What are complex polysaccharides?
oligosaccharides w/ more than one type of sugar residue ex: Glycoproteins GPI anchors Glycosylated Lipids GAGs Proteoglycans
What are GPI anchors?
these anchor soluble proteins to the exterior of the plasma membrane
GAGs with proteins are called what?
proteoglycans
2 important classes of enzymes form complex polysaccharides. What are they?
- Glycosyltransferases
2. Glycosidases
What is dolichol phosphate?
a lipid
an isoprenoid compound w/ 16-20 isoprene units embedded in the ER membrane
How do glycosyltransferases work?
they transfer saccharides from a donor to an acceptor
*different families of this enzyme based off of the donor, acceptor, sugar transferred, site where it is transferred etc.
A cyclized sugar is considered alpha if the OH is above/below the plane. & beta if the OH is above/below the plane.
Alpha–OH below the plane
Beta–OH above the plane
How do glycosidases work?
they remove specific sugar residues/break bonds
usu in the lysosome
if involved in synthesis–>in the Golgi apparatus
What are glycoproteins? How are they synthesized? What is their destination?
They are proteins with saccharides covalently bonded to them.
- *synthesized by RER via secretory pathway
- *Destination: secretion, membrane, lysosome
What’s the deal with the carbs on glycoproteins?
the amount attached is very variable usu 12-15 residues branched heterogeneity evenly distributed to crazily distributed
What is an example of a glycoprotein that has sugars unevenly distributed on the protein?
Glycophorin A
the sugars are mainly clustered on the portion that is outside the cell
What are some examples of glycoproteins?
Glycophorin A IgG Human ovarian cyst glycoprotein Human gastric glycoprotein FSH LH Immunoglobulins
What are 4 things that make glycoproteins super important?
Their function in the cell membrane–they mediate a lot of things & touch a lot of things…
Mucus constituents
Secreted Hormones
Plasma Proteins
What are the 2 basic types of glycoproteins?
The N-linked to asparagine
The O-linked to serine or threonine
What is always the first sugar residue of a glycoprotein?
N-acetyl glucosamine
What are the 3 types of glycoproteins that can be formed?
High Mannose Type
Hybrid
Complex
What is the common core of all N-linked oligosaccharides?
N-acetyl glucosamine NAG Mannose Mannose Mannose
Explain how you get an N-linked glycoprotein via Dolichol phosphate.
Dolichol phosphate accepts GlcNAc on the cytoplasmic side.
It is glycosylated on the cytoplasmic side
Flippase on the ER flips it into the ER for more synthesis.
Glycosyl transferases in the ER add 4 more mannoses & 3 glucoses
A glycosyl transferase in the ER also transfers the unit onto asparagine.
The asparagine is being made by a ribosome that is attached to the ER.
then the protein folds & a glucosidase takes away the 3 extra glucoses.
After the 3 glucoses have been removed on the N-linked glycoprotein in the ER…what happens next?
It is packaged into a Clathrin-coated vesicle & it moves to the Golgi apparatus. Cis–>trans direction.
Could go thru the Golgi & exit to be secreted or put into the membrane–>will meet more glycosidases & transferases
OR could be phosphorylated & put into lysosomes.
In collagen–>what are saccharides O-linked to?
5-OH lysine
What must O-linked glycoproteins be attached to?
serine or threonine
**this is so it can be accepted by GalNAc transferase.
How are O-linked glycoproteins made?
The protein is synthesized in the ER w/o any glycosylation.
Once it moves to the Golgi a bunch of glycosidyl transferases go at it.
Describe the structure of mucin & its fcn.
Mucin is secreted or membrane-bound. Its fcn is to provide a gel barrier.
It has a core protein with lots of serine & threonine.
It has a bunch of oligosaccharide chains off of that that are sulfated & have sialic acid/NANA, which is negatively charged.
The neg. charges repel each other–helps w/ function.
Which conditions are there a lot of mucins in?
cancers asthma bronchitis COPD Cystic Fibrosis
What are disorders of glycoprotein synthesis called?
Congenital Disorders of Glycosylation
Which steps are affected in N-linked congenital disorders of glycosylation–>Type I?
Type I:
the early steps are affected…the steps before the ER
Deficiency in phosphomannosemutase II is the most common disorder
Which steps are affected in N-linked congenital disorders of glycosylation–>Type II?
Type II:
defects in the enzymes in the Golgi that process the glycoproteins
What’s the deal with disorders of O-linked glycosylation?
These are defects in O-mannosylation & O-fucosylation
What is Walker Warburg Syndrome?
This is a Congenital Disorder of Glycosylation
O-linked
O-mannosyltransferase I is deficient
**causes congenital muscular dystrophy
Congenital Disorders of Glycosylation that are N & O-linked involve what?
a deficiency of the sialic acid transporter
What are general symptoms of congenital disorders of glycosylation?
psychomotor retardation
malfunction of organ systems: nervous, muscle, & intestinal in infants
What is the term for diseases of glycoprotein degradation? What’s the deal with these diseases? What are the symptoms?
Lysosomal Storage Diseases
a deficiency in a hydrolytic enzyme in the lysosome–>accumulation of that glycoprotein in the tissues & urine
Symptoms: skeletal abnormalities, hepatosplenomegaly, cataracts, mental retardation
What 2 types of enzymes are involved in lysosomal storage diseases?
lysosomal exoglycosidases
lysosomal endoglycosidases
What’s another name for I cell disease? What’s the deal with this disease? What are its symptoms?
