Glycosylation

Glycosylation - Importance in Maintaining protein structure, stability and function

1. GLYCOSYLATION – IMPORTANCE IN MAINTAINING PROTEIN STRUCTURE, STABILITY AND FUNCTION Assignment 1 Submitted by Sourik Dey (18610023), M.Sc Biotechnology, 1 st Year, 2018-2019 Cell and Developmental Biology(BTN-516) Submitted to Dr. R. P. Singh

2. 1.) Glycosylation is the reaction in which a carbohydrate , i.e. a glycosyl donor, is attached to a hydroxyl or other functional group of another molecule (a glycosyl acceptor). 2.) Glycans serve a variety of structural and functional roles in membrane and secreted proteins.The majority of proteins synthesized in the rough endoplasmic reticulum undergo glycosylation. 3.) It is an enzyme-directed site-specific process, as opposed to the non-enzymatic chemical reaction of glycation. Glycosylation is also present in the cytoplasm and nucleus as the  O -GlcNAc modification. 4.) The biophysical properties of proteins are a consequence of the features of the underlying energy landscapes that govern their folding. 5.) Glycosylation can adjust the thermodynamic, kinetic, and structural features of proteins .

3. TYPES OF GLYCOSYLATION There are two main types of glycosylations conducted inside the ER Lumen:- a ) N -linked glycans attached to a  nitrogen of asparagine or arginine  side-chains .  N -linked glycosylation  requires participation of a special lipid called dolichol phosphate . b ) O -linked glycans attached to the hydroxyl  oxygen of  serine, threonine, tyrosine,  hydroxylysine or  hydroxyproline side-chains, or to oxygens on lipids such as ceramide.

4. FUNCTIONS OF OLIGOSACCHARIDE MOEITIES ON THE PROTEINS Recognition and adhesion Carbohydrates can modulate the activity of enzymes. Glycosylation also affects better water solubility of proteins. Glycosylation of proteins can maintain their structure. Receptor Signalling.

5. Recognition and adhesion The surface carbohydrates on a cell serve as points of attachment for other cells, infectious bacteria and viruses, toxins, hormones, and many other molecules. This ability depends on the presence of these sugars, their accessibility and their mode of presentation. The carbohydrates are recognized by the cell  adhesion molecules  which are glycoproteins expressed on the cell surface. Bacteria adhere to tissues selectively  due to their fimbriae ( containing lectins) which bind exclusively to certain surfece carbohydrates. These interactions determine which tissues are susceptible for the invasion (for example  E.coli  causes urinary tract infections while  Streptococci  of A group colonize upper respiratory tract and skin ).

6. 1.) Carbohydrates can be strong  antigens  which may trigger harmful immune reactions. For example the  ABO blood group antigens  are the carbohydrate moieties attached to the lipids or proteins on the surface of erythrocytes and many others cell types. A, B and O antigens are structurally related. 2.) All people have enzymes which synthesize the O antigen (H substance). In people with  B antigen  there is an enzyme which adds a  galactose  residue to the H substance and people with  A antigen  have an enzyme adding  GalNac  residue. SURFACE RECEPTORS

7. Enzymatic Functions 1.) Many  enzymes  are glycoproteins. For example the hydrolase  sucrase - isomaltase  from microvilli of intestinal epithelial cells have many sugar residues. 2.) Such hydrolases belong to the  glycocalyx  (the loose network built up of the carbohydrates of glycoproteins and glycolipids) which cover the outer membrane surface.  Negatively charged sugar residues prevent the close contacts between different enzymes. 3.) Other glycosylated enzymes are the chaperones  calnexin  and c alreticulin . 4.) Some glycoproteins act as  carriers . They can bind to certain molecules such as vitamins, hormones, cations. For example  ceruloplasmin  is a transport glycoprotein found in vertebrate plasma. It mobilizes iron into the serum.

8. Glycosylation of proteins can maintain their structure In  human immunoglobulins  the carbohydrate chain wraps around one of the protein domains preventing its contact with the adjacent domain. When the carbohydrate chain is removed, the domain could no longer perform its ordinary function. Because immunoglobulin function is determined to a large extent by its structure, the carbohydrate removing affected the structure of the molecule. IgG  is unique among the serum glycoproteins because it is associated with at least 30 different biantennary N-linked oligosaccharides. This extremely high microheterogenity is probably produced because human individuals have series of B-cell clones equipped with different sets of glycosyltransferases. 

9. Thermodynamic and kinetic aspects of glycosylation The covalent binding of the bulky hydrophilic glycans is known to increase the solubility of the protein enhancing the thermodynamic and kinetic stability of the glycoprotein, as well as providing protection from proteolysis. This increase in thermodynamic stability is manifested by the increase in the melting temperature along with the free energy difference between the two forms. Thermodynamic analysis to investigate the origin of the enhanced protein stabilization by glycosylation indicates that the difference mainly arises from the higher amount of destabilization of the unfolded state in comparison with stability gain of the folded state. The analysis indicated that the enhanced protein stabilization originated by destabilization of the unfolded state reasonably than stabilization of the folded state. This is mediated by limiting the formation of a residual structure besides enthalpically inducing a higher free energy to the unfolded state 

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