This document discusses glycoproteins, which are proteins that contain oligosaccharide chains covalently attached to their polypeptide side chains. There are three major classes of glycoproteins based on the nature of the linkage between the polypeptide and oligosaccharide chains. Glycoproteins play important roles in various biological functions and processes like fertilization. Several methods are used to study the structure and functions of glycoproteins, including using specific glycosidases and lectins.

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2. o GLYCOPROTEIN
o DIFFERENCE BETWEEN GLYCOPROTEIN AND GAG’AND PROTEOGLYCAN
o MAJOR CLASSESOF GLYCOPROTEINS
o BIOMEDICAL IMPORTANCE
o SUGARS IN HUMAN GLYCOPROTEIN
o FUNCTIONS OF GLYCOPROTEINS
o SUGARS IN HUMAN GLYCOPROTEIN
o PRINCIPLE SUGARS FOUND IN GLYCOPROTEINS
o SOME FEATURES OF MUCIN
o EXO- & ENDOGLYCOSIDASES FACILITATE STUDY OF GLYCOPROTEINS
o SOME GLYCOSIDASES USED TO STUDY THE STRUCTURE AND FUNCTION OF GLYCOPROTEINS
o SOME GLYCOSIDASES USED TO STUDY THE STRUCTURE AND FUNCTION OF GLYCOPROTEINS
o LECTINS CAN BE USED TO PURIFY GLYCOPROTEINS & TO PROBE THEIR FUNCTIONS
o SOME IMPORTANT METHODS USED TO STUDY GLYCOPROTEINS
o INHIBITORS OF GLYCOPROTEIN
o GLYCOPROTEINS ARE IMPORTANT IN FERTILIZATION
o REFERENCES

3.  Glycoproteins are proteins that contain oligosaccharide
chains (glycans) covalently attached to polypeptide side-
chains
 Glycosylation is the enzymatic attachment of sugar to
protein
 The carbohydrate is attached to the protein during the
following modifications:
 Co translational modification &
 Post-translational modification

4.  In proteins that have segments extending extracellularly, the
extracellular segments are often glycosylated
 The carbohydrate content of glycoprotein ranges from 1% to
85% by weight-Glucose
 Carbohydrate may be:
 Simple sugars- Glucose, Galactose, Mannose, Xylose
 Amino sugar -The sugar that have an amino group such as
N-acetyl glucosamine or N-acetyl galactosamine
 Acidic Sugar-Sugar that have carboxyl group such as
sialic acid

7. GLYCOPROTEINS GLYCOSAMINOGLYCANS
Length of carbohydrate chain
is short(2-10 sugar units)
Carbohydrate chain is long
Chains may or may not be
negatively charged
Chains are negatively charged
Carbohydrate contain is
variable(4-85%)
Carbohydrate contain is more
than 95%
Storage diseases are called
oligosaccharidosis
Storage diseases are called
mucopolysaccharidosis

12. Based on the nature of the linkage between their polypeptide
chains and their oligosaccharide chains, they are of three
Types:-
(1) O-glycosidic linkage
 hydroxyl side chain of serine or threonine and a sugar such
as N- acetylgalactosamine (GalNAc-Ser[Thr])
(2) N-glycosidic linkage
 amide nitrogen of asparagine and N-acetylglucosamine
(GlcNAc- Asn)

13. (3) Those linked to the carboxyl terminal amino acid of a protein
via a phosphoryl-ethanolamine moiety
Joined to an oligosaccharide (glycan),which in turn is linked
via glucosamine to phosphatidyl inositol (PI)
This latter class is referred to as glycosyl phosphatidyl
inositol- anchored (GPI anchored, or GPI linked)
glycoproteins
 Other minor classes of glycoproteins also exist.

14.  In N-glycosylation, the addition of sugar chains can happen
at the amide nitrogen on the side-chain of the asparagine.
 They can be of 2 types
Complex oligosaccharide- diverse group of sugar like
GlcNac, L-fucose, NANA are present
 High mannose oligosaccharide- contain primarily
mannose

15.  In O- glycosylation, the addition of sugar chains can happen
on the hydroxyl oxygen on the side-chain of hydroxy lysine,
hydroxy proline, serine, or threonine

16.  The biosynthesis of N-linked glycans occurs via 3 major steps:
Synthesis of dolichol-linked precursor oligosaccharide
En bloc transfer of precursor oligosaccharide to protein
Processing of the oligosaccharide

17. Synthesis, en bloc transfer and initial trimming of
precursor oligosaccharide occurs in the endoplasmic reticulum (ER)
Subsequent processing and modification of the oligosaccharide
chain is carried out in the Golgi apparatus
The type of N-glycan synthesized, depends on its accessibility to
the different enzymes present within these cellular compartments
 The core glycan structure is essentially made up of two N-acetyl
glucosamine and three mannose residues

18. The process of N-linked glycosylation starts with the
formation of dolichol-linked GlcNAc sugar.
Dolichol is a lipid molecule composed of repeating isoprene
units.
This molecule is found attached to the membrane of the ER.
The precursor molecule, ready to be transferred to a protein,
consist of 2 GlcNAc, 9 mannose and 3 glucose molecules

19. Sugar molecules are attached to the dolichol through a
pyrophosphate linkage
The assembly of this precursor oligosaccharide occurs in
two phases
 Phase I : cytoplasmic side of the ER
 Phase II : luminal side of the ER

