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高等生化學
Advanced Biochemistry
Carbohydrates and
Glycobiology
陳威戎
Carbohydrates and Glycobiology
Preface
Ah! Sweet mystery of life…..
- Rida Johnson Young (lyrics) and Victor Herbert (music), 1910
I would feel more optimistic about a bright future for man if he
spent less time proving that he can outwit Nature and more
time tasting her sweetness and respecting her seniority.
- E. B. White, “Coon Tree”, 1977
Structure and Functional Roles of Carbohydrates
Most abundant biomolecules on Earth.
- Photosynthesis: cellulose and other plant products
- Dietary staple: sugar and starch
- Oxidation: energy-yielding pathway
- Structural and protective elements
- Lubricate skeletal joints
- Recognition and adhesion between cells
- Signals determine the intracellular location or
metabolic fate
Properties and Classification of Carbohydrates
Polyhydroxy aldehydes or ketones, or substances that
yield such compounds on hydrolysis.
Three major size classes:
- Monosaccharides: glucose (dextrose)
- Disaccharides: sucrose (cane sugar)
- Polysaccharides: cellulose, glycogen
Glycosidic linkage (glycosidic bond)
Carbohydrates and Glycobiology
1. Monosaccharides and Disaccharides
2. Polysaccharides
3. Glycoconjugates: Proteoglycans,
Glycoproteins, and Glycolipids
4. Carbohydrates as Informational Molecules:
The Sugar Code
5. Working with Carbohydrates
Monosaccharides and Disaccharides
1. The two families of monosaccharides are aldoses and
ketoses.
2. Monosaccharides have asymmetric centers.
3. The common monosaccharides have cyclic structures.
4. Organisms contain a variety of hexose derivatives.
5. Monosaccharides are reducing agents.
6. Disaccharides contain a glycosidic bond.
The two families of monosaccharides are
aldoses and ketoses
Colorless, crystalline solids, freely soluble in water.
Backbone: unbranched carbon chains linked by
single bonds
Open chain form: one carbonyl group and many
hydroxyl groups
- aldose and ketose
- simplest: two trioses
- with four to seven carbon atoms:
Representative monosaccharides- two trioses
Representative monosaccharides- two hexoses
The pentose components of nucleic acids
Monosaccharides have asymmetric centers
Contain one or more asymmetric carbon: optically
active isomeric forms
With n chiral centers: have 2n stereoisomers
D, L designation: chiral center most distant from the
carbonyl carbon
- hydroxyl group on the right: D isomer
- hexoses of living organisms: most are D isomers
Three ways to represent the two stereoisomers of
glyceraldehyde
Three ways to represent the two stereoisomers of
glyceraldehyde
Three ways to represent the two stereoisomers of
glyceraldehyde
Aldoses
Aldoses
Aldoses
Ketoses
Ketoses
Epimers
Common monosaccharides have cyclic structures
Sugars in aqueous solution occur predominantly as
cyclic structures.
Carbonyl + hydroxyl group: hemiacetals and hemiketals
Additional chiral center at C-1 or C-2: a and b isomers
Pyranose: six-membered ring compound
Furanose: five-membered ring compound
C-1 or C-2 isomers: anomers, mutarotation
Formation of hemiacetals and hemiketals
Formation of the two cyclic forms of D-glucose
Pyranoses and Furanoses
Haworth perspective formulas:
Stereochemistry of ring forms
of monosaccharides.
The ring is not planar, but tends
to assume either of two “chair”
conformations.
Conformational formulas of pyranoses
Monosaccharides are reducing agents
Monosaccharides can be oxidized by mild oxidizing
agents such as ferric (Fe3+) or cupric (Cu2+) ion.
Carbonyl carbon is oxidized to a carboxylate group.
Sugars capable of reducing ferric or cupric ion are called:
reducing sugars.
Fehling’s reaction: a qualitative test for the presence of
reducing sugars.
Sensitive methods for measuring blood and urine glucose.
Sugars as reducing agents
Disaccharides contain a glycosidic bond
Disaccharides consist of two monosaccharides joined by
an O-glycosidic bond, formed between a hydroxyl of
one sugar and the anomeric carbon of the other.
N-glycosyl bonds: anomeric carbon + N in glycoproteins
and nucleotides.
Sugars with anomeric carbon involved in glycosidic bond:
nonreducing sugar.
Reducing end
Formation of maltose
Naming rules for reducing disaccharides
1) Place nonreducing end to the left
2) Give configuration to the anomeric carbon (left)
3) Name the nonreducing residue, distinguish ring forms
4) Indicate the two carbons joined by glycosidic bond
(in parenthesis with an arrow connecting the numbers)
5) Name the second residue
6) Use 3-letter abbreviations to shorten the description
Maltose: Glc(a1→4)Glc ; Lactose: Gal(b1→4)Glc
Some common disaccharides- Lactose
Naming rules for nonreducing disaccharides
1) - 3) The same as above.
