GLYCOBIOLOGY Flashcards
Glycoconjugates are complex carbohydrates:
hetero-oligosaccharides or hetero-polysaccharides that contain multiple species of monosaccharides.
What are the 3 major classes of glycoconjugates?
Glycoproteins
Proteoglycans
Glycosphingolipids
How do you get from the Fischer projection of D-glucose to the Haworth closed projection? What is formed?
React the 5’OH group with the 1’aldehyde carbonyl group
Forms: B-D-Glucopyranose or a-D-Glucopyranose
Which form of D-glucose is more abundant?
The cyclic closed forms (a- and B-D-glucopyranoses).
The open chain forms are toxic
How do you convert the D-Glucopyranose sugar into L-idopyranose?
Epimerize carbon 5 of D-glucose –> the carbon 5 H is up, and the carbon 6 (CH2OH) is down in both anomers.
- a-L-Idopyranose: carbon 1-OH is up
- B-L-Idopyranose: carbon 1-OH is down
The a-anomeric configs of D- or L- sugars have the carbon 1 -OH group and the carbon 5 H in the __ direction
For a, it’s the same
For B-configurations, they’re opposite
Structure of D-glucose (Glc)
This is the only one you should be able to draw; the others, just know by words
C2 epimer of glucose
C4 epimer of glucose
N-acetylglucosamine (GlcNAc)
C2 hydroxyl gorup modified into an N-acetyl group
D-glucuronic acid (GlcA)
Converting the CH2OH at carbon 6 of D-glucose with a carboxylic acid (COOH) group
L-iduronic acid (IdoA)
Epimerization of carbon 5 of D-glucuronic acid
N-acetylgalactosamine (GalNAc)
C4 epimer of N-acetylglucosamine
Sialic acids / Acylated-neuraminic acids
What are they?
What enzyme cleaves them?
What infection is the enzyme associated with?
Monosaccharides often attached to glycoconjugates
Cleaved from glycoconjugates by sialidase (or Neuraminidase), which are associated with the influenza virus infection
Where does biosynthesis of glycosidic linkages between monosaccharides occur?
What are the requirements?
ER & Golgi
- Activated Nucleotide-Sugar donor
- Sugar acceptor
- Glycosyltransferase
Biosynthesis of lactose in the mammary gland
In the presence of a-lactalbumin (produced in mammary gland), glucose accepts B-galactosyltransferase and forms a glycosidic linkage with galactose –> lactose
In the absence of a-lactalbumin, N-acetylglucosamine accepts B-galactosyltransferase and forms N-acetyllactosamine
N-acetyl glucosamine & glucose are…
sugar acceptors.
Glucose is the one in mammary glands, where a-Lactalbumin is made
N-Acetylglucosamine is the normal acceptor in ordinary tissue
Lactose intolerance
Lactase (B-galactosidase) deficiency -> can’t break down lactose into glucose and galactose
Lactaid contains B-galactosidase
Glycoproteins
glycosylated proteins
may occur via an O-glycosidic linkage (glycosyltransferase sequentially adds sugars to Ser or Thr) or an N-glycosidic linkage (N-glycan changed added to an Asn)
Processing of the N-glycan in the golgi
Canbe processed to either high mannose (neutral) or complex (acidic, containing sialic acid)
type 1 congenital disorders of glycosylation (CDG)
vs
Type 2
- Type 1 CDG disrupts the normal synthesis of the N-linked oligosaccharide precursor or its transfer to proteins
- Type 2 CDG disrupts the processing of the protein-bound oligosaccharide chain
Glycation (nonenzymatic addition of carbs) of HbA creates ___.
The most common and stbale form is ____
Glycation occurs proportionate to the ____
Glycation of HbA –> HbA1
HbA1C is the most stable
Glycation occurs proportionate tot he mean plasma blood [glucose]
Clinical relevance of HbA1C
- Reflects the average blood [glucose] over a period of 2 months (the half-life of an RBC).
