Transcript GAG structure

Extracellular Macromolecules
Glycosaminoglycans; proteoglycans;
glycoproteins; mucins
Glycoprotein synthesis; plasma proteins
Molecular immunology:
innate immunity; inflammation
Molecular immunology:
adaptive (acquired) immunity
Fibrous proteins: keratin, collagen and elastin
Extracellular Macromolecules
1. Glycosaminoglycans Proteoglycans
Glycoproteins
Mucins
Extracellular Macromolecules
macromolecule
% carb.
glycosaminoglycans* (GAGs)
proteoglycans*
90-95
glycoproteins
2-30
fibrous proteins
1-2
100
Examples of functions:
mechanical supportlubrication
cushioning adhesives
cell spacers selective filters
1 * aka mucopolysaccharides, mucoproteins, respectively
Extracellular matrix in tissues
ground substance + fibers
 macromolecules between cells

ground substance molecules
GAGs/proteoglycans (mostly carbohydrate)
– fibers
fibrous proteins:
structural
epithelial cells
adhesive
•
especially abundant
in connective tissue
adhesion
molecules
basal
lamina
extracellular
matrix
underlying cells
2
Adapted from Hypercell
GAG structure

A sugar
exist as:
independent molecules
e.g., hyaluronate & heparin
parts of larger structures
e.g., in proteoglycans

B sugar
heteropolysaccharides
repeating structure:
disaccharide (AB)n ABABAB…
where A is usually 1 uronic acid (hexose with C6 as COO– )
& B is 1 glycosamine (amino sugar) derivative

unbranched
glycosidic linkage
anomeric C of 1 unit linked to hydroxyl of adjacent unit
3
GAG structure: repeating units
GAG
A sugar
B sugar
hyaluronate glucuronate N-acetyl glucosamine
*
5
2
4
GAG structure: repeating units
GAG
A sugar
4
B sugar
hyaluronate glucuronate N-acetyl glucosamine
*
chondroitin sulfate
dermatan sulfate
heparan sulfate
heparin
keratan sulfate
5
2
glucuronate N-Ac galactosamine 4-SO4
iduronate
"
glucuronate glucosamine N-SO3, 6-SO4
iduronate 2-SO4
"
galactose
N-Ac glucosamine 6-SO4
*opposite configuration in iduronate
glucuronate/iduronate: epimers at C5
glucose/galactose: epimers at C4
Hyaluronate (aka hyaluronan)
5
mol wt: 106 – 107 (5000 – 50,000 monosaccharide units)
very polar: 2 hydroxyls/unit
6 heteroatoms/unit
COO– every other unit
Display of HA
binds cations: Na+, Ca++

in motion
A
B
A
B
A
B
–
–
–
1
2
3
4
5
(glucuronate–N-acetyl glucosamine)3 (glcUA–glcNAc)3
6
6
Hyaluronate: structure & properties
extended structure (charge repulsion)
 hydrophilic: binds 10 –100 × wt in H2O
 additional, loosely associated H2O, so that volume occupied ~1000 ×
weight
Display of HA with
glcUAs in CPK

–
–
–
1
(glcUA–glcNAc)3
2
3
4
5
glcUAs in space-filling form (CPK)
6
Hyaluronate
solutions viscous, gel–like,

compression-resistant
 occurrence: EC matrix,
esp. in
developing tissue
healing wounds
synovial fluid
 functions: lubricant
shock absorber
flexible cement
attachment site
path for cell migration
 made by fibroblasts
 degraded by hyaluronidase
hyaluronidase
bacterial hyaluronidase facilitates
spread of infection
7
Alberts et al. Fig. 19-37
Heparin
mol wt ~ 104
 ~ 40 monosaccharide units
 made & released from mast cells in lungs & liver

heparin
cell
8
Heparin
mol wt ~ 104
 ~ 40 monosaccharide units
 made & released from mast cells in lungs & liver
 also associated with luminal surface of endothelium
 anticoagulant
heparin

forms complex with antithrombin III
this complex binds to thrombin, inactivating it
as a result, clot growth is limited
fast-acting, making it therapeutically useful
8
cell
Extracellular Macromolecules
macromolecule
% carb.
glycosaminoglycans* (GAGs)
proteoglycans*
90-95
glycoproteins
2-30
fibrous proteins
1-2
100
Examples of functions:
mechanical supportlubrication
cushioning adhesives
cell spacers selective filters
* aka mucopolysaccharides, mucoproteins, respectively
Proteoglycans (PGs)
composed of as many as 200 GAG chains covalently bonded to a core protein via serine side
chains
5
7
molecular weight range: 10 – 10
GAG chains:
chondroitin sulfate, heparan sulfate,
dermatan sulfate, keratan sulfate

Examples
decorin

many connective tissues
binds type I collagen, TGF-β
perlecan

basal laminae
structural & filtering function
aggrecan
syndecan (slide 13)