4) Indicate the two carbons joined by glycosidic bond
(use double-headed arrow connecting to numbers)
5) Name the second sugar residue as glycoside
6) Use 3-letter abbreviations to shorten the description
Sucrose: Glc(a1 2b)Fru
Trehalose: Glc(a1 1a)Glc
Some common disaccharides- Sucrose
Some common disaccharides- Trehalose
Carbohydrates and Glycobiology
1. Monosaccharides and Disaccharides
2. Polysaccharides
3. Glycoconjugates: Proteoglycans,
Glycoproteins, and Glycolipids
4. Carbohydrates as Informational Molecules:
The Sugar Code
5. Working with Carbohydrates
Polysaccharides (Glycans)
1. Some homopolysaccharides are stored forms of fuel.
2. Some homopolysaccharides serve structural roles.
3. Bacterial and algal cell walls contain structural
heteropolysaccharides.
4. Glycosaminoglycans are heteropolysaccharides of
the extracellular matrix.
Homo- and heteropolysaccharides
Some homopolysaccharides are stored forms
of fuel
The most important storage polysaccharides:
1) Starch in plant cells (tubers and seeds)
2) Glycogen in animal cells
Both occur intracellularly as large clusters or granules.
Both are heavily hydrated- why?
Electron micrographs of starch granules
Electron micrographs of glycogen granules
Some homopolysaccharides are stored forms
of fuel - Starch
Starch contains two types of glucose polymer:
1) amylose: unbranched ; thousands to millions
(a1→4) linkage
2) amylopectin: highly-branched ; up to 100 million
main chain: (a1→4) linkage
branch points: (a1→6) linkage
(every 24-30 residues)
The polysaccharides of starch- amylose
The polysaccharides of starch- amylopectin
A cluster of amylose and amylopectin
Some homopolysaccharides are stored forms
of fuel - Glycogen
Like amylopectin, glycogen is a polymer of (a1→4)linked glucose, with (a1→6)-linked branches.
Glycogen is more extensively branched (every 8-12
residues) and more compact than starch.
Abundant in the liver and skeletal muscle.
One reducing end with as many nonreducing ends as
it has branches.
Why not store glucose in its monomeric form?
Some homopolysaccharides are stored forms
of fuel - Dextrans
Dextrans are bacterial and yeast polysaccharides:
main chain: (a1→6)-linked poly-D-glucose
branches: all (a1→3); some (a1→2) or (a1→4)
Dental plaque: rich in dextrans
Synthetic dextrans: ex: Sephadex
- Chemically cross-linked insoluble materials of
various porosities
- Size-exclusion chromatography
Some homopolysaccharides serve structural
roles - Cellulose
A fibrous, tough, water-insoluble substance found in the
cell walls of plants.
Linear and unbranched poly D-glucose (10,000-15,000
units) like amylose, but with (b1→4) linkage.
Most animals: a-amylases only.
Termites: readily digest cellulose with symbiotic
Trichonympha that secretes cellulase.
Ruminants: only vertebrate able to use cellulose with
bacteria and protists in rumen that produce cellulase.
The structure of cellulose
The structure of cellulose
Cellulose breakdown by wood fungi
Some homopolysaccharides serve structural
roles - Chitin
A linear homopolysaccharide composed of
N-acetylglucosamine residues in (b1→4) linkage.
Only chemical difference from cellulose:
replacement of the –OH group at C-2 with an
acetylated amino group.
Forms extended fibers and is the principle component of
the hard exoskeleton or nearly a million species of
arthropods.
Chitin
Chitin
Bacterial and algal cell walls contain structural
heteropolysaccharides - Peptidoglycan
The rigid component of bacterial cell wall, heteropolymer
of alternating (b1→4)-linked N-acetyl-glucosamine
and N-acetylmuramic acid residues.
Cross-linked by short peptides, prevents swelling and
lysis due to osmolarity changes.
Lysozyme: hydrolyze the (b1→4)-linkage.
Penicillin and related antibiotics: prevent synthesis of the
cross-links.
Peptidoglycan
Peptidoglycan
Bacterial and algal cell walls contain structural
heteropolysaccharides - Agar
Certain marine red algae and seaweeds have cell walls
that contain agar.
Agar: Poly-[D-Gal(b1→4)3,6-anhydro-L-Gal2S]
Two major component: agarose and agaropectin.
