Glycans: Structure and Role | Carbohydrates

In this article we will discuss about:- 1. Nature of the Carbohydrate Part 2. Nature of the Glycan-Protein Linkage 3. Biosynthesis of Glycoproteins 4. Importance of Glycoproteins 5. Role of Glycan Groups.

Nature of the Carbohydrate Part:

1. Monosaccharide Constituents of Glycans:

The monosaccharides constituting the glycans belong to the 4 categories described above.

The most common are:

1. Neutral monosaccharides: D-galactose, D-mannose, L-fucose, L-rham- nose, D-xylose;

2. Osamines (in N-acetylated form): D-glucosamine, D-galactosamine , D-muramic acid;

3. Uronic acids: D-glucuronic acid, L-iduronic acid;

4. Sialic acids: N-acetylneuraminic acid, N-glycolylneuraminic acid, N.O- acetylneuraminic acids.

2. Structure of Glycans:

There are linear glycans and branched glycans. As examples, we will indi­cate the structures of some glycans belonging to each of these two categories, and briefly describe the glycoproteins themselves.

A. Sub-Maxillary Mucins:

One of the best known is the sub-maxillary mucin of sheep which contains about 40% carbohydrates, mainly consisting of N-acetylneuraminic acid and N-acetyigalactosamine.

These two sugars are conjugated in disaccharide units, and about 800 of these units are linked to the peptide chain in the following manner:

R = H (serine) or CH₃ (threonine).

Mucins give highly viscous solutions, but this viscosity disappears under the action of a neuraminidase (or sialidase).

B. Proteoglycans of the Fundamental Substance:

They result from the association — by O-glycosidic linkages with serine residues — of proteins with glycans having an acid character and called acid mucopolysaccharides. These glycans are composed of linear chains, of molecular weight varying from 50 000 (chondroitin sulphuric acid A) to several millions (hyaluronic acid), resulting from the polymerization of disaccharide units.

These units always contain a N-acetylhexosamine associated either with a neutral monosaccharide (galactose in keratan sulphate), or with a uronic acid. Esterification by sulphuric acid confers on these molecules a highly acid character which reinforces that of uronic acid.

As an indication, one may represent the structure of the disaccharide repeti­tion unit (called hyalobiuronic acid) of hyaluronic acid:

and the general structure diagram of the proteoglycan of chondroitin sulphuric acid A (or chondroitin-4-sulphate):

C. Muropeptides of Murein:

Murein is one of the constituents of bacterial cell walls. It is composed of muropeptides joined by their peptide chains. These muropeptides result from the linkage of short peptides with glycan chains consisting of disaccharide repetition units; these units are composed, for example, of N-acetyl-D- glucosamine and N-acetyl-D-muramic acid in the case of the muropeptide of Staphylococcus aureus.

D. Complex Glycans:

Glycans of very complex structures are found in numerous animal glycoproteins like ovalbumin, human serum transferrin, thyroglobulin, etc.

In transferrin there are two identical glycans having the following structure:

Such a structure is called “bi-antennate” and of the N-acetyl-lactosaminic type, due to the presence of the N-acetyl-lactosamine pattern: β-Gal-(1 → 4)- GlcNac. In other glycoproteins, there are glycans which contain only N-acetyl- glucosamine and mannose, the residues of which are grouped in an oligoman-nosidic structure.

Such glycans, like the one of unit A of thyroglobulin illustrated in the diagram below, are said to be of the oligomannosidic type:

Nature of the Glycan-Protein Linkage:

The above examples show that there are mainly two types of linkages.

1) The Amide Linkage is Present:

i. On the one hand, in numerous animal glycoproteins, where it results from the conjugation of the reducing group of N-acetylglucosamine with the amide group of asparagine:

This type of linkage, called “N-glycosylaminic” linkage, defines the class of N-glycosylproteins;

ii. On the other hand, in the murein of bacterial cell walls, where it results from the condensation of the carbonyl group of N-acetyl-muramic acid with the amino group of alanine:

2) The O-glycosidic linkage, characteristic of O-glycosylproteins where the terminal reducing group of a glycan (generally, that of N-acetyl-D-galac- tosamine) is involved with the hydroxyl of a serine (R = H) or threonine (R = CH₃), as mentioned in the case of sub-maxillary mucins, for example:

Biosynthesis of Glycoproteins:

The biosynthesis of glycan groups involves glycosyltransferases and nucleotides-monosaccharides, in conditions and which are to a great extent common to the synthesis of free and conjugated polysaccharides.