- Demonstrates long-term blood [glucose] maintenance
- At normal blood glucose (100mg/dL), HbA1C should be in only 5% of total adult hemoglobin
- Diabetics have > 6.5% of HbA in the HbaA1C form
- Unaffected by day-to-day diabetic compliance or fluctuations in blood glucose levels.
Influenza A / Swine flu subtypes
Subtypes are based upon antigen H/Hemagglutinin (18 subtypes) and antigen N/Neuraminidase (11 subtypes), which are virally encoded envelope proteins
Ex) H1N1
Hemagglutinin/antigen H
A lectin (carbohydrate-binding protein) that specifically binds sialic acid in the membrane of a host cell (respiratory endothelial cells).
Antigen N/ Neuraminidase
Also in the viral envelope, it cleaves sialic acid (N-Acetylneuraminic acid) from the host cell surface
Interaction of viral hemagglutinin and neuraminidase
- Hemagglutinin on the viral envelope interacts with sialic acid on the respiratory endothelial cell’s surface –> entry of virus into the cell
- Virus replicates inside
- Once a new viral particle is released, the hemagglutinin on the viral surface can attach to the host cell membrane
- Neuraminidase cleaves the sialic acid from the host cell surface, allowing the virus to spread to and infect, new hosts
Oseltamivir (Tamiflu)
&
Zanamivir (Relenza)
neuraminidase inhibitors –> prevents the release of new viral particles from the surface of infected cells –> prevents infection spread
Glycosaminoglycans (GAGs) / Mucopolysaccharides
Linear heteropolysaccharides composed of a repeating disaccharide unit: [acidic sugar-amino sugar]n
Usually linked to a core protein to form a proteoglycan, except in hyaluronic acid.
5 divisions of glycosaminoglycans
Hyaluronic acid (HA/hyaluronan)
Chondroitin sulfate (CS)
Dermatan sulfate (DS)
Keratan sulfate (KS)
Heparin & Heparan sulfate (HS)
Proteoglycans - what are they and where are they found?
Core protein + GAGs
in the extracellular matrix
Aggrecan
A chondroitin-sulfate proteoglycan.
- Core protein + ~100 CS + ~30-50 KS
- Can be attached to hyaluronic acid/hyaluronan to form an aggregate
Aggregate
Aggrecan attached to hyaluronic acid
- Extremely hydrophilic & gel-like for the ECM
Mucopolysaccharidosies (MPSs)
lysosomal storage diseases caused by deficiency of enzymes for GAG degradation (DS, HS, and KS)
Coarse facial features, hepatosplenomegaly, bone deformity, intellectual disability
Scheie syndrome
Milder form of Hurler’s syndrome
Name the repeating disaccharide found in DS, HS, and KS
DS: L-IdoA 2-sulfate linked to GalNAc 4-sulfate
HS: L-IdoA 2-sulfate linked to GlcN 2-sulfate
KS: Gal 6-sulfate linked to GlcNAc 6-sulfate
Hunter & Hurler syndromes are associated with ___
What enzyme deficiency is associatd with each?
defects in the degradation of DS & HS –> buildup of modified forms of DS & HS in the lysosome
- Hunter’s lacks idouronate sulfatase (removes sulfate from 2-S-L-IdoA) in the first reaction
- Hurler’s lacks L-uronidase (removes Iduronic acid) for the second reaction
Elaprase is a recombinant iduronate sulfatase. Which syndrome woud it help?
Aldurazyme is a recombinant a-L-Iduronidase. Which syndrome would it help?
Elaprase - Hunter syndrome
Aldurazyme - Hurler’s synrdome
*Note: neither can treat the neurological symptoms of these syndromes
Sanfillippo syndrome is associated with the degradation of ___
GlcNS
which is formed after 2-S-LIduronic acid is cleaved off HS
Name the deficiencies associated with Sanfilippo types A-C
Type A - Heparan N-sulfatase
Type C- Acetyl-CoA acetyltransferase
Type B- a-N-acetylglucosaminidase
The buildup products in all cases are HS with modifications to the exposed GlcNS
Morquio syndrome & types
Can’t degrade KS
Type A: deficiency in galactose 6-sulfatase
Type B: deficiency in B-galactosidase
Vimizim
N-Acetyl galactosamine 6-sulfatase, which uses Galactose 6-sulfate as a substrate (like galactose 6-sulfatase would)
Enzyme replacement therapy for Morquio A
Morquio type B is a B-Galactosidase deficiency.
