Lecture 6: Structure and function of carbohydrates Flashcards
Digestion of dietary carbs by what enzymes?
Mouth - Salivary α-amylase
GI tract - Pancreatic α-amylase to small oligosaccarides
then epithelial brush border enzymes tomonosaccarides
Lactose intolerance lacks what enzyme?
Lactase, which turns lactose to glucose and galactose
Enzyme that digests sucrose
Isomaltase-sucrase
Where does monosaccharide uptake from the intestinal lumen take place?
Absorption takes place in the small intestine
Glucose and galctose transporter?
SGLT1
Fructose transporter
GLUT-5
The glucose transporter in red blood cells and the blood-brain barrier (on the endothelial cells of the brain blood vessels).
GLUT-1
The glucose transporter in main bidirectional transporter in liver, kidney, pancreas.
GLUT-2
The glucose transporter used by neurons
GLUT-3
The glucose transporter pecific for muscle and adipose tissue. (the only insulin-regulated glucose transporter! )
GLUT-4
Composed of repeating disaccharide units of an acidic sugar (glucuronic or iduronic acid) and an N-acetylated amino sugar.
Glycosaminoglycans (GAGs)
What do GAGs behave like due to their negative charges?
Like sponges, the negative charge attracts a lot of water
GAG
Localized in cartilage, tendon, ligaments, aorta
Chondroitin sulfate
GAG
Localized in skin, blood vessels, heart valves
Dermatan sulfate
GAG
Localized in cartilage, cornea
(No acidic sugar present)
Keratan sulfate
GAG
Localized in basement membranes, cell surfaces
Heparan sulfate
GAG
Localized in mast cells
Heparin
GAG
Localized in joints, cartilage, umbilical cord, vitreous humor of the eye
(Not attached to protein, not sulfated)
Hyaluronic acid
Proteins to which at least one glycosaminoglycan (GAG) chain is attached.
Proteoglycans
Through what linkages are GAGs attached to proteoglycans?
Either a hydroxyl of Ser or an amide of Asn.
Cartilage proteoglycan, Large, contains ~ 100 GAG chains.
Aggrecan.
The large GAG content of cartilage provides the tissue with resilience under compressive forces.
Diseases that are accompanied by excessive degradation of cartilage proteoglycans, which leads to defective cartilage function.
Osteoarthritis, rheumatoid arthritis and systemic lupus erythematosus (SLE)
Explain GAG degradation
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 called what
mucopolysaccharidoses – a group of lysosomal storage diseases. ex. Hurler syndrome, Hunter syndrome
Syndrome with
-Complete deficiency in the alpha-L-iduronidase
-Corneal clouding,mental retardation, dwarfing, coarse facial features, upper airway obstruction
-degradation of dermatan sulfate and heparan sulfate are affected
-deposition in coronary artery leads to ischemia and early death
-treated by bone marrow or cord blood transplantation
enzyme replacement therapy
Hurler syndrome (MPS I H) Most severe form
Syndrome with
- Iduronate sulfatase deficiency
- x-linked deficiency
- wide range of severity, no corneal clouding, but physical deformity and mental retardation mild to severe
- Degradation of dermatan sulfate and heparan sulfate are affected.
- enzyme replacement therapy available
Hunter syndrome (MPS II) Less severe form
Proteins with oligosaccharides attached to them covalently
Glycoproteins
Function of glycoproteins
- Cell surface recognition molecules/receptors (including binding sites for pathogens)
- Blood group antigens
- Extracellular matrix molecules (e.g. laminins, collagens, fibronectin)
- Mucins (lubricants, surfactants)
- Plasma proteins (oligosaccharide chains increase the solubility of the protein)
Oligosaccaride type:
- attached to the hydroxyl group of Ser or Thr.
- mostly linear structure
O-linked
Oligosaccaride type:
- attached to the amide group of asparagine.
- branched structure
- can be either mannose-rich or complex.
N-linked
Influenza virus binds to specific sugars present on cell surface glycoproteins on target cells, what are these sugars called?
Sialic acids
Designations of influenza viruses: HxNy
What does the H stand for?
Hemagglutinin
- Binds to sialic acid on target cells
- Facilitates viral entry into target cell
Designations of influenza viruses: HxNy
What does the N stand for?
Neuraminidase (enzyme)
- Cleaves sialic acid from target cell surface
- Facilitates release of new viruses from infected cells
How does tamiflu fight flu symptoms?
Tamiflu is a competitive enzyme inhibitor that attaches to Neuraminidase, preventing the cleavage and spread.
Delivering the lysosomal enzymes into the lysosomes requires what?
Phosphorylation of a mannose on an N-linked oligosaccharide of the lysosomal enzyme.
What causes Mucolipidosis II (I-cell disease)?
- 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)
Characteristics of Mucolopidosis II (I-cell disease)
Skeletal abnormalaties, restricted joint movement, coarse facial features, severe psychomotor impairment.
Death usually occurs by 8 years of age.
Glycolipids function
Glycolipids are essential components of the outer layer of the plasma membrane.
Brain and the peripheral nervous system are very rich in glycolipids.
Glycolipids are a major component of myelin sheets that protect the axons and help the effective communication between neurons.
Glycolipids (together with glycoproteins) are also blood group antigens (i.e antigens on the surface of red blood cells).
Defective lysosomal degradation of glycolipids leads to this lysosomal storage diseases - frequently has neurological consequences.
Sphingolipidoses
Lysosomal storage disease
Inability to degrade glycosaminoglycans
Mucopolysaccharidoses
Lysosomal storage disease
Inability to degrade plasma membrane sphingolipids
Sphingolipidoses
Lysosomal storage disease
Inability to degrade glycogen
Pompe disease
Lysosomal storage disease
Inability to deliver acid hydrolases into lysosomes
Mucolipidosis II (I-cell)
antibody B
N-acetylgalactosamine transferase
antibody A
galactosyltransferase