There are signals on the peptide chain which are recognized by the glycosyltransferase fixing N-acetylglucosamine on the protein. For example, in the case of an “asparaginyl-N-acetylglucosamine” linkage, the monosaccharide is fixed on Asn only if a Ser or Thr residue is in position β towards the C-terminal end.

It may be said that the recognition sequence is therefore:

— Asn—X —Ser — (Thr).

Importance of Glycoproteins:

Glycoproteins are very widely distributed in animal and plant tissues, as well as in micro-organisms and viruses.

The fundamental substance which, in the connective tissue, fills the mesh of glycoprotein fibers of elastin, reticulin and collagen, contains a large propor­tion of acid mucopolysaccharides linked to proteins (for example, chondroitin- sulphuric acids).

Numerous hormones are of glycoprotein nature: pituitary (LH, FSH) and chorionic gonadotropins, pituitary thyrotropic hormone (TSH). Thyroglobulin, precursor of the thyroid hormone is also a glycoprotein.

In cell membranes, there are numerous glycoproteins which play an impor­tant role in the social life of the cell and in the immune phenomena: for example, the specific antigens of blood groups A, B and O, present in the walls of human erythrocytes (and capable of triggering an agglutination if they come in contact with the corresponding agglutinin, a plasma antibody which does not exist in the same individual but may be present in the plasma of an individual belonging to another blood group) are glycoproteins or glycolipids.

Biological fluids are generally very rich in glycoproteins (saliva, urine, bile, milk, tears, blood). For example, in blood, practically all proteins — except serumalbumin — are glycoproteins: antibodies, orosomucoid, transferrin (form of transport of iron), ceruleoplasmin, haptoglobins, prothrombin and fibrinogen (the latter two substances play a fundamental role in blood coagulation).

Role of Glycan Groups:

Our present knowledge of the primary structure and spatial conformation of glycans has enabled us to lay the bases of the molecular biology of glycoproteins and show that glycans play an important role in the following mechanisms:

1) Maintenance of the structure of the protein in a biologically active conformation, through glycan-glycan or glycan-protein secondary linkages (hydrogen bonds, ionic bonds and hydrophobic interactions);

2) Protection of the peptide chain against attack by proteolytic enzymes;

3) Decrease in antigenic power of proteins by the masking of epitopes;

4) Control of the imbibition of membranes by-water and, consequently, of the diameinbrane passage of mineral ions and small organic molecules. In this con­nection, the observation that the glycan fractions of membrane glycoconjugates are profoundly modified in cancerous cell (increase in molecular weight of glycans of the glycoproteins and in their rate of sialylation) could explain the modifications noted in the permeability of the membranes of these cells;

5) Action in the cellular phenomena of recognition: recognition and associa­tion of cells involving proteids which specifically recognize oligosaccharide structures and are called lectins; recognition of target-cells by hormones, toxins, bacteria and viruses and also by other glycoproteins (the desialylation of circulating glycoproteins causes their instantaneous capture by a lectin present in the membranes of hepatocytes and which specifically recognizes the residues of galactose in non-reducing terminal position, this capture being followed by the internalization of asialoglycoproteins and their destruction by the lysosomial enzymes).

The above mentioned fact that glycans of the glycocon­jugates of cancerous cell membranes undergo profound modifications on the one hand, and the observation that glycans play a role in the contact inhibition phenomenon (arrest of mitoses when normal cells come in contact with each other), on the other hand, could explain (a) the perturbations manifest in the social life of cancerous cells: loss of contact inhibition and metastatic diffusion and (b) the appearance of surface neo-antigens capable of inducing the immune reaction of the organism.