Glycosylation is an important post-translational modification that affects protein structure, stability, and function in various ways. It involves attaching carbohydrate chains to proteins. There are two main types of glycosylation - N-linked and O-linked. Glycosylation helps in cell recognition and adhesion, can modulate enzyme activity, improves protein solubility and stability, and is involved in receptor signaling. It can adjust the thermodynamic, kinetic and structural properties of proteins. Glycosylation increases protein stability by destabilizing the unfolded state more than it stabilizes the folded state.

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

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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.

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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.

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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.

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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 ).

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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

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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 calreticulin.
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.

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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.

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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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Thank You