Glycoproteins and Mucins B.Sopko

Content Glycoproteins: Structures and Linkages Interconversions and activation of dietary sugars Other pathways of sugar nucleotide metabolism Biosynthesis of oligosaccharides Functions of the oligosaccharide chains of glycoproteins Mucins Pathological glycosylation

Glycoproteins: Structures and Linkages Glycoproteins and Proteoglycans Glycoproteins Protein >> carbohydrate Proteins conjugated to saccharides lacking a serial repeat unit Proteoglycans Carbohydrate >> protein Repeat unit HexN and HexUA Glycosaminoglycans and Mucopolysaccharides

Glycoproteins: Structures and Linkages Structures of the oligosaccharides attached to proteins

Interconversions and activation of dietary sugars

Fructose metabolism

UDP-glucose

GDP-mannose

Synthesis of amino-sugars and sialic acids

Interconversions and activation of dietary sugars - overview

Biosynthesis of oligosaccharides N-linked glycoproteins

N-linked glycoproteins

O-linked glycoproteins

Route from the Golgi complex to the final destination

Functions of the oligosaccharide chains of glycoproteins N-linked oligosaccharides have an important role in protein folding High-mannose oligosaccharides target some proteins to specific sites in the cell The oligosaccharide chains of glycoproteins increase the solubility and stability of proteins Both N- and O-linked glycan structures are involved in recognition processes

High-mannose oligosaccharides target some proteins to specific sites in the cell

N-linked oligosaccharides have an important role in protein folding

The oligosaccharide chains of glycoproteins increase the solubility and stability of proteins

Both N- and O-linked glycan structures are involved in recognition processes

Lectins Lectins are defined as proteins that do not have enzymatic activity but that reversibly bind monosaccharides and oligosaccharides with high specificity.

Mucins - characterization Complex glycoproteins synthetized in epithelial cells Components of mucus secretions covering epithelial cells in gastrointestinal, urogenital, tracheobronchial, ocular and auditory systems of all vertebrates (but they can be found in all eukaryotes) Very rich in carbohydrates (50-90% of mucin mass is composed by sugars) and saccharides are linked to protein via O-glycosidic bond O-glycans are linked to serine/threonine in specific domain called tandem repeat Some mucins can also contain N-glycosidic oligosaccharides, but they are bound only in cysteine-rich domain Marked MUC1 ….

Mucins - structure

Mucins - types Membrane-tethered with TR (membrane mucins) – e.g. MUC1, MUC4 Secreted, cysteine-poor with TR (gel forming cystein-poor mucins) - e.g. MUC7 Secreted, cysteine-rich with TR - MUC2, MUC5AC Mucins without TR

Mucins synthesis

Secreted mucins in the airways mucus gel and their sites of synthesis.

Secreted mucins in the airways mucus gel and their structure

Pathological glycosylation and glycation

I-CELL DISEASE I-cell disease results from an enzyme deficiency so that lysosomal enzymes do not aquire the targeting signal, mannose 6-phosphate. Fibroblasts in this disease have dense inclusion bodies (I-cells) and are deficient in many lysosomal enzymes. The lysosomes become engorged with indigestible substrates, leading to death in infancy.

N-linked oligosaccharides have an important role in protein folding

Carbohydrate- deficient glycoprotein syndromes (CDGSs)

DiseaseEnzyme DeficiencySymptoms/Comments Aspartylglucosaminuria aspartylglucosaminidase (N-aspartyl-β-glucosaminidase) progressive mental retardation, delayed speech and motor development, coarse facial features β-Mannosidosisβ-mannosidase primarily neurological defects, speech impairment α-Mannosidosisα-mannosidase mental retardation, dystosis multiplex, hepatosplenomegaly, hearing loss, delayed speech G M1 Gangliosidosisβ-galactosidase also identified as a glycosphingolipid storage disease or lysosomal storage disease Sandhoff diseaseβ-hexosaminidases A and B also identified as a glycosphingolipid storage disease or lysosomal storage disease Sialidosis Sialidosis (also identified as Mucolipidosis I) neuraminidase (sialidase) myoclonus, congenital ascites, hepatosplenomegaly, coarse facial features, delayed mental and motor development Fucosidosisα-fucosidase progressive motor and mental deterioration, growth retardation, coarse facial features, recurrent sinus and pulmonary infections

Cystic fibrosis Decreased Cl - concentration results in thicker and less fluid mucous secretion

Glycation (spontaneus glycosylation)

Extracellular proteinsIntracellular proteins Long-lived proteins Collagen Type I, III and IV, cartilage, elastin, myelin, proteoglycan Lens crystallins, Neurofilaments Short-lived proteins Plasma proteins such as Apo B, LDL, albumin, immunoglobulins Hemoglobin, enzymes, lysosomes, ribonuclease and several membrane proteins

Glycation (spontaneus glycosylation)