Proteoglycans and Glycoproteins

Differentiate between proteoglycans and glycoproteins

How do we build complex carbohydrates? List out the nucleotides we use for the different saccharides

• Monosaccharides added one at a time

• have to be in a nucleotide-activated form.

• most frequently = UDP, but can be GDP or CMP.

• The nucleotide-activated form is specific for a given monosaccharide.

Describe Glycosyltransferases; Function? Why are we using these nucleotides?

• catalyzes the addition of monosaccharides to other molecules.

• Specific for a given monosaccharide.

• hydrolysis of the nucleotide-linkage provides energy for the formation of the glycosidic bond.

Describe Glycosaminoglycans (GAGs)

(Composition? Attachment? Synthesis? Charge? Function of charge

• repeating disaccharide (acidic sugar (glucuronic or iduronic acid) + N-acetylated amino sugar.)

• All (but hyaluronic acid) are attached to a protein core

• All (but hyaluronic acid) are synthesized in the Golgi. Hyaluronic acid is synthesized at the plasma membrane

• All negatively charged.

• All (but hyaluronic acid) are sulfated.

• negative charges attract water →“sponges”.

  • They confer resilience to tissues under compressive forces.

Describe Hyaluronic Acid

(Importance? location? Function? Clinical importance?

• Widespread structural GAGs in the human body
  

• Main locations: cartilage, synovial fluid, vitreous humor of the eye.
  

• Function:

  • Major lubricant of the joints.

  • Essential for embryogenesis, morphogenesis, and wound healing.
    

• Clinical applications:

  • used in eye surgery (Healon)

  • treatment of osteoarthritic pain (intraarticular Synvisc injection)

Describe Chondroitin Sulfates

(Sulfation? Location? Function? )

• sulfated in the 6th and/or the 4th (*) position of N-acetyl- galactosamine

• Largest concentration is in cartilage, (but found in many other tissues.)

• Main GAG in aggregating proteoglycans (that aggregate with hyaluronic acid)

What is the most abundant of GAGs in the human body?

Chodroitin Sulfates

Describe Dermatan Sulfate

(Difference? Location? clinical importance?

Differs from chondroitin-4-sulfate:

• orientation of carboxyl group

Location:

• Mainly found in skin, blood vessels, and heart valves

Clinical importance:

Increased accumulation of dermatan sulfate in mitral valves → thickening and abnormal displacement of the valve into the left atrium of the heart (mitral valve prolapse).

Describe Keratan Sulfate

(exception? types? clinical importance?)

• only GAG that does not contain an acidic sugar.
  

• two types of keratan-sulfate (KS):

  • KS I: linked to the amide of Asn (mainly found in cornea).

  • KS II: linked to the hydroxyl of Ser (mainly found in cartilage).
    

Clinical Importance:

• Carbohydrate sulfotransferase 6 (CHST6) deficiency →undersulfated KS in cornea → macular corneal dystrophy (cloudy corneas, vision impairment)

differentiate between Heparin and Heparan Sulfate

• differ in sulfation pattern (heparin = more sulfated).

Heparan sulfate:

• Part of a major proteoglycan (perlecan) in basement membranes

• Frequent component of cell surface molecules/receptors
  

Heparin:

• stored in and released by mast cells

• Used as anticoagulant in medicine (management of myocardial infarction and deep vein thrombosis)

What are proteoglycans? What type of molecules are they? How are they attached?

• Proteins where at least one (GAG) chain is attached.
  

• either cell surface receptors or extracellular matrix molecules.
  

Attachment:

• GAGs are attached to the protein core through a specific linkage region.

  • either a hydroxyl of Ser or an amide of Asn.

What is aggrecan?

• main cartilage proteoglycan (aggrecan) contains ~ 100 GAG chains

  • mainly chondroitin sulfates with some keratan sulfate.

  • has a characteristic bottle-brush structure.

How is Aggrecan retained? Function of GAG?

• Aggrecan is retained in the cartilage ECM through non- covalent interactions with hyaluronic acid and link protein (a small glycoprotein).
  

