generated through this sequence of reactions share a common pentasaccharide core, (Man)₃(GlcNAc)₂, linked to the NH group of asparagine:

Various external oligosaccharide chains of different structures are attached to the specified pentasaccharide. Depending on the monosaccharide residue composition and chain architecture, these outer branched oligosaccharide fragments are classified into complex, hybrid, and high-mannose (or mannose-rich) oligosaccharides.

Glycoproteins formed through O- and N-glycosylation processes either remain bound to membrane structures as integral Membrane Proteins or are secreted into the extracellular space.

3. Synthesis of Glycolipids

Glycolipids are crucial Structural components of Biomembranes that, alongside membrane glycoproteins, determine the antigenic properties of Cells.

Similar to glycoprotein formation, the synthesis of glycolipids, particularly glycosphingolipids, proceeds via the sequential addition of oligosaccharides mediated by specific glycosyltransferases. Nucleotide sugars serve as donors of MONOSACCHARIDES AND THEIR derivatives, transferring sugar residues onto a molecular scaffold—ceramide (acylsphingosine). The process follows this pathway:

The synthesis of erythrocyte membrane glycolipids, which comprise the antigenic determinants of human Blood Groups According to the ABO system, follows this established mechanism. Genetically determined differences in blood groups are driven by the molecular STRUCTURE OF THE oligosaccharide components found in membrane glycolipids and glycoproteins, specifically the major integral erythrocyte membrane protein—Glycophorin.

Individuals with blood group O possess a functional Gene (H) whose expression results in Erythrocyte membranes containing a glycolipid with a core oligosaccharide terminated by fucose—known as the type O oligosaccharide or the H antigen. Individuals with blood groups A and B carry allelic genes (I^A and I^B, respectively) responsible for synthesizing specific glycosyltransferases. These enzymes modify the base O-oligosaccharide by attaching N-acetylgalactosamine (type A) or galactose (type B) residues.

Heterozygous individuals whose erythrocytes express both types of oligosaccharides (A and B) have blood group AB. Over 30 distinct blood groups are currently recognized, differing in their erythrocyte antigenic determinants, which reflects variations in The structure of membrane glycolipids and glycoproteins.

Catabolism of Glycoconjugates

The Cleavage of heteropolysaccharide fragments from glycoconjugates—such as proteoglycans, glycoproteins, and glycolipids—is carried out by glycosidases localized within Lysosomes, which exhibit Specificity toward Different types of glycosidic bonds.

Lysosomal enzymes involved in the degradation of the carbohydrate components of proteoglycans (glycosaminoglycans) and glycolipids are the most thoroughly studied. Congenital defects in The breakdown of these compounds (enzymopathies) manifest as glycogenoses (or Mucopolysaccharidoses/glycolipidoses), characterized by the intracellular accumulation of specific heteropolysaccharide forms.

The degradation of proteoglycan heteropolysaccharide structures is catalyzed by enzymes that, based on their substrate specificity, are classified into Endoglycosidases, exoglycosidases, and sulfatases. Glycosidases involved in glycosaminoglycan catabolism include:

Hyaluronidase is a widely distributed tissue endoglycosidases that acts on hyaluronic acid and chondroitin sulfates, cleaving them into tetrasaccharide fragments.

β-Glucuronidase is an exoglycosidase that cleaves glucuronic and iduronic acids from tetrasaccharides (see above) as well as from Heteropolysaccharides such as dermatan sulfates, heparan sulfates, chondroitin sulfates, and hyaluronic acid. Hereditary deficiency of this enzyme leads to the urinary excretion of glycosaminoglycans.

β-Galactosidases are a group of multiple enzymes that cleave internal β-galactosidic bonds within the oligosaccharide residues of proteoglycans, glycoproteins, and glycolipids.

β-D-Acetylhexosaminidase is an exoglycosidase that removes terminal N-acetylhexosamines (GlcNAc and GalNAc) from the carbohydrate moieties of glycoconjugates. Hereditary deficiency of this enzyme is observed in glycolipidoses such as Tay-Sachs and Sandhoff diseases.

α-L-Iduronidase is an exoglycosidase that cleaves terminal L-iduronic acid residues from polysaccharide chains. Several forms of inherited disorders associated with a deficiency of this enzyme are known.

There are several lysosomal sulfatases that remove sulfate groups from various monomeric units within glycosaminoglycans, notably arylsulfatase A, arylsulfatase B, and arylsulfatase C. Sulfatase deficiencies manifest as Various Forms of lysosomal storage diseases.