Dental Biochemistry Lecture 45 Carol Lutz, PhD Complex Carbohydrates: Glycoproteins and Proteoglycans

LECTURE OUTLINE Differences between glycoproteins and proteoglycans Functions and structures of glycoproteins and proteoglycans Synthesis and degradation of glycoproteins and proteoglycans Pathology related to glycoproteins and proteoglycans

Suggested Reading Medical Biochemistry, third edition, edited by Baynes and Dominiczak. Chapter 26, pages 351-366 on glycoproteins Chapter 28, pages 384-388 on proteoglycans

Differences between glycoproteins and proteoglycans Proteins conjugated to saccharides lacking a serial repeat unit Glycoproteins Protein>>carbohydrate Carbohydrate>>protein Proteoglycans Proteins conjugated to polysaccharides with serial repeat units Glycosaminoglycans Mucopolysaccharides Repeat unit HexN and HexUA

These molecules function in the extracellular matrix (ECM)

Overview of glycoproteins: --carbohydrate chain short --no serial repeats --often branched, not linear --variable amounts of carbs --wide range of functions

Function Glycoprotein 1. Structural molecule Collagens Some Functions of Glycoproteins _________________________________________________ Function Glycoprotein 1. Structural molecule Collagens 2. Lubricant Mucins 3. Transport molecule e.g. Transferrin, Ceruloplasmin 4. Immune system Immunoglobulins, Histocompatibility antigens, Blood group determinants 5. Hormone e.g. HCG, TSH 6. Enzymes e.g. Alkaline phosphatase 7. Blood clotting e.g. Fibrinogen 8. Cell surface recognition Lectins __________________________________________________

STRUCTURE OF GLYCOPROTEINS One or more carbohydrate chains--covalently linked to a protein. The chains may be neutral or negatively charged. They are frequently branched. There are two types of glycosidic links: 1. O-glycosidic link O-glycosidic link between galactose or glucose and the hydroxyl group of hydroxylysine (i.e. collagen). Other O-linked glycoproteins have a glycosidic link between N-acetyl galactosamine and either serine or threonine (i.e. blood group substances and salivary mucins). 2. N-glycosidic link N-glycosidic links exist between N-acetylglucosamine and asparagine. There are two types: A. High mannose B. Complex. For example, in addition to mannose they may contain N-acetylglucosamine, galactose, fucose and N-acetylneuraminic acid (sialic acid)

Lippincott

SYNTHESIS OF GLYCOPROTEINS Synthesized on ribosomes attached to the RER, then transported via vesicles to the Golgi for sorting The units in the saccharide chains are added from UDP-glucuronic acid, UDP-N-acetylgalactosamine and GDP-mannose. Sialic acid in glycoproteins is added from CMP-NANA. These additions are catalyzed by specific glycosyltransferases. For synthesis of O-linked glycoproteins, addition is direct. For N-linked glycoproteins, the chain is formed on dolichol pyrophosphate and then transferred to the protein.

Defects can lead to a number of diseases/disorders DEGRADATION OF GLYCOPROTEINS Degradation of the saccharide chains is achieved by hydrolytic enzymes present in lysosomes. The enzymes act on the ends of the chains on a last-on-first-off basis. Defects can lead to a number of diseases/disorders

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.

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Proteoglycans are usually structural components of the extracellular matrix; some have a lubricant role. --bind large amounts of water --cell/cell signalling and adehsion roles Heparin is normally intracellular and it inhibits blood clotting.

The proteoglycans include: Hyaluronic acid Chondroitin sulfate Dermatan sulfate Heparan sulfate Heparin Keratan sulfate Proteoglycan monomers typically are bound non-covalently to a hyaluronic acid molecule in association with linker proteins in a “bottle brush” arrangement.

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STRUCTURE OF PROTEOGLYCANS (GAGs) Proteoglycans usually consist of a core protein covalently linked to a glycosaminoglycan. The glycosaminoglycan typically consists of a long polysaccharide chain with a repeating disaccharide motif. Proteoglycans are polyanionic. The negative charge comes from the presence of carboxyl and/or sulfate groups. The carboxyl group is on either D-glucuronic acid or its epimer L-iduronic acid. The repeating disaccharide is glycosidically linked to a serine residue on the protein through a galactose-galactose-xylose- serine sequence.

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SYNTHESIS OF PROTEOGLYCANS Synthesized in Golgi The units in the saccharide chains are elongated in alternating acidic/amino sugars, donated from UDP derivatives. Last step is sulfation of some amino sugars. For glycosaminoglycan synthesis and synthesis of O-linked glycoproteins, the addition is direct. For N-linked glycoproteins, the chain is formed on dolichol pyrophosphate and then transferred to the protein.

Defects can lead to a number of diseases/disorders DEGRADATION OF PROTEOGLYCANS Some proteoglycans must be phagocytosized first Degradation of the saccharide chains is achieved by hydrolytic enzymes present in lysosomes. The enzymes act on the ends of the chains on a last-on-first-off basis. Defects can lead to a number of diseases/disorders

MUCOPOLYSACCHARIDOSES (MPS) Rare inborn errors in the degradation of glycosaminoglycans result in a series of diseases called mucopolysaccharidoses; characterized by mental retardation and/or structural defects. MPS Type I Hurler’s syndrome results from a deficiency of alpha-L-iduronidase. Heparan sulfate and dermatan sulfate accumulate. There is growth and mental retardation with characteristic facial changes. MPS Type II Hunters syndrome is similar to Hurler’s syndrome but the enzyme deficiency is for iduronate sulfatase and the inheritance is X-linked. MPS Type III Sanfilipo’s syndrome is caused by a deficiency of one of four enzymes of which three are hydrolases and one is an N-acetyltransferase. There is severe mental retardation but only mild structural features. Other MPS Types are IV, VI and VII. There is no MPS Type V.

MPS I (Hurler Syndrome) A deficiency of L-iduronidase leads to mental retardation and structural changes due to accumulation of dermatan sulfate and heparan sulfate 26

MPS II (Hunter Syndrome) X-linked disease due to a deficiency of iduronate sulfatase 27

MPS III (Sanfilippo Syndrome) Deficiency in one of four degradative enzymes leads to severe mental retardation but little structural change 28

MPS IV (Morquio Syndrome) Deficiency of a galactose-6-sulfatase or a beta-galactosidase leads to accumulation of keratan sulfate with normal intelligence but severe deformity 29

Summary Glycoproteins and proteoglycans are distinct: --functions/structures --synthesis/degradation --associated pathologies