Lecture 18: glycoconjugates

Glycoconjugates

Carbohydrates covalently linked to other biomolecules

Function

Cell communication, labeling of proteins, recognition sites for extra cellular molecules

Proteoglycans

Part of ECM, one or more sulfates glycosaminoglycan chain are bound to protien

Proteoglycan shape

Rod protein backbone with lots of carbohydrates linked

Proteoglycans: Glycosaminoglycans (GAGs)

Linked carbohydrates to proteins backbone

Proteoglycans: GAGs monomers

Amino sugar, N-acetyl-glucosamine or n-acetyl-galactosamine, and a negatively charged monomer

Proteoglycans: Sulfate esters in GAGs

Some have sulfate esters that are highly negative

Proteoglycans: GAGs and ECM

Forms a mesh work of fibrous proteins that make up the ECM

Proteoglycans: Heparin And heparan sulfate

Heparin is linear and heparan sulfate is heparin like polysaccharide but attached to proteins, prevents blood clotting and regulates blood vessel formation

Proteoglycans: glycosaminoglycans linked to core

Different types linked through carbohydrate polymer tetrasaccharide bridge, link from anomeric carbon of xylose to serine hydroxyl

Proteoglycans: tetrasaccharide bridge sequence

Ser-gly-x-gly

Proteoglycans: syndecans

Protien has a single transmembrane domain (integral)

Proteoglycans: glypicans

Protein is anchored to lipid membrane (GPI anchor) in the membrane

Proteoglycan aggregates

Supramolecular assemblies of many core proteins bound to hyaluronan

Proteoglycans: hyaluronan and aggrecan

Form huge non covalent aggregates, hold a lot of water and provide lubrication, covers join surfaces to reduce rubbing

Extra cellular matrix

Elastic, serves as physical barrier, Proteoglycan aggregates, collagen, elastin are main components

ECM and cell interaction

Syndecans and integrins allow signal transmission and reception

Glycoproteins

Protein with small oligosaccharide, branched or unbranched attatched via anomeric carbon to amino acid on protein (Ser, Thr, and Asn are common)

Glycoproteins: linkage

Anomeric carbon attatched through glycosidic bond, O linked to hydroxyl group of Ser or Thr and N linked to amide nitrogen of Asn

Glycolipids

Lipids with covalently bound oligosaccharides, components of cell membranes act as lectins binding site

Glycolipids: in vertebrates

Ganglioside carbohydrate composition is blood group

Glycolipids: gram neg bacteria

Lipopolysaccharides cover peptidoglycan layer, lipid A is endotoxin in some species, core region, and O specific chain region

Glycolipids and glycosphingolipids

Often on cell envelope with oligosaccharide chains exposed on cells surface