20. Once the precursor oligosaccharide is formed, the completed
glycan is then transferred to the nascent polypeptide in the lumen of
the ER membrane.
There are three conditions to fulfill before a glycan is transferred
to a nascent polypeptide
Asparagine must be located in a specific consensus sequence in
the primary structure

21. Asparagine must be located appropriately in the three
dimensional structure of the protein
Asparagine must be found in the luminal side of the
endoplasmic reticulum for N-linked glycosylation to be
initiated.
oligosaccharyltransferase is the enzyme responsible for the
transfer of the precursor glycan to a polypeptide acceptor

23. N-glycan processing is carried out in endoplasmic reticulum
and the Golgi body.
Upon transferring the completed glycan onto the nascent
polypeptide, three glucose residues are removed from the
structure.
Enzymes known as glycosidase remove some sugar residues.
Three main types of glycans:
 High mannose,
 Hybrid and
 Complex glycans.

26. O-linked glycosylation is the attachment of a sugar
molecule to an oxygen atom in an amino acid residue in
a protein
 O-linked glycosylation is a form of glycosylation that
occurs in the Golgi apparatus in eukaryote
The enzyme helps for glycosylation is
glycosyltransferase
The enzymes involved are located in various sub
compartments of the Golgi apparatus

27. Each glycosylation reaction involves the appropriate nucleotide-
sugar.
 Dolichol-P-P-oligosaccharide is not involved, nor are
glycosidases; and the reactions are not inhibited by tunicamycin.
 O-Glycosylation occurs post translationally at certain Ser and Thr
residues

28. Carboxyl terminal amino acid of a protein via a phophoryl –
ethanolamine moiety joined to an oligosaccharide (glycan) which
in turn is linked via glucosamine to phophatidyl inositol

30.  It may allow greatly enhanced mobility of a
protein in the plasma membrane
It connect with signal transduction pathways
GPI structures can target certain proteins to apical
domains and also basolateral domains

31. Acetylcholinesterase (red cell membrane)
Alkaline phosphatase (intestinal, placental)
 Decay-accelerating factor (red cell membrane)
5’-Nucleotidase (T lymphocytes, other cells)

32. Almost all the plasma proteins of humans—except
albumin—are glycoprotein.
Many proteins of cellular membranes contain substantial
amounts of carbohydrate.
A number of the blood group substances are glycoproteins,
whereas others are glycosphingolipids.

33. Certain hormones are glycoprotein.
•Hormones that are glycoprotein include:
Follicle-stimulating hormone
 Luteinizing hormone
 Thyroid-stimulating hormone
Human chorionic gonadotropin
 Alpha-fetoprotein
 Erythropoietin (EPO)

36.  200 monosaccharides are found in nature
 8 are commonly found in oligosaccharide chains of
glycoproteins
 N-Acetylneuramic acid(NeuAc) is usually found at terminal
of oligosaccharide chain
 It attached to subterminal galactose(Gal) or
Nacetylgalactosamine(Gal-Nac) residue
 The other sugars are found in more internal positions
 Sulphate is often found in glycoproteins, usually attached to
Gal, GalNac, or GlcNac

38. ❄ Found in secretions of the gastrointestinal, respiratory, and
reproductive tracts and also in membranes of various cells
❄ Exhibit high content of O-glycan chains, usually containing
NeuAc
❄ Contain repeating amino acid sequences rich in serine,
threonine, and proline

39. ❄ Extended structure contributes to their high visco elasticity
❄ Form protective physical barrier on epithelial surfaces
❄ Involved in cell-cell interactions
❄ Mask certain surface antigens

40.  A number of glycosidases of defined specificity have
proved useful in examining structural and functional
aspects of glycoproteins
 These enzymes act at either external (exoglycosidases) or
internal (endoglycosidases) positions of oligosaccharide
chains.

42.  Lectins are carbohydrate-binding proteins that
agglutinate cells or precipitate glycoconjugates
 Number of lectins are themselves glycoproteins
 Immunoglobulins that react with sugars are not
considered lectins.
 Lectins contain at least two sugar-binding sites

43.  Proteins with a single sugar-binding site will not
agglutinate cells or precipitate glycoconjugates
 The specificity of a lectin is usually defined by the sugars
 They are best at inhibiting its ability to cause
agglutination or precipitation
 Enzymes, toxins, and transport proteins can be classified
as lectins if they bind carbohydrate

47.  To reach the plasma membrane of an oocyte, a sperm has to
traverse the zona pellucida (ZP), a thick, transparent, non
cellular envelope that surrounds the oocyte
 The zona pellucida contains three glycoproteins ZP1–3
 ZP3:-an O-linked glycoprotein that functions as a receptor
for the sperm.

48. ❄ A protein on the sperm surface, galactosyl transferase,
interacts with oligosaccharide chains of ZP3
❄ This interaction, by transmembrane signaling, induces the
acrosomal reaction
❄ Enzymes such as proteases and hyaluronidase and other
contents of the acrosome of the sperm are released
❄ Liberation of these enzymes helps the sperm to pass
through the zona pellucida
❄ Reach the plasma membrane (PM) of the oocyte

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(29th ED).
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edition.W.H.Freeman and Company, New york
 H. Robert Horton, Laurence A. Moran, K. Gray Scrimgeour,Marc D. Perry,
J. David Rawn : Principles of Biochemistry-(4 thED)
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(4th ED)
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