glycoproteins
DESCRIPTION
GlycoproteinsTRANSCRIPT
Glycoproteins(N & O Glycosylation ).
Presented By: Dr. Kapil Dev
OBJECTIVES:
• Glycoproteins
• Biological Roles Of Glycoproteins
• Glycosylation (N & O linkage)
• Diseases
Glycoproteins• They are proteins that contain oligosaccharide
(glycan) chains covalently attached to their polypeptide backbones.
• Eight sugars predominate in human glycoproteins
• Glucose, Galactose, Mannose, N-Acetyl neuraminic acid, Fucose, N-Acetylgalactosamine, N-Acetylglucosamine, Xylose .
Functions Served by Glycoproteins:
Classes of Glycoproteins:
O-Linked Glycoproteins
GalNAcSer(Thr) linkage
GlcNAc-Ser[Thr] linkage
N-Linked Glycoproteins
Complex
Hybrid
High-mannose
GPI-Linked Glycoproteins
Other minor groups
• O-glycosidic linkage-hydroxyl side chain of serine or threonine and a sugar such as Nacetylgalactosamine (GalNAc-Ser[Thr])
• N-glycosidic linkage-amide nitrogen of asparagine and N-acetylglucosamine (GlcNAcAsn)
• Glycosylphosphatidylinositol-anchored (GPI-anchored, or GPI-linked)- 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 phosphatidylinositol
O-linkage (N -acetylgalactosamine to serine)
O-linkage
1. The GalNAcSer(Thr) linkage is the predominant linkage. This type of linkage is found in mucins
2. Proteoglycans contain a Gal-Gal-Xyl-Ser trisaccharide
3. Collagens contain a Gal-hydroxylysine (Hyl) linkage. 4. Many nuclear proteins (eg, certain transcription
factors) and cytosolic proteins contain side chains consisting of a single GlcNAc attached to a serine or threo-nine residue (GlcNAc-Ser[Thr]).
• Carrier systems (permeases, transporters) are necessary to transport nucleotide sugars across the Golgi membrane.
• Systems transporting UDP-Gal, GDP-Man, and CMP-NeuAc into the cisternae of the Golgi apparatus
antiport systems; influx of one molecule of nucleotide sugar is balanced by the efflux of one molecule of the corresponding nucleotide (eg,UMP, GMP, or CMP)
Biosynthesis of O-Linked Glycoproteins:
• Stepwise donation of sugars from nucleotide sugars such as:– UDP-GalNAc– UDPGal– CMP-NeuAc
• Enzymes involved: membrane-bound glycoprotein glycosyltransferases.
• Enzymes located in various subcompartments of Golgi apparatus.
Schematic diagram of a mucin
N-linkage(N -acetylglucosamine to asparagine)
The boxed area encloses the pentasaccharide core common to all N-linked glycoproteins
Dolichol-P-P-Oligosaccharide:
Structure of Dolichol (polyisoprenol)
Structure of dolichol-P-P-oligosaccharide
Dol-P-P-GlcNAc2Man9Glc3.
Biosynthesis of DolicholP-P-oligosaccharide:
• Dolichol Dolichol phosphate (Dol-P)
• Dol-P + UDP-GlcNac Dol-P-P-GlcNac +UMP
dolichol kinase
ATP ADP
Biosynthesis of DolicholP-P-oligosaccharide (contd.)
1= oligosaccharide: protein transferase2= -glucosidase I3= -glucosidase II 4= endoplasmic reticulum 1,2-mannosidase 5=Golgi apparatus -mannosidase II= N-acetylglucosaminylphosphotransferase II= N-acetylglucosamine-1-phosphodiester α-N-acetylglucosaminidase
6= N-acetylglucosaminyltransferase I7= Golgi apparatus -mannosidase II8= N-acetylglucosaminyltransferase II 9= fucosyltransferase10= galactosyltransferase11= sialyltransferase.
Differences between O- & N- linked Glycoproteins:
N-Linked GlycoproteinsSynthesis:-
• Cotranslationally• En-bloc transfer of
Oligosachharide chain on the protein and further modification.
• Enzymes not membrane bound.
• Dolichol P-P-Oligosaccharide involved.
• Inhibited by Tunicamycin
O-Linked GlycoproteinsSynthesis:-
• Post- translationally• Oligosaccharide chain
synthesized on the protein.
• Enzymes membrane bound.