Mucolipidosis II
deficiency in GlcNAc-P glycosyltransferase
**this is the enzyme that marks a glycoprotein to go to a lysosome…
**when this is deficient–> a bunch of glycoproteins accumulate outside the cell–>become dense bodies in fibroblasts
Symptoms: psychomotor retardation, skeletal abnormalities, coarse facial features, restricted joint movement, death by age 8
What does GPI stand for? What is the structure of the GPI anchor?
glycosylphosphatidylinositol
core tetrasaccharide + protein + PI (w/ 2 FA that anchor everything in the plasma membrane)
What is the tetrasaccharide of the GPI composed of?
GlcNH2 + 3 mannoses
Explain the synthesis of GPI.
N-acetyl glucosamine is added to PI on the cytoplasmic side of the ER.
Then it is put into the ER.
3 mannoses & ethanol amine are attached.
A protein is being made into the ER by an attached ribosome.
An amino group of GPI attaches to a portion of the protein near the C-terminus…a part of it is cleaved.
The glycoprotein ends up on the exterior of the plasma membrane.
Aside from glycoproteins…what else do glycosyltransferases synthesize? What type?
Glycolipids
type: sphingolipids
built on a molecules of sphingosine w/ 2 FA
What are the 4 principle classes of sphingolipids?
Cerebrosides
Sulfatides
Globosides
Gangliosides
What are sulfa tides?
cerebrosides w/ galactose + sulfate
these are common in neural cells
What are gangliosides really important in?
the brain
What is the following process the synthesis process for? (2 things)
First rxn:
UDP-Gal + glucocerebroside → lactosyl ceramide (β (1→4))
Then glycosyltransferases add the remaining hexoses.
Globosides
Gangliosides
What types of diseases are there for glycolipids?
No diseases for glycolipid synthesis, but there are diseases for glycolipid degradation.
These are lysosomal storage diseases.
**often these affect the nervous system (mental retardation & motor deterioration)
What is the most common glycolipid degradation/lysosomal storage disease?
Tay Sachs
What deficiency causes Tay Sachs? What happens as a result?
hexosaminidase A
accumulation of ganglioside GM2 in lysosomes
milky halo in eye from accumulation of GM2 in ganglion cells
What’s the deal with all this ABO stuff?
ABO are the types of antigens on the RBCs. These are complex polysaccharides. These polysaccharides include sphingolipids & glycoproteins.
Type O: just the core H antigen made by a fucosyltransferase
Type A: Type O + N-acetyl galactose
Type B: Type O + extra galactose
So…what’s the deal with the Lewis blood group system?
Some epithelial & secretory tissues lack the FUT2 gene that produces the H antigen.
So, instead they have FUT3 that has a loose specificity & creates a different type of bond.
This produces: Leb & Legamma
What are the diseases/antigens associated with the Lewis antigens?
Leb (receptor for heliobacter pylori)
Legamma (elevated in cancers)
When lewis antigens are absent–>associated w/ Leukocyte Adhesion Deficiency Syndrome
What are 2 other names for glycosaminoglycans?
When you add a protein to glycosaminoglycans–>what does it become?
GAGs, mucopolysaccharides
GAG + Protein = proteoglycan
What substances make up the extracellular ground substances?
GAGs
proteoglycans
glycoproteins
What are some important properties of GAGs?
long & unbranched heteropolysaccharides
repeating disaccharide units w/ hexosamine & uronic acid
has sulfate groups w/ ester & amide bonds
highly neg. charged–>attract Na+ & K+ & therefore water–>not easily compressed
in ground substance in extracellular spaces
What’s the deal with hyaluronate?
GAG w/ the repeating unit N-acetyl glucosamine
Uronic Acid: D-glucuronic acid
Hexosamine: GlcNac
NOt sulfated
Binds cations tightly
occupies a large volume & produces a clear, highly viscous soln
Where is hyaluronate found?
It is a lubricant & shock absorber & it is found in important places. Cartilage, tendons, synovial fluid, vitreous humor of eye, umbilical cord
List some important GAGs. Which of these are sulfated? Which are found extracellularly?
hyaluronate chondroitin sulfate dermatan sulfate heparin heparan sulfate keratan sulfate **all are sulfated except hyaluronate **all are extracellular except heparin (in granules in mast cells) released to inhibit clotting
What’s the deal with heparin?
anti-coagulant
binds antithrombin III, activating it
Then antithrombin III complexes with other factors & causes anticoagulation…
This would happen w/o heparin, but happens a lot better w/ heparin.
What’s the deal with the biosynthesis of GAGs?
It involves glycosyltransferases
Sulfation occurs as the chain grows, requires molecules called PAPS (if lacking, genetic diseases-short stature, bowed limbs)
Sometimes made on the back of a protein like with chondroitin sulfate
What are proteoglycans?
large molecules w/ large protein core w/ a bunch of GAGs hanging off
neg. charged, attracts cations
major contributor to ground substance
Different carbs attach to different parts of the protein in the aggrecan proteoglycan. Describe the differences.
At the top: N-linked w/ Asn
In the middle: O-linked w/ keratan sulfate
At the C-terminus: chondroitin sulfate
Aside from aggrecan, what are 2 other proteoglycans that are found interacting w/ hyaluronic acid?
versican
neurocan
What are some proteoglycans that are integral membrane proteins or membrane anchored proteins?
Syndecan
CD44
Thrombomodulin
Glypican
What are some proteoglycans that are in the nervous system?
neurocan
brevican
cerebrocan
phosphacan
What are some diseases associated w/ proteglycans?
only diseases associated w/ not being able to break them down... mucopolysaccharidoses lysosomal storage disease Hurler Hunter Sanfilippo
At what level of proteglycan synthesis is there heavy regulation?
at the beginning…at the level of creating hexosamine & uronic acid