GAG chains
core
protein
9
PG in basal lamina of renal glomerulus

adapted from
Alberts et al.,
3 ed., Fig. 19-56
network of
fibrous
proteins &
perlecan
PG forms
filter
entactin
perlecan
laminin
10
type IV collagen
Proteoglycans: aggrecan
~100 GAG chains/molecule
~100 monosaccharides/GAG chain
each "bristle" = 1 GAG chain
each GAG chain is either chondroitin sulfate
or keratan sulfate
GAG chains linked to ser side chains of core protein

core
protein
11
GAG chains
An aggregate of aggrecans & hyaluronan
major GAG–PG
in cartilage
 link proteins bind
noncovalently
 with bound H2O,
disperses shocks,
compressive force
 ~ cell size
 adhesion proteins
link to collagen &
hyalurcells
onan
 degraded by
chondroitin sulfatase,
keratan
etc

sulfate
12
ç 1μm è
core protein
link proteins
chondroitin
sulfate
Alberts et al. Fig. 19-41
Proteoglycans:
syndecan
cell-surface PG
 core protein domains

intracellular
transmembrane
extracellular
5 GAGs attached

GAG chains
outside
functions
interactions
cell-cell
cell-matrix
growth factor receptor
13
inside
core
protein
Lehninger et al.
Fig. 9-22
GAG synthesis & breakdown

–UDP
synthesis
activated precursors: UDP–monosaccharide derivatives
e.g., UDP–glucuronate
residues added one at a time in Golgi complex
sulfate moieties
 donor:

PAPS (active sulfate)
degradation
–
–
adenine
lysosomes
–
specific glycosidases & sulfatases
–
mucopolysaccharidoses
 genetic disorders
 accumulation of GAG due to absence of a specific glycosidase
or sulfatase
14
Extracellular Macromolecules
macromolecule
% carb.
glycosaminoglycans (GAGs)
proteoglycans
90-95
glycoproteins*
2-30
fibrous proteins
1-2
100
* polypeptide with 1 or more oligosaccharide side chains
15
Glycoproteins: functions of glyco moieties
increase protein’s solubility & hydrophilicity (sl 19)
stabilize protein against


denaturation
proteolysis
markers

direct protein's destination



Glycosylation:
one kind of
post-translational modification
others: phosphorylation
carboxylation
organelle
plasma membrane
export (secretion)
indicate protein's lifetime (sl 21)
part of the protein's receptor recognition site (sl 23)

signal factors such as hormones, cytokines
 cell-cell adhesion proteins
16
Glycoprotein structure
polypeptide with 1 or more oligosaccharide side chains
oligosaccharide linked to polypeptide in two ways:

type
linked to side chain of
organelle where sugars
are added to protein
O-linked
serine (ser), threonine (thr), Golgi complex lumen
(O-glycoside)
hydroxylysine (in collagen)
N-linked
asparagine (asn)
rough ER lumen
(N-glycoside)
17
Glyco moiety structure
oligosaccharide chain extends away from protein surface
 units mostly hexoses in pyranose (6-atom ring) form
 branched
 glycosidic links varied:
α or β
1,2; 1,3; 1,4
 terminal
sugar
often
sialate

2
asn
7
2
7
asn
18
Stryer 4ed., p. 463
Mucins: salivary glycoproteins
mol wt ~ 106
 ~800 short
(disaccharide)
side chains
 terminal sugar is sialate
anionic sugar
at end of glyco chains
of many glycoproteins
 very hydrophilic,
extended structure

19
~
~
galNAc
sialate
–
2
Mucins: modification & aggregation

sialidase (neuraminidase)
catalyzes hydrolysis
of sialates from mucins
secreted by oral bacteria
 products:
less hydrophilic,
less H2O-soluble,
more folded,
more aggregated
part of the enamel pellicle
& dental plaque matrix
20
~
~
x H2O
sialidase
x
~
~
~
~
galNAc
sialate
Role of glyco moiety in controlling protein lifetime
many blood proteins have glyco chains with terminal sialate
 endothelial surface sialidases slowly remove sialates from these circulating proteins
 rate of sialate removal depends on protein's structure

sialoglycoprotein:
sugars]–protein
now-exposed
gal–glcNAc… sia–gal–glcNAc–[core
residues bind to asialoglycoprotein
receptor on liver cell
surface
asialoglycoprotein:
gal–glcNAc–[core sugars]–protein
 protein is then endocytosed & broken down

21
Blood group types
core sugars
Type O cell surface:
gal–glcNAc–gal–glc–protein†
Type A cell surface:
galNAc–gal–glcNAc–gal–glc–protein†
|
fucose*
|
fucose
Type B cell surface:
gal–gal–glcNAc–gal–glc–protein†
|
fucose
A: have
– enzyme to add galNAc to core sugars
– antibody to type B antigen
 B: have
– enzyme to add gal to core sugars
– antibody to type A antigen
 O: have
– neither enzyme
antibodies
 AB: have
– both enzymes (eitherboth
galNAc
or gal
added to core sugars)

22
†
neither antibody
or lipid
* 6-deoxygalactose
Glyco moiety-binding proteins: lectins
contain sites that bind specific glyco structures
 e.g., asialoglycoprotein receptor described on sl 21
 important in intercell adhesion (i.e., lectins are CAMs: cell
adhesion molecules)
 selectins:

plasma
membrane
lectins that
mediate
cell-cell
recognition
& adhesion
23
Lehninger et al.
Fig. 7-37