Glycobiology I - Werlen 3/9/16 Flashcards
lysosomal storage diseases
can occur due to
- ER level : protein misfolding, transporter defects
- Golgi level : defects in protein processing, transport preventing exit from Golgi; defective lysosomal targeting
- lysosomal level : accumulation of unmetabolized substrates in lysosome
two types of proteins that might be implicated in LSDs
glycosaminoglycans (GAGs)
glycoproteins
- both glycosylated : proteins with sugar moieties
- key diff: GAGs are mostly (90%) sugar moieties; glycoproteins are mostly proteins
glycosaminoglycans
heteropolysacch chains (repeating disacch units: one acidic sugar, one amino sugar) covalently associated with proteins
- sugars tend to be long, unbranched, negatively charged
- amino sugars usually acetylated → negative charge
- slip/slide past each other (repulsion), giving slippery consistency of mucus/synovial fluid
- can absorb significant quantities of water
glycoproteins
oligosacchs covalently associated with proteins
- mostly protein
- don’t contain repeating units
- sugars are short, branched, not necessarily negatively charged
present in a lot of cell surface proteins: signalling, immune response
GAG classification
6 major groups classified based on:
- monomeric composition
- types of glycosidic linkages
- location of sulfate units
acidic sugars
- glucuronic acid : GlcUA
- iduronic acid : IdUA
- galactose : Gal
N-acetylated amino sugar
- N-acetylgalactosamine : GalNAC
- N-acetylglucosamine : GlcNAC
- glucosamine : GlcN
chondroitin 4-sulfate
chondroitin 6-sulfate
most abundant GAG in body
fx: cartilage (bind collagen), tendons, ligaments, aorta
deficiency causes osteoarthritis
acidic sugar: GlcUA
N-acetylated amino sugar: GalNAC
beta 1,3 glycosidic linkage
dermatan sulfate
fx: skin, blood vessels, heart valves
disease: Hunter/Hurler, Sanfilippo, Sly
acidic sugar: IdUA
N-acetylated amino sugar: GalNAC
beta 1,3 glycosidic linkage
keratan sulfate I
keratan sulfate II
most heterogeneous (contain addtl monosacchs)
fx: cornea (KS1), connective tissues (KSII)
disease: Morquio syndrome A, B
acidic sugar: Gal
N-acetylated amino sugar: GlcNAC
beta 1,4 glycosidic linkage
heparin
heparin sulfate
anticoagulant
fx:
Hep: intracellularly in mast cells in arteries - esp liver, lungs, skin
HepS: membranes, cell surfaces
disease: Hunter/Hurler, Sanfilippo, Sly
heparin
acidic sugar: IdUA
N-acetylated amino sugar: GlcN
alpha 1,4 glycosidic linkage
heparin sulfate
acidic sugar: GlcUA
N-acetylated amino sugar: GlcNAC
alpha 1,4 glycosidic linkage
hyaluronic acid
no sulfate, doesn’t form proteoglycan monomers; only GAG not ltd to animals
fx: lubricant, shock absorber, synovial fluid, vitreous humor, umb cord, loose CT, cartilate
acidic sugar: GlcUA
N-acetylated amino sugar: GlcNAC
beta 1,3 glycosidic linkage
GAG structure/fx
negative charges, extended conformation → high viscosity when solubilized
- jt lubricating fluid
ability to bind lots of water, become gel-like → good ground substance for body
- major component of ECM
hydrated GAGs can provide flexible support and molecular sieve fx
- structural integrity for cells and passageways between cells
structure of proteoglycan monomers
all GAGs except hyaluronic acid attach to proteins through covalent bonds to form proteoglycans (“bottle brush” look)
- side chains repel one another due to like charges
protein and GAG are linked by a trihexoside bond
GAG – Gal-Gal-xylose – Ser side chain on core protein
lysosome structure and fx
major digestive compartment of mammalian cells
- resp for degrading internalized extracellular material and intracellular material through autophagocytosis
contain hydrolytic enzymes (pH optimum = 5)
- optimum pH protects cytosolic components from degradation in case of leak (enzymes would be active, but not very active in cytosol pH7.2 environment)
GAG degradation
most have short half-lives (3-10 days)
exception: keratan sulfate (>120 days)
degradation
- extracellular or cell surface molecules → phagocytosis
- phagocytic vesicle fuses with a lysosome, hydrolytic enzymes do their thing
