Glycoproteins and lectin ( Conjugated Carbohydrate)

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 .

3. 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.

4. 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.

5. 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

6. 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 .

7. 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.

8. 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.

9. Lectin - Structure

10. 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 .

11. 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.

12. 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.

13. 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.

14. Trafficking of lysosomal hydrolases

15. 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 ,

16. 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 ) G lycoproteins 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)