The gel-forming property of agarose makes it useful in
the biochemistry laboratory:
- Agarose gel: for electrophoretic separation of N.A.
- Agar: for the growth of bacterial colonies.
- Capsules: for vitamin and drug package.
The structure of agarose
Glycosaminoglycans are heteropolysaccharides
of the extracellular matrix
Extracellular matrix is composed of:
- heteropolysaccharides: glycosaminoglycans
- fibrous proteins: collagen, elastin, fibronectin, laminin.
Glycosaminoglycans: a family of linear polymers of
repeating disaccharide units. One is always either Nacetylglucosamine or N-acetylgalactosamine; the other
is a uronic acid, usually D-glucuronic acid or L-iduronic
acid. Some are esterified with sulfate.
Repeating units of some common
glycosaminoglycans - Hyaluronate
Repeating units of some common
glycosaminoglycans - Chondroitin 4-sulfate
Repeating units of some common
glycosaminoglycans – Keratan sulfate
Repeating units of some common
glycosaminoglycans - Heparin
Glycosaminoglycans are heteropolysaccharides
of the extracellular matrix
1) Hyaluronic acid - glassy and translucent
- lubricants in joints, cartilage, and tendons
- hyaluronidase in pathogenic bacteria and sperm
2) Chondroitin sulfate
- cartilage, tendon, ligament, and walls of the aorta
3) Dermatan sulfate
- skin, blood vessels, and heart valves
4) Keratan sulfate
- cornea, bone, horn, hair, hoofs, nails and claws
5) Heparin - natural anticoagulant made in mast cells
- bind antithrombin, then bind and inhibit thrombin
Interaction between a glycosaminoglycan and its
binding protein
Fibroblast
growth factor
(FGF)
Heparin
Carbohydrates and Glycobiology
1. Monosaccharides and Disaccharides
2. Polysaccharides
3. Glycoconjugates: Proteoglycans,
Glycoproteins, and Glycolipids
4. Carbohydrates as Informational Molecules:
The Sugar Code
5. Working with Carbohydrates
Glycoconjugates: Proteoglycans, Glycoproteins,
and Glycolipids
1. Proteoglycans are glycosaminoglycan-containing
macromolecules of the cell surface and extracellular
matrix.
2. Glycoproteins have covalently attached
oligosaccharides.
3. Glycolipids and Lipopolysaccharides are membrane
components.
Biological roles of glycoconjugates
1) Destination labels
2) Mediators of specific cell-cell interactions
3) Cell-cell recognition and adhesion
4) Cell migration during development
5) Blood clotting
6) Immune response
7) Wound healing
Biologically active molecules of glycoconjugates
1) Proteoglycans - glycosaminoglycans + proteins
- glycosaminoglycans: main site of biological activity
- hydrogen bonding and electrostatic interactions
- major components of connective tissues
2) Glycoproteins - oligosaccharides + proteins
- outer face of membrane ; extracellular matrix, blood
- Golgi complexes ; secretory granules ; lysosomes
3) Glycolipids – oligosaccharides + membrane lipids
- oligosaccharides: hydrophilic head groups
- specific sites for recognition
Proteoglycans are glycosaminoglycan-containing
macromolecules of the cell surface and extracellular matrix
Proteoglycan superfamily: at least 30 types
- tissue organizers
- influence the development of specialized tissues
- mediate activities of growth factors
- regulate extracellular assembly of collagen fibrils
Basic proteoglycan unit:
core protein + trisaccharide linker + glycosaminoglycan
Secreted into the extracellular matrix (basal lamina)
Integral membrane protein (syndecans ; glypicans)
Proteoglycan structure, showing the trisaccharide
bridge
Proteoglycan structure of an integral membrane
protein - Syndecan
Proteoglycan structure of an integral membrane
protein
Four types of protein interactions with S domains of
heparan sulfate
1) Conformational activation
2) Enhanced protein-protein interaction
3) Coreceptor for extracellular ligands
4) Cell surface localization/concentration
Four types of protein interactions with S domains of
heparan sulfate
Four types of protein interactions with S domains of
heparan sulfate
Four types of protein interactions with S domains of
heparan sulfate
Four types of protein interactions with S domains of
heparan sulfate
Proteoglycan aggregate of the extracelluar matrix
Interactions between cells and the extracellular matrix
- Anchor cells to the
extracellular matrix
- Direct cell migration in
developing tissue
- Convey information in both
directions across the
plasma membrane
Glycoproteins have covalently attached oligosaccharides
Carbohydrate moiety in glycoproteins are smaller but
more structurally diverse than the glycosaminoglycans
of proteoglycans.