• Dolichol not involved.• Not inhibited by Tunicamycin
e.g. Mucin e.g. Calnexin
Some Diseases Due to Glycoproteins:
DiseaseHEMPAS
Leukocyte adhesion deficiency, type II
Paroxysmal nocturnal hemoglobinuria
I-cell disease
Congenital disorders of glycosylation
I-Cell Disease:
Summary of the causation of I-cell disease
Congenital Disorder of Glycosylation• Previously called carbohydrate-deficient glycoprotein
syndrome.
• Glycosylation of a variety of tissue proteins and/or lipids is deficient or defective.
• CDG-I mainly in ER & related to steps prior processing of glycan chains and transfer of oligosac chain to protein.
• CDG-II includes all defects localized in the processing of N-glycans on the glycosylated protein. These are situated mainly in the Golgi compartment.
• Often cause serious, sometimes fatal, malfunction of several different organ systems (especially the nervous system, muscles, and intestines) in affected infants.
Diseases due to O-linked Glycoproteins:
• Inborn errors of O-glycan metabolism can lead to – severe autosomal recessive multisystem
syndrome with neurologic involvement,
– some defects, for example, those in persons with the Bombay blood group do not produce a clinical phenotype.
Defects of O glycosylationName Defect Clinical features
Defects in mucin-type o-glycan biosynthesis(familial tumoral calcinosis (FTC)
UDP-GalNAc transferase 3 (ppGalNAc T3) abnormal cleavage and secretion of the phosphaturic factor FGF23.
massive calcium deposits in skin and subcutaneous tissues and unresponsiveness to parathyroid hormone
Defects in o-galactosyl glycan biosynthesisLysyl hydroxylase-1 deficiencyEhlers–Danlos syndrome type Via
Deficiency or mutation in Lysyl hydroxylase-1
neonatal kyphoscoliosis,
generalized joint laxity, skin fragility, and severe muscle hypotonia at birth
Defects in o-mannosyl glycan biosynthesis
Walker–Warburg syndrome (WWS) and limb-girdle muscular dystrophy type 2K
Muscle–eye–brain disease (MEB)
HEMPAS:• Hereditary Erythroblastic Multinuclearity With A Positive
Acidified Lysis Test .
• Also known as Congenital Dyserythropoietic Anemia Type II.
• Claimed to be due to defects in alpha–mannosidase II
• Characterized by– Ineffective erythropoiesis– Hemolysis & erythroblast morphological abnormalities– Hypoglycosylation of some red blood cell (RBC) membrane proteins.
• Treatment consists of frequent blood transfusions and chelation therapy
Leukocyte adhesion deficiency (LAD) II:
• A congenital disorder of glycosylation
• Mutations affecting the activity of a Golgi-located GDP-fucose transporter
• Marked decrease in neutrophil rolling
• Subjects suffer
– Life-threatening, recurrent bacterial infections
– Psychomotor and mental retardation
• The condition appears to respond to oral fucose
Paroxysmal Nocturnal Hemoglobinuria:
Scheme of causation of paroxysmal nocturnal hemoglobinuria
SUMMARY
• Glycoproteins • Functions Served by Glycoproteins • Classes of Glycoproteins• N and O linkage• Glycoproteins in Disease:
• I-Cell Disease• CDGs• others
References:• Harper's Illustrated Biochemistry, Twenty-Eighth Edition.
• Lehninger’s Principles of Biochemistry, Fifth Edition.
• Stryer’s Biochemistry, Sixth Edition.
• Dennis JW, Nabi IR, Demetriou M. Metabolism, cell surface organization, and disease. Cell. 2009 Dec 24;139(7):1229-41. Review.
• Vagin O, Kraut JA, Sachs G. Role of N-glycosylation in trafficking of apical membrane proteins in epithelia. Am J Physiol Renal Physiol. 2009 Mar;296(3):F459-69. Epub 2008 Oct 29.
• Luther KB, Haltiwanger RS. Role of unusual O-glycans in intercellular signaling. Int J Biochem Cell Biol. 2009 May;41(5):1011-24. Epub 2008 Oct 8. Review.
O- mannosyl glycan• O-Mannosyl glycans are a less common type of protein
modification
• present on a limited number of glycoproteins in the brain, nerves
and skeletal ms.
• Eg α-Dystroglycan– It is a laminin receptor– 2 subunits– α-DG interacts with laminin-2 in the basal lamina and
β-DG binds to dystrophin– induces Ach receptor clustering at neuromuscular junctions