Download UNIT 4. CARBOHYDRATES

UNIT 4.
CARBOHYDRATES
OUTLINE
4.1. Introduction.
4.2. Classification.
4.3. Monosaccharides.
Classification. Stereoisomers. Cyclic structures. Reducing sugars.
Sugar derivatives
4.4. Oligosaccharides. Disaccharides.
4.5. Polysaccharides
Homopolysaccharides: starch, glycogen, cellulose, chitin.
Heteropolysaccharides: peptidoglican, glycosaminoglycans,
glycoconjugates (Proteoglycans, Glycoproteins, Glycolipids).
4.6. Lectins.
4.1 INTRODUCTION:
• Carbohydrates are the single most abundant class of organic
molecules found in nature.
• Polyhydroxyaldehydes or polyhydroxyketones.
• Basic molecular formula: (CH2O)n. Some of them contain phosphate,
sulphate or amino groups.
• Biological roles:
- Nutrients and energy storage.
- Structural and protection roles.
- Bones lubricants.
- Cellular adhesion.
- Glycoconjugates: intracellular signalling, cell recognition.
4.2. CLASSIFICATON:
WHAT DO YOU HAVE TO KNOW?
-Three groups classification: monosaccharides, oligosaccharides and
polysaccharides.
- Structure and chemistry of monosacharides:
- Aldoses or ketoses (aldehyde function or ketone group)
- Trioses, tetroses, pentoses, hexoses.
4.3. MONOSACCHARIDES:
• Single bounds between C.
• They contain a carbonyl group.
• Most
abundant
and
important
aldohexose: GLUCOSE.
• Most
FRUCTOSE.
abundant
Glyceraldehyde: aldose Dihydroxyacetone: ketose
(carbonyl at the end of (carbonyl in the middle of
the chain)
the chain)
ketohexose:
1
2
1
2
3
3
4
4
5
5
6
6
D-Glucose
D-Fructose
4.3. MONOSACCHARIDES:
Aldoses
4.3. MONOSACCHARIDES:
Ketoses
4.3. MONOSACCHARIDES:
WHAT DO YOU HAVE TO KNOW?
-What is a chiral centre?
- Why is Stereochemistry a prominent feature of monosaccharides?
- How many stereoisomers do the aldoses have? And the ketoses?
- What is an enantiomer? Give examples
- Which kind of enantiomers are the most frequent within the carbohydrates
group?
- What is a diastereomer? Give examples
- What is an epimer? Give examples
- What is an anomer? Give examples
- How are the sugars represented by means of Fischer and Haworth projections?
4.3. MONOSACCHARIDES:
WHAT DO YOU HAVE TO KNOW?
- How does the linear form of a monosaccharide undergo an intramolecular
reaction to form a cyclic hemiacetal?
- Which are the differences between hemiacetal and hemiketal cycles?
- How many conformations of a pyranose sugar are there (chair, boat)?
- Which is the meaning of the 'axial bond' and equatorial bond'
(pyranose/furanose conformations)?
- Which kind of process is called mutarotation?
4.3. MONOSACCHARIDES: REDUCING SUGARS
• Sugars with free anomeric carbon atoms that are reasonably good
reducing agents and will reduce hydrogen peroxide, ferricyanide or
certain metals such as Cu2+.
4.3. MONOSACCHARIDES: SUGAR DERIVATIVES
A variety of chemical and enzymatic reactions produce derivatives of the
simple sugars:
1) Reductions: alditols and deoxy sugars:
Alditols: carbonyl
group is reduced to
alcohol (-itol).
4.3. MONOSACCHARIDES: SUGAR DERIVATIVES
1) Reductions: alditols and deoxy sugars:
Deoxy sugars: one or more
hydroxyl groups are replaced
by hydrogens.
(they are part of glycoproteins and glycolipids)
4.3. MONOSACCHARIDES: SUGAR DERIVATIVES
2) Amino sugars: amino group at the C-2 position.
• The amino groups can accept
an acetyl group. The final
products are sugars derivatives
with important biological roles:
(They are part of the bacterial wall)
4.3. MONOSACCHARIDES: SUGAR DERIVATIVES
3) Oxidations: Oxidation at the aldoses C-end.
Aldonic
acids
Uronic
acids
Aldaric
acids
4.3. MONOSACCHARIDES: SUGAR DERIVATIVES
4) Sugar esters: Phosphate and sulphate ester of monosaccharides.
4.4. OLIGOSACCHARIDES.DISACCHARIDES.
O-Glycosidic bond: two monosaccharide units are linked by a covalent bond.
Nonreducing
end
Reducing
end
Maltose (Component of malt).
Reducing disaccharide
4.4. OLIGOSACCHARIDES.DISACCHARIDES.
Component of milk.
Reducing disaccharide.
Sacarosa
α-D-glucopiranosil-(12)-β-D-fructofuranosa
Glc(α12β)Fru
Sucrose
(table
sugar).
Component of many higher
plants.
No free anomeric C.
Nonreducing disaccharide.
4.5. POLYSACCHARIDES:
• Classification: - Homopolysaccharides.
- Heteropolysaccharides.
Linear chain
Branched
chain
glucans, fructans, mannosans, galactans…
Linear chain:
two types of
monomers
Branched chain:
several types of
monomers
4.5. POLYSACCHARIDES: HOMOPOLYSACCHARIDES. STARCH:
• Storage polysaccharide in plants.
• Contains two polymers of glucose: Amylose, linear chain; and
Amylopectin, branched chain.
Amylose Fragment
Branched point in
amylopeptin
4.5. POLYSACCHARIDES: HOMOPOLYSACCHARIDES. STARCH:
Amylose chains (blue), and
amylopeptin chains (pink)
Helical conformation of amylose
• The enzyme α-amilase catalyses the
α(1-4) bonds digestion.
4.5. POLYSACCHARIDES: HOMOPOLYSACCHARIDES. GLYCOGEN:
• Storage polysaccharide in animals.
• It is stored in the liver and skeletal muscle.
• Its structure is similar to amylopeptin structure, but with higher
branched points (high compact structure).
Glycogen granules
4.5. POLYSACCHARIDES: HOMOPOLYSACCHARIDES. CELLULOSE:
• Structural polysaccharide in plants.
• Linear chains constituted by D-glucose units β(1-4) linked.
• No hydrolysable by human beings enzymes. Hydrolysable by cellulases.
• Insoluble in water, fibrous and resistant.
• Cellulose chains interact by means of hydrogen bonds forming cellulose
microfibres. The hydrogen bonds increase the strength of the structure.
Cellulose microfibres
4.5. POLYSACCHARIDES: HOMOPOLYSACCHARIDES. GLYCOGEN:
Structural differences due to the nature of the bond: α(14) or β(14).
Amylose helical conformation (a), or sheets composed by several cellulose
chains (b).
4.5. POLYSACCHARIDES: HOMOPOLYSACCHARIDES. CHITIN:
• Structural polysaccharide present in the cell walls of fungi and in the
exoskeletons of crustaceans, insects and spiders.
• Linear chain constituted by N-acetyl-D