essentials of glycobiology lecture 33 may 28, 2002 jeff esko bacterial polysaccharides: structure,...
TRANSCRIPT
Essentials of Glycobiology
Lecture 33
May 28, 2002
Jeff Esko
Bacterial Polysaccharides: Structure, Biosynthesis, Biological Functions and Significance
…or I never met a glycan I didn’t like
Overview
General structure of bacterial cell walls
Structure, function and assembly of peptidoglycan (murein)
Periplasmic -glucans (MDO)
Lipopolysaccharide (LPS) - endotoxin
Capsular polysaccharides - mimicry
Bacterial glycoproteins - a new area
Peptidoglycan
AGDA
AGDA
AGDA
AGDA
AGDA
AGDA
AGDA
AGDA
A A
AGDA
AGD
AGDA
AGDA
AGD
AGDA
AGDA
AGDA
AGDA
AGDA
AGDA
AGDA
AGDA
AGDA
AGDA
[GlcNAc1,4MurNAc1,4]n
Heijenoort (2001) Glycobiology 11:25R
Murein Glycopeptide
O
NHAc
O
CH2OH
HC CH3
C
O
NHAc
O
CH2OH
O OO
L-Ala
D-Glu
DAP
D-Ala
D-Ala
O
Notice D-amino acids
DAP = diaminopimelic acid
Peptidoglycan
AGDA
AGDA
AGDA
AGDA
AGDA
AGDA
AGDA
AGDA
A A
AGDA
AGD
AGDA
AGDA
AGD
AGDA
AGDA
AGDA
AGDA
AGDA
AGDA
AGDA
AGDA
AGDA
AGDA
The final step involves cleavage between the D-Ala-D-Ala unit and transpeptidation to the amino group of DAP of another unit
A AA A
Membrane Derived Oligosaccharides (MDO)
Branched -glucans Represent about 1-5%
of dry weight Charged substituents
act as an osmolyte and protects the inner membrane against the large difference in osmolarity inside the cell compared to outside the cell
Lipopolysaccharide (LPS)
LPS consists of three domains
– Lipid A, otherwise known as endotoxin
– Core region composed of KDO (K), heptoses (H), and hexoses (open hexagons
– Highly variable outer O-antigen region
Initiates by acylation of UDP-GlcNAc at C3, followed by N-deacetylation, and N-acylation
HOO
ONH
O H
CO
HO
OO
P
O
OO H
HO
P
O
OO H
Uridin e
HO
O
HOAcN
HO
OP
O
OO H
P
O
OO H
Uridin e
Wyckoff et al. (1998) Trends Microbiol. 6:154
Lipid A Assembly
HOO
ONH
OH
CO
HO
OO
P
O
OOH
HO
P
O
OOH
UridineHO
O
ONH
OH
CO
HO
O OP
O
OOH
HO
HOO
ONH
OH
CO
HO
O
O
HO
HOO
ONH
CH2
CO
HO
O OP
O
OOH
HO
+
Diacylglucosamine-1-P condenses with another molecule of UDP-diacylglucosamine to form the tetraacyl disaccharide core
Lipid A Assembly
KDO transferases initiate the formation of the core
Additional C12 fatty acids added to -hydroxy groups (wax)
Lipid A translocates to the outer leaflet of the outer membrane by msbA (ABC transporter)
Doerrler et al. (2001) J Biol Chem. 276:11461
Lipid A Biology
Lipid A, the heat stable endotoxin of gram negatives
Resistant strains of mice defined a locus, lps, which as positionally cloned. This turned out to be homologous to toll receptors in Drosophila, which were known to be involved in innate immunity to fungal infection (anitmicrobial peptides)
lps turns out to beTlr4, a member of a family of signaling receptors (10 members known) that induce . Tlr4 binds to Lipid A. Tlr2 apparently binds and responds to muramyl-dipeptide
Core region contains unusual sugars
The inner core contains 1-4 KDO residues, which look like an analog of sialic acid.
The core also contains heptopyranoses, which can vary stereochemically
The rest of the core consists of various combinations of Glc and Gal
OH O COOH
OHOH
HOHCCH2OH
3-deoxy-D-manno-octulosonic acid (KDO)
L-glycero-D-manno-heptulose
O
OHOH
CHOHCH2OH
OHOH
O-antigens
O-antigens consist of 2-8 sugars, repeated ≤50 times
O-antigens gives rise to different serotypes and some are correlated with disease
Mucoid strains contain a polysaccharide capsule >80 different capsules types are known just in E. coli Extraordinary diversity of structure
Bacterial Glycoproteins
Surface-layer (S-layer) glycoproteins
Prevalent in Bacteria and Archaea, but structures differ, e.g., N-linked glycosylation only in Archaea
Structural analyses have revealed unusual carbohydrate-linkage regions….
….and unusual nucleoside diphosphate-linked oligosaccharides