Glycoprotein - Introduction, Structure, Significance. Lectin - Introduction, Structure, Significance. Lipid definition, Some review questions related to Glycoprotein and lectins

1. Glycoproteins and lectin
Presented by:
Jasmine Juliet (Teaching Assistant)
Biochemistry, ACRI, Madurai.

2. Glycoconjugates
 Glycoconjugates is the general classification for carbohydrates .
 Glycoconjugates are covalently linked with other chemical species such
as proteins, peptides and other compounds.
 Glycoconjugates consist of many different categories such as:
 glycoproteins,
 glycopeptides,
 peptidoglycans,
 They are involved in cell–cell interactions, including cell–cell recognition; in
cell–matrix interactions; in detoxification processes.

4. Glycoproteins
 The carbohydrate is an oligosaccharide chain (glycan) that is covalently
bonded to the polypeptide side chains of the protein.
 Sugars in glycoproteins are attached either to the amide nitrogen atom of
asparagine (termed as N-linkage) or to the oxygen atom of serine or
threonine (termed as O-linkage).
 Erythropoietin is a glycoprotein hormone present in blood serum that
stimulates production of RBC.
 Most blood group antigens are glycoproteins and their specificity is
mostly determined by the oligosaccharide present on the surface of
RBC.

6. Glycoprotein - Structure
 Glycoproteins are proteins that contain covalently attached sugar
residues.
 The hydrophilic and polar characteristics of sugars may
dramatically change the chemical characteristics of the protein to
which they are attached.
 The addition of sugars is often required for a glycoprotein to
function properly and reach its ultimate destination in the cell or
organism.

7. Glycoprotein - Function
 Glycoproteins serve many functions in the body.
 Some glycoproteins provide structure e.g. collagens,
 Some glycoproteins are involved in immunity e.g. immunoglobulins (such as
IgG).
 Mucins are secreted into mucus of the respiratory and digestive tracts where the
specific mucins can retain water thus allowing mucus to serve as an effective
lubricant.
 Specific glycoproteins present on the surface of red blood cells determine blood
group type

8. Lectins
 Lectins are a type of protein that can bind to specific carbohydrates.
 They may bind to a soluble carbohydrate or to a carbohydrate moiety
that is a part of a glycoprotein or glycolipid.
 Lectin usually contains two or more binding sites for carbohydrate
units.
 They typically agglutinate red blood cells. Therefore, lectins are also
called as hemagglutinins.
 Some of the plant lectins are Concanavalin A, Ricin,Abrin, Wheat
germ agglutinin, Soybean lectin and Peanut lectin.

9. Lectin – Structure
Two important classes of lectins are the C-type
and P-type lectins.
The C-type lectins require calcium ion for binding
to a carbohydrate. The calcium ion will link the
sugar residue to the lectin.
P-Type lectins contain a phosphate group.

10. Lectin – Functions
 The chief function of lectins is to facilitate cell-cell contact and
communication.
 β - Carbohydrates on the surface of one cell bind to the binding sites of lectins
on the surface of another cell.
 Binding results from numerous weak interactions which come together to form
a strong attraction.
 A lectin usually contains two or more binding sites for carbohydrate units.
 In addition, the carbohydrate-binding specificity of a certain lectin is
determined by the amino acid residues that bind the carbohydrate.

11. Lectin - Structure

12. Lectin - Structure
 Collectins(C-type lectins) are a part of the innate
immune system. Their function is to bind to
oligosaccharides on the surface of microorganisms.
 Binding of collectins to microorganisms may trigger
elimination of microorganisms by aggregation,
complement activation, opsonization, or activation of
phagocytosis.

13. Lectin - Structure
 Selectins (C-type lectins) play a role in the immune system by
recruitment of white blood cells into inflamed tissue.
 White blood cells normally do not interact with the endothelium of
blood vessels. However, inflammation causes the expression of
selectin on the surface of the blood vessel wall.
 Selective carbohydrate units on the surface of white blood cells
present in flowing blood can interact with the selectin.
 This interaction results in "rolling" of the white blood cell on the
endothelial cell surface followed by stable adhesion and migration of
the white blood cell into the inflamed tissue.

15. Lectin - Functions
 Lectins stored in seeds function as storage proteins.
 They also function in defense. When an invading microbe or insect
disrupts the plant’s cellular contents, the stored lectin, may
agglutinate or sicken the pathogen or predator.
 Ricin is a toxic lectin present in castor seeds which inhibit protein
synthesis at ribosomes.
 Legume lectins generally bind either glucose, mannose or galactose.
They are involved in the attachment of nitrogen-fixing bacteria to
legumes and in the protection against pathogens.

16. Lectin - Functions
 Lectins are very essential in trafficking lysosomal hydrolases
synthesized in the golgi bodies to lysosomes.
 Binding of the lectins to mannose-6-phosphate residue present on
lysosomal hydrolases mediate the transport of these enzymes to
lysosomes.
 When the phosphotransferase enzyme is defective, the mannose
phosphate residue is not added to the lysosomal hydrolases and they
are not targeted to the lysosomes. They are excreted outside the cells
and also accumulate in cytosol leading to the I-cell disease.

17. Trafficking of lysosomal hydrolases

18. Review questions
A1.O-linked oligosaccharide are attached to the protein via
a)OH group of serine and tyrosine b) OH group of serine and threonine
c) OH group of threionine and tyrosine d) OH group of threonine only
A2. Lectins are preferentially bind to
a)more polar region of the carbohydrate
b) less polar region of the carbohydrate
c) more and less polar region of the carbohydrate
d)All of the above
B1. What are lectins? (1 mark)
Lectins are a type of protein that can bind to specific carbohydrates.
B2. Give any one example for lectin?
Examples for lectins are Concanavalin A, Ricin,Abrin,

19. Review questions
C1.Write any two functions of lectin?(2 mark)
(i) Lectins stored in seeds function as storage proteins.
(ii) They also function in defense. When an invading microbe disrupts the plant’s
cellular contents, the stored lectin, may agglutinate predator.
C2.What are the significance of glycoproteins?
(i) Glycoproteins provide structure e.g. collagens.
(ii)Some glycoproteins are involved in immunity e.g. immunoglobulins (such as
IgG).
D1. Elaboratly explain the structure of lectin and their significance?(5 mark)