What other lysosomal disorder shares this enzyme defect?
GM 1 gangliosidosis
Gram positive bacteria have a thicker
layer of peptidoglycan in their cell well than gram-negative.
Describe the structure of a peptidoglycan strand
Repeating disaccharide unit of N-acetylmuramic acid (Mur2Ac) and N-Acetylglucosamine (GlcNAc)
-
Tetrapeptide chain attached to Mur2NAc composed of..
- L-alanine
- D-glutamine
-
L-lysine
- 5 Glycines attached to L-lysine
- D-alanine
Describe the peptidoglycan network of the bacterial cell wall
Transpeptidase crosslinks the pentaglycine of one strand with a neighboring strand’s D-alanine
Penicillin and Cephalosporins
B-Lactam antibiotics that inhibit transpeptidase –> weakened bacterial cell wall
Resistant bacteria produce B-lactamase to degrade the antibiotic.
Structure of glycosphingolipids (GSLs)
Hydrophilic sugar chain linked to a hydrophobic ceramide
Ceramide: sphingosine backbone acylated to an FA
Functions of glycosphingolipids (GSLs)
- Major constituent of membranes
- Signaling molecule for differentiation, proliferation, and apoptosis
- Water permeability in skin
Cerebrosides vs Gangliosides
Cerebrosides: neutral GSLs
Gangliosides: acidic GSLs that contain sialic acid
Name the two main neutral GSLs that contain only one sugar residue
Galactosylceramide / Galacocerebroside
Glucosylceramide / Glucocerebroside
Lactosylceramide is another neutral GSL.
If you add a second galactose to it, what does it become?
Globotriaosylceramide (Gb3)
Fabry disease
defect in Gb3 degradation
How can you go from Gb3 to Gb4?
Add GalNAc to the terminal galactose
What enzymes can turn Gb4 back to Gb3?
B-Hexosaminidase A or B-Hexosaminidase B
can remove the terminal B-linked GalNAc
Type O Tay-Sachs disease / Sandhoff
Deficiency of both A & B forms of B-Hexosaminidase, which turn Gb4 back to Gb3 –> buildup of Gb4
How do you go from Gb3 back to lactosylceramide?
a-Galactosidase A hydrolyzes the a-linked terminal Galactose to release Galactose and Lactosylceramide
Fabry disease
Deficiency in a-Galactosidase A, –> accumulation of Gb3
- X-linked; fatal from renal & cardiac failure
- Hemizygous males may have skin lesions, fever, burning pain in the extremities, and renal dysfunction
recombinant a-galactosidase A is used for what?
Also called “fabrazyme” because it can slow the progression of Fabry’s
Gaucher disease
-
Deficiency of Glucocerebrosidase/B-Glucosidase to hydrolyze glucosylceramide into glucose + ceramide.
- –> buildup of glucosylceramide in tissue macrophages –> engorged Gaucher cells
- Type1: adult, nonneuropathic
- type2: acute, neuropathic
- type3: subacute, neuropathic
- Treatment: cerezyme, a recombinant B-glucosidase
Krabbe disease / Globoid cell leukodystrophy
- Defect in Galactocerebrosidase (GALC, a B-galactosidase) to hydrolyze galactosylceramide
- NO build up of galactosylceramide
- Build up of psychosine/galactosylsphingosine in the brain (galactosylceramide without the acylated FA residue)
- –> Severe demyelination
Why doesn’t GalCer build up in Krabbe patients (lack galactocerebrosidase)?