- endoglycosidases cleave polysacchs → oligosacchs, followed by more degradation
- “last on, first off” order of degradation
mucopolysaccharidoses (MPS)
- autosomal recessive diseases (exception: Hunter syndrome, X linked)
- progressive (normal at birth, progressively worse)
defective lysosomal hydrolases → defective degradation of heparan sulfate, dermatan sulfate
- accumulation of GAGs → skeletal/ECM deformities, mental retardation, death
- incomplete degradation of GAGs → presence in urine (used in diagnosis)
Hunter syndrome
enzyme: iduronate sulfatase deficiency
affects degradation of: dermatan sulfate, heparan sulfate
sx
- no corneal clouding
- mild to severe physical deformity, mental retardation
Hurler syndrome
enzyme: alpha L iduronidase deficiency
affects degradation of: dermatan sulfate, heparan sulfate
sx
- corneal clouding
- dysmorphic facial features
- upper airway obst
- hearing loss
- deposition in coronary artery → ischemia/early death
Sanfilippo syndrome
enzyme: four enzymes
affects degradation of: heparan sulfate
sx
- severe system disorders
- profound mental retardation
glycoprotein fx
- cell surface recognition
- cell surface antigenicity
- ECM
- mucins (protective lubricant)
- acidic hydrolases in lysosomes
two types of glycoproteins
oligosacch attached to protein via N-glycosidic link or O-glycosidic link
glycoproteins can have N-link, O-link, both
N-glycoproteins
Asn amide residue → high mannose or complex (add other sugars) conformations
O-glycoproteins
Ser, Thr hydroxyl residue → variety of sugars arranged in linear, branched pattern
synthesis of O-linked glycoside
- transfer of N-acetyl galactosamine from UDP-GalNAC → R group of Ser, Thr
- extension of oligosacch via attachment of nt sugar residues [glucosyltransferase]
synthesis…
protein - ER
addition of sugar - posttrans in Golgi
RBC surface antigens
antigens are linked to O-linked glycoproteins
default antigen : H [type O]
addition of GalNAC via GalNAC transferase : antigen A [type A]
addition of Gal via Gal transferase : antigen B [type B]
individual makes antibodies against the MISSING antigens → Landsteiner’s rule! (blood type compatibility)
synthesis of N-linked glycoside
in ER, branched oligosacch is synthesized on dolichol phosphate
oligosacch is transferred to Asn residue of polypeptide chain
as molecule moves through ER and Golgi, oligosacch is trimmed, monosacchs are added
synthesis…
protein - ER
addition of sugar - co- and posttrans in ER, Golgi
glycoprotein processing in Golgi
- default : secreted from the cell : don’t become associated with vesicular membrane
- become associated with the portion of cell membrane that their vesicle becomes
- sent to lysosomes
N-linked glycoprotein transport to lysosomes
defect in transport
acid hydrolases get targeted to lysosome by phosphorylation of the mannose residues of their N-linked oligosacch → mannose-6-phosphate [Golgi-bound phosphotransferase]
once in lysosome, they can do perform catabolic fx
defect in phosphotransferase leads to improper/absent targeting → improper secretion of all hydrolytic enzymes
- substance buildup in lysosomes and inclusion cell phenotype (I Cell)
I-cell disease
- skeletal abnormalities
- restricted joint movement
- coarse/dysmorphic facial features
- severe psychomotor impairment
diagnosis: high conc of acid hydrolases in blood (usually only in lysosomes in cells)
lysosomal degradation of glycoproteins
similar to that of GAGs (last on, first off)
- lysosomal acid hydrolases typically exoenzymes that are resp for removal of one glycoprotein component (reverse order)
- one missing, degradation by others is stalled
glycoprotein storage disease
(oligosaccharidoses)
very rare, autosomal recessive caused by deficiency of any degradative enzyme → accumulation of partially degraded structures in lysosomes
ex. alpha mannosidosis type I
- progressive, fatal deficiency of alpha mannosidase
- presentation similar to Hurler
- immune deficiency
- mannose-rich oligos present in urine
- diagnosis via enzyme assay