O-linked: -OH of Ser or Thr (GalNAc)
N-linked: amide nitrogen of Asn
(GlcNAc)
Membrane protein: glycophorin A
Secreted proteins: immunoglobulins and certain
hormones, lactalbumin, ribonuclease, etc.
Proteins in lysosomes, Golgi complexes, and ER.
Oligosaccharide linkages in glycoproteins
Glycolipids and lipopolysaccharides are membrane
components
Gangliosides: membrane lipids whose polar head group
is a complex oligosaccharide containing sialic acid
and other monosaccharides.
Blood group type determination: identical
oligosaccharide moieties as those found in certain
glycoprotein, also are blood group type determinants.
Lipopolysaccharides: dominant surface feature of the
outer membrane of G(-) bacteria.
- prime targets of the antibodies
- determinants of the serotypes of bacterial strains
Bacterial lipopolysaccharides
Bacterial lipopolysaccharides
Carbohydrates and Glycobiology
1. Monosaccharides and Disaccharides
2. Polysaccharides
3. Glycoconjugates: Proteoglycans,
Glycoproteins, and Glycolipids
4. Carbohydrates as Informational Molecules:
The Sugar Code
5. Working with Carbohydrates
Carbohydrates as Informational Molecules:
The Sugar Code
1. Lectins are proteins that read the sugar code and
mediate many biological processes.
2. Lectin-carbohydrate interactions are very strong
and highly specific.
Glycobiology and the sugar code
Glycobiology: the study of the structure and function of
glycoconjugates.
20 different monosaccharides:
- 1.44 x 1015 different hexameric oligosaccharides
20 common amino acids:
- 6.4 x 107 (206) different hexapeptides
4 nucleotides:
- 4,096 (46) different hexanucleotides
Oligosaccharides are enormously rich in structural
information- the sugar code.
Lectins are proteins that read the sugar code and
mediate many biological processes
Lectins: proteins that bind carbohydrate with high
affinity and specificity.
Cell-cell recognition, signaling, adhesion processes,
and intracellular targeting of newly synthesized
proteins.
Useful reagents for detecting and separating
glycoproteins with different oligosaccharide moieties.
Lectins are proteins that read the sugar code and
mediate many biological processes
1) Lectin receptor of hepatocytes- peptide hormones
2) Asialoglycoprotein receptors- ceruloplasmin
3) Mechanisms for removing old erythrocytes
4) HA of influenza virus- bind sialic acid- viral infection
5) HS-1 and HS-2 of HSV- bind heparan sulfate
6) Selectins- cell-cell recognition and adhesion
ex: Movement of T lymphocytes through the capillary wall, from
blood to tissues, at sites of infection or inflammation.
Role of lectin-ligand interactions in T lymphocyte
movement to the site of an infection or injury
Lectins of microbial pathogens
1) Mediate bacterial adhesion to host cells
- Helicobacter pylori vs. Leb oligosaccharide on the
membrane glycoproteins of gastric epithelial cells
- Treatment- chemically synthesized Leb
2) Mediate toxin entry into host cells
- Cholera toxin (Vibrio cholerae) vs. GM1 ganglioside
on the surface of intestinal epithelial cells
- Pertussis toxin (Bordetella pertussis)
- Treatment- toxin analogs as vaccines
Role of lectin-ligand interactions in lymphocyte
movement to the site of an infection or injury
Lectins acting intracellularly
Oligosaccharides containing mannose 6-phosphate
marks newly synthesized proteins in the Golgi
complex for transfer to the lysosome.
Recognized by the cation-dependent mannose 6phosphate receptor, a membrane-associated lectin
on the lumenal side of the Golgi complex.
Many degradative enzymes (hydrolases) of the
lysosome are targeted and delivered by this
mechanism.
Lectin-carbohydrate interactions are very strong
and highly specific
1) Sialoadhesin (siglec-1) of mouse macrophages vs.
certain sialic acid-containing oligosaccharides.
- b sandwich domain
- each ring substituent is involved in the interaction
2) Bovine mannose 6-phosphate receptor vs. mannose
6-phosphate
- specificity of the binding
- necessity for a divalent cation in the interaction
Sialic acid-specific lectin vs. sialic acid
Each ring substituent is involved in the
interaction between sugar and lectin
Details of lectin-carbohydrate interaction
Details of lectin-carbohydrate interaction
Lectin-carbohydrate interactions are very strong
and highly
Many carbohydrates have a more polar and a less
polar side.
General interactions- more polar side: hydrogen bonds with lectin
- less polar side: hydrophobic interactions with
nonpolar amino acid residues
Hydrophobic interactions of sugar residues
Roles of oligosaccharides in recognition and
adhesion at the cell surface
Methods of
carbohydrate
analysis