Polysaccharides and glycoconjugates

Functions of Glycoconjugates and An Introduction to Antibiotics

10 monosaccharides as building blocks for human fly and

• glucose

• galactose

• mannose

• N-acetylgalactosamine

• N-acetylglucosamine

• N-acetylmuramic acid

• Xylose

• glucuronic acid

• fucose

• iduronic acid

• N-acetylneuraminic acid

Are pyranoses or furanoses preferred?

Six Membered Rings Predominate

steric clashing

Will influence whether bulkier groups are “axial” or “equatorial” - prefer equatorial

anomers

depending of the side that hydroxyl attack comes from, makes two different rings

anomeric position

C1

alpha anomer

end group and hydroxyl opposite oxygen are trans (different sides)

beta anomer

end group and hydroxyl opposite oxygen are cis (same sides)

polysaccharides

• glucose homopolymers

  • Cellulose. starch, glycogen

• Disaccharide heteropolymers

  • chitin, keratan sulfate

cellulose

beta-1,4 linkages

not digested by humans (dietary fibre)

principal component of plant structure

most common biopolymer on Earth

starch

alpha-1,4 linkages

digested by humans w/ amylase

amylose

linear starch

amylopectin

branched starch

homopolysaccharides

same monosaccharide repeated; may be branched or unbranched

heteropolysaccharides

two or more monosaccharides; may be branched or unbranched

Overview of saccharide nomenclature

• L-sugars, D-sugars

• Triose/tetrose/pentose/hexose

• Aldose/ketose

• Furanose/pyranose

• Epimers

• Anomers: alpha-, beta-

• Monosaccharide, disaccharide, oligosaccharide, polysaccharide

• Reducing sugar/non-reducing sugar

• Linear vs branched polysaccharides

Synthetic chemistry of oligosaccharides

No straightforward automated solid state synthesis strategy* has been developed for carbohydrates, similar to methods for nucleic acids or peptides, but there is progress by Seeberger and coworkers

Are glycans biosynthesized using template-directed synthesis?

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glycoconjugates

• glycoproteins

  • glycosyltransferases, mucin

• proteoglycans

  • aggrecan, petidoglycan

• glycolipids

  • blood antigens, mebrane anchors

glycoconjugates in cells

• cell surfaces

• mediate cellular interactions

Lectins

glycan-binding proteins

Intrinsic binding

binding of glycans to lectins within the same host cell (intracellular) or between host cells (intercellular)

Extrinsic

binding between glycans/lectins on human and pathogen cells

ex. FimH - E.coli causes UTIs - binds mannose residues on the human glycoprotein uroplakin found on bladder cells & facilitates bacterial adhesion

Human milk oligosaccharides (HMOs)

affect human- bacterial interactions

Common GI bacteria can metabolize HMOs as an energy source

HMOs compete with bacterial glycans for human receptors

Some oligosaccharides might function as antivirals

Protein glycosylation

50% of all proteins are glycoslyated
N-glycans are added to secreted and membrane-bound glycoproteins at Asn-X-Ser/Thr “sequons” (where X = any amino acid except Pro).
About 70% of eukaryotic proteins contain this sequon and ~70% of the sequons contain an N-glycan.

N-linked glycopoteins

2 N-acetylglucosamine + 3 mannose attached to Asn

O-linked glycoproteins

N-acetylgalactosamine attached to Ser/Thr

N-glycan biosynthesis location

endoplasmic reticulum or ER make proteins (ribosomes on rough ER)

Golgi apparatus modifies them and are transported to surface for attachment/secretion

Nucleotide sugars

building blocks of glycans; usually NDP-sugars but may be NMP-sugars

Glycosyltransferases

enzymes that transfer sugars onto other molecules

ex. fucosyltransferase 

inverting Glycosylation

S_n2 like; opposite stereochem

retaining Glycosylation

double displacement mechanism, same sterochem

Blood groups

determined by glycosylation patterns and expression of GTs; each only differ by few amino acids but are indentifiably different; literally same evolutionary origin

Techniques for “sequencing” glycoproteins

1. Antibody-based

2. Lectin-based

3. Liquid chromatography

4. Mass spectrometry

Lectin microarrays

rapid identification of glycan sequences; contain hundreds of different, covalently linked lectin proteins on a solid surface (different specificities for glycans).

peptidoglycan

alternating GlcNAc-MurNAc polysaccharide component of bacterial cell walls; a type of proteoglycan; highly conserved structure

antibiotics

bacteriostatic - restrict growth and reproduction

batericidal - cause cell death

inhibit DNA/RNA (precursor) or protein or peptidoglycan synthesis

Gram-negative bacteria

outer membrane; thin peptidoglycan; no stain

Gram-positive

only inner membrane; thick peptidoglycan; stain

Peptidoglycan (PG) biosynthesis stages

1. lipid I

2. lipid II

3. transpeptidation and glycosyltransfer

Penicillin-binding proteins (PBPs)

major antibiotic targets; two major regions; A single bacterium has many that perform various functions

Transpeptidase reactions

acyl-enzyme intermediate

beta-lactams

most successful class of antibiotics; “suicide inhibitors’; square ring very reactive; substrate mimics (similar distances between backbone atoms in the terminal dipeptide and beta-lactams)