• GAG content provides the tissue with resilience under compressive forces

Describe the Homeostasis of Cartilage Proteoglycan

Which diseases are associated with degradation of cartilage?

• Osteoarthritis, rheumatoid arthritis, and systemic lupus erythematosus (SLE)

• leads to defective cartilage function.

Describe the Synthesis of Proteoglycans

1. Synthesis of amino sugars (cytosol)

   • Amino sugars derive from glucose.

   • Glutamine provides the amino group.

2. Synthesis of glucuronic acid (cytosol)

   • From UDP-glucose by oxidation

3. Synthesis of core protein (endoplasmic reticulum)

4. Synthesis of the GAG chain (Golgi, but for hyaluronic acid – plasma membrane)

   • synthesized on trisaccharide links at serine residues and sometimes asparagine residues of the core protein.

   • Glycosyltransferases produce the growing GAG chain by the addition of the appropriate UDP-sugars.

5. Sulfation (Golgi)

   • Sulfotransferases add sulfate groups to the GAG chains.

     • The sulfate donor is PAPS (3’ phosphoadenosyl-5’-phosphosulfate)

Describe the Degradation of Glycosaminoglycans; Deficiency?

• GAGs are taken up by the cell through endocytosis.
  

• GAGs are transported to lysosomes where they are degraded by specific enzymes
  

• Deficient lysosomal degradation of GAGs can cause severe diseases (mucopolysaccharidoses – a group of lysosomal storage diseases)

Draw out the steps of the degradation of Glycosaminoglycans and the relevant diseases/symptoms

Describe glycoproteins? How are they different from proteoglycans? Function?

• Proteins with oligosaccharides attached to them covalentl

• The oligosaccharides are smaller than GAGs of the proteoglycans.
  

Function of glycoproteins:

• Cell surface recognition molecules/receptors

  • Includes binding sites for pathogens

• Blood group antigens

• ECM molecules

  • laminins, collagens, fibronectin

• Mucins

  • lubricants, surfactants

• Plasma proteins

  • oligosaccharide chains increase the solubility of the protein

What are the two types of Oligosaccarides? Compare/Contrast them

• O-linked

  • attached to the hydroxyl group of Ser or Thr.

  • linear structure
    

• N-linked

  • attached to the amide group of asparagine.

  • branched structure

  • can be either mannose-rich or complex.

Describe the synthesis of Glycoprotein

1. protein synthesized in ER
   

2. Monosaccharides have to be in nucleotide activated form to be added (usually UDP-linked)
   

3. Monosaccharides are added by glycosyltransferases

Differentiate between synthesis of O-linked glycoproteins and N-linked glycoproteins

O-linked glycoproteins:

• chain built in Golgi.

N-linked glycoproteins:

• chain starts building on lipid (dolichol) in the membrane of ER

  • (Dolichol is a side- product of cholesterol synthesis)

• oligosaccharide precursor transferred to the protein in the ER.

• Oligosaccharide synthesis is completed in the Golgi.

Describe the Role of N-Linked Oligosaccharides in Lysosomal Targeting of Proteins

Delivering enzymes into lysosomes requires the
phosphorylation of a mannose on an N-linked oligosaccharide of the lysosomal enzyme.

Describe Mucolipidosis II (I-Cell Disease); Symptoms?

• Deficient mannose phosphorylation on lysosomal enzymes

• Lysosomal enzymes are not delivered to the lysosomes (instead they are secreted).

• Macromolecules are not degraded (enlarged lysosomes, Inclusion bodies)

What does Osteonectin do?

• Bone glycoprotein

• Interacts with bone minerals and collagen fibers.

• Facilitates collagen fiber mineralization.

What does link protein do?

• Cartilage glycoprotein

• Stabilizes the interaction between hyaluronan and aggrecan.

What does Dystroglycan do

• Skeletal muscle cell membrane glycoprotein
  

• Binds to laminins and mediate cell attachment to the adjacent basement membrane.
  

• Defective glycosylation of dystroglycan leads to muscular dystrophy