GALC degrades both GalCer & Psychosine, so GALC deficiency causes elevation of psychosine
–> Psychosine buildup destroys oligodendroglia, which produce GalCer.
If Morquio Type B is also due a defect in B-galactosidase, why is it separate from Krabbe?
- Two different enzymes from different two different loci:
- B-Galactosidase deficiency in Morquio type B is encoded by GBL1 gene
- Galactocerebrosidase (also a B-galactosidase) deficienciy in Krabbe is encoded by GalC gene
-
In Morquio, the substrate is Keratan sulfate; In Krabbe, the substrate is Galactocerebroside.
- Diff substrate specificities, diff active sites, diff buildup
What is the sialic acid that all gangliosides contain?
N-Acetylneuraminic Acid (NeuAc)
How are GM3, 2, and 1 formed?
GM3: a-link NeuAc to the terminal galactose of lactosylceramide
GM2: B-link GalNAc to termianl galactose of GM3
GM1: B-link galactose to GalNAc of GM3
which GM is the major ganglioside of the CNS and is a receptor for cholera toxin?
GM1
How to go from GM1 to GM2?
B-Galactosidase hydrolyzes the terminal B-linked galactose, releasing GM2 & Galactose
GM1 gangliosidosis / Generalized gangliosidosis
Deficiency of B-Galactosidase –> accumulation of GM1 in lysosome
Because this disease state is due a mutation in the same gene as that for Morquio type B (GLB1) they’re pretty much the same disease
Defect in B-galactosidase results in the buildup of both __ and ___
KS & GM1
because Morquio type B and GM1 gangliosidosis have a mutation in the same gene (GLB1)
How do you go from GM2 to GM3?
What’s the disease associated with the inability to do this?
B-Hexosaminidase A in the presence of a GM2-activator removes the terminal GalNAc, generating GM3
Classical Tay-Sachs; causes GM2 buildup
Classical Tay-Sachs
Hex A deficiency
Hex A & B can cleave both B-linked GlcNAc and B-linked GalNAc, but only Hex A can hydrolyze the terminal GalNAc in GM2.
Hex __ : heterodimer of aB; requires GM2 activator cofactor
Hex __: homodimer of BB; can’t cleave Gal NAc in GM2
HexA (aB) can cleave GalNAc in GM2
HexB (BB)
3 variants of Tay-Sachs
-
Classical Tay-Sachs / variant B / GM2-gangliosidosis:
- mutation in a chain –> defective Hex A
- –> can’t digest GM2
-
Tay-Sachs variant O / Sandhoff:
- mutation in B chain –> defective Hex A & B
- –> can’t digest Gb4 and GM2; can’t digest GalNAc
- More severe
-
Tay-Sachs variant AB:
- mutation in gene for GM2-activator
- GalNAc in GM2 can’t be hydrolyzed by Hex A
The H-antigen is expressed in what blood group on RBCs?
What makes it an H antigen?
Blood group O
It has L-fucose linked N-Acetyllactosamine
What makes an group A-antigen, and thus blood group A?
GalNAc attached to H antigen
GalNAc is the temrinal sugar
What makes a B-antigen, and thus blood group B?
Galactose attached to the H-antigen
Blood type A
RBCs either have
- only A-antigens (homozygous A/A)
- both A- and H-antigens (heterozygous A/H)
Blood group B
RBCs either have
only B-antigens (homozygous B/B)
both B- and H-antigens (heterozygous B/H)
Ex) Individuals with blood group B (B/B or B/H) possess the B antigen and have antibodies against
antibodies against antigen A (anti-A)
Group AB individuals can accept ___ and have antibodies against__
Acccepts transfusiosn from all blood types because they don’t have anti-A or anti-B antibodies.
Universal recipient.
Bombay (h/h) blood group
- Can’t add L-Fucose to create teh H-antigen, so they also can’ tmake type A or type B antigens.
- They generate antibodies against all 3 major antigens: A, B, and H.
- They can’t receive ANY blood unless its h/h
