biomolecules

1. Photo by Markus Spiske · CC-License: CC BY · www.temporausch.com

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

49.  Robert K. Murray and David A. Bender .Harper’s Illustrated Biochemistry.
(29th ED).
 Jeremy M Berg , John M. Tymoczko and Lubert Stryer (2002):Biochemistry.
( 5th ED).W.H Freeman and Company,New York
 David Michael M. Cox .Lehninger Principles of biochemistry – 5th
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)
 Devlin, Thomas M: Textbook of Biochemistry : With Clinical Correlations-
(4th ED)
 Voet, Donald; Voet, Judith G.; and Pratt, Charlotte W. (2002). Fundamentals
of Biochemistry, updated edition. New York: Wiley