Glycosylation

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Slide1: 

Covalent Modification of protein termini and side chains: overview

Slide2: 

Protein Topogenesis: Overview

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. http://www.chem.qmul.ac.uk/iupac/misc/glycp.html A glycoprotein is a compound containing carbohydrate (or glycan) covalently linked to protein. The carbohydrate may be in the form of a monosaccharide, disaccharide(s). oligosaccharide(s), polysaccharide(s), or their derivatives (e.g. sulfo- or phospho-substituted). One, a few, or many carbohydrate units may be present.

The most common modifications in glycoproteins occurs through N- or O- glycosidic bonds: 

The most common modifications in glycoproteins occurs through N- or O- glycosidic bonds O-linkage: Ser, Thr (GalNAc) N-linkage: Asn (GlcNAc) ~ 50% of all eukaryotic proteins are glycosylated. Of these, 90% contain N-linked glycans (motif Asn-X-Ser/Thr; X == P or D). The predominant sugars found in glycoproteins are glucose, galactose, mannose, fucose, GalNAc and GlcNAc.

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Proteoglycans (modified by glycosaminoglycans) GPI-linked proteins N-linked glycoproteins. We are going to look at three types of carbohydrate modifications of proteins:

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Proteoglycans An extreme form of protein glycosylation in which the carbohydrate units are polysaccharides that contain amino sugars (glycosaminoglycans). http://www.chem.qmul.ac.uk/iupac/misc/glycp.html The difference between proteoglycans and glycoproteins resides in the level and types of carbohydrate modifications (ie proteoglycans are andgt;andgt; complex) A proteoglycan is a protein glycosylated by one or more (up to about 100) glycosaminoglycans. The glycosaminoglycans are repeating disaccharide chains of three types: chondroitin sulfate/dermatan sulfate, heparan sulfate/heparin, or keratan sulfate.

Glycosaminoglycans: 

Glycosaminoglycans

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http://web.indstate.edu/thcme/mwking/glycans.html Also see table 19-4 from Alberts Glycosaminoglycans

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Glycosaminglycans and proteoglycans can associate to form huge polymeric complexes in the ECM. Syndecan, an integral membrane proteoglycan

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GPI anchors ('glypiation')

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N-linked Glycosylation of Asn residues Helenius andamp; Aebi (2001) Science 291, 2364 N-linked glycoproteins all contain a common core carbohydrate consisting of 3 mannose groups and two GlcNAc groups.

The precursor oligosaccharide is synthesized in a multi-step process and transferred to a protein en bloc: 

The precursor oligosaccharide is synthesized in a multi-step process and transferred to a protein en bloc

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Helenius andamp; Aebi (2001) Science 291, 2364 Every N-linked glycan is subjected to extensive modification while traversing the Golgi by a battery of mannosidases, glycosyltransferases and glucosidases All of the diversity of N-linked oligosaccharide structures on mature glycoproteins results from later modifications of the original precursor.

Modified N-linked glycans are divided into three families:: 

Modified N-linked glycans are divided into three families: Lots of heterogeneity amongst different proteins and even within a population of the same protein

N-linked glycans play a role in at least three different stages during the existence of a glycoprotein:: 

N-linked glycans play a role in at least three different stages during the existence of a glycoprotein: Structure and function of mature glycoprotein (many examples) Intracellular transport and targeting eg, mannose-6 receptors for lysosomal transport Sorting of proteins in ER, Golgi and PM/secretion Protein folding and quality control in the ER Helenius and Aebi, Annu. Rev. Biochem, 2004, 73: 1019-1049

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Sears andamp; Wong (1998) Cell. Mol. Life Sci. 54, 223 Effects of glycosylation: • Rigidity

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Effects of glycosylation: • Rigidity • Molecular recognition Saxon andamp; Bertozzi (2001) Ann. Rev. Cell. Dev. Biol. 17, 1

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Rudd et al. (2001) Science 291, 2370 ‘immunological synapse’ * Model * Antigen Presenting Cell T-Cell Orient binding faces Provide protease protection Restrict nonspecific lateral protein-protein interactions Structural rigidity Modulate geometry and spacing of interactions

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Effects of glycosylation: • Rigidity Molecular recognition • Solution properties: Stokes radius, viscosity, solubility Secondary structure/nucleate b turns? Folding ◊ Rate ◊ Disulfide bond formation ◊ Proteolytic processing (This effect is highly variable: for some proteins glycosylation is essential, for others dispensable.) Stability • Activity • Serum half-life (asialoglycoprotein receptor) • Interaction with chaperones

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95% larger Rudd et al. (2001) Science 291, 2370 constrained to biantennary CHOs * Model * Structural roles for glycosylation in antibodies

Glycosylation as a marker for protein quality control: 

Glycosylation as a marker for protein quality control ERAD: ER-associated degradation pathway COP: Coatomer protein ERGIC: ER-Golgi intermediate compartment TGN: Trans Golgi network Many mechanisms for protein folding and quality control are found in the ER Protein folding quality control (QC) occurs co-translationally in the lumen of the ER. N-linked glycosyl groups are utilized to help direct QC pathway

Glycosylation as a marker for protein quality control: 

Glycosylation as a marker for protein quality control Calnexin and calreticulin are ER chaperones of the lectin superfamily, bind to terminal glc group Glucosyl transferase recognizes misfolded glycoprotein, adds terminal glucose group.