Bacterial Cell Walls Contain Peptidoglycans

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Glycoconjugates
Carbohydrates covalently linked to a protein or
lipid
Proteoglycans Glycoproteins Glycolipids
act as informational carrier in cell-cell
recognition, cell-cell adhesion, cell
migration, immune response, .
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Glycoconjugates 1. Proteoglycans

• Proteoglycans major components of connective
  tissues
• Macromolecules of the cell surface and
  extracellular matrix
• Composition
• A core protein (integral membrane protein or
  extracellular)
• Glycosaminoglycans covalently attached to the
  protein (through Ser)
• Main fraction by mass
• Main site of biological activity
• Example syndecan core protein with 3 Heparan
  sulfate and 2 Chondroitin sulfate chains
  (bind cell surface receptors)

Trisaccharide linker
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Proteoglycan aggregates
Associate with collagen in extracellular matrix
of cartilage, contribute to the development and
tensile strength of cartilage

• Proteoglycan aggregates
• Supramolecular assemblies
• Many core proteins bound
• to a single molecule of hyaluronate

Mr gt 2x108
e.g. Aggrecan core protein bound to chondroitin
sulfate keratan sulfate Many copies bound to
a single molecule of Hyaluronate
Take up a volume equivalent to a bacteria cell
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• Extracellular meshwork
• Formed by extracellular proteoglycans interwoven
  with fibrous matrix proteins (collagen, elastin,
  fibronectin) and plasma membrane proteins
• Roles
• Anchor cells to extracellular matrix
• Direct migration of cells in developing tissues
• Convey information in and out of cells across the
  plasma membrane

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2. Glycoproteins

• Glycoproteins
• Carbohydrate-protein conjugates
• Carbohydrate moieties are smaller (in many cases)
  more structurally diverse
• Linking point
• anomeric C of reducing end N- or O-linked to
  protein
• N-linkage Asn
• O-linkage Ser/Thr
• N-linked oligosaccharides contain a common core
• 2 N-acetylglucosamine (GlcNAc)
• 3 mannose (Man)
• Additional sugars attached to the core in many
  different ways
• High mannose type
• Complex type
• GlcNAc, galactose, L-fucose
• and Sialic acid (negatively charged)

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Common core 2 GlcNAc 3 Man
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High mannose type
Complex type
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Glycoprotein - Glycophorin
An erythrocyte plasma membrane protein 16
glycoxylation sites 15 O-glycoxyl linkage
(Ser/Thr) and 1 N-glycoxyl linkage (Asn)
Rich in sialic acid
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Biological Roles of Glycans in Proteins

• In early secretory pathway of glycoproteins in ER
• Promote protein folding
• Some proteins are completely dependent on
  glycosylation
• Some are partially dependent
• Others are not dependent on glycans
• Some are glycan-dependent in one cell type but
  not in another
• Some glycosylation sites are more important than
  others
• Aid in certain sorting events
• In later secretory pathway of glycoproteins in
  Golgi
• Structural features of glycans act as destination
  labels
• (secreted, membrane proteins, lysosomal
  proteins)
• In final mature forms (intra- or extracellular
  locations)
• Cell-cell recognition and adhesion

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3. Glycolipids and Lipopolysaccharides

• Glycolipids
• e.g. Gangliosides
• Membrane lipids of eukaryotic cells
• Head group is a complex oligosaccharide
  containing sialic acid in addition to other
  monosaccharides
• Oligosaccharides contribute to the different
  blood groups
• Lipopolysaccharides
• Occurs on the outer membrane of gram-negative
  bacteria
• (e.g. E. coli, Salmonella typhimurium)
• Prime targets of antibodies
• Some bacterial polysaccharides are toxic to humans

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Bacterial Lipopolysaccharides
Principal determinant of serotype (immunologic
reactivity)
Endotoxin inherent to Gram-negative bacteria
(E.coli Salmonella typhimurium)
Permeability barrier Selective passage of
nutrients and exclusion of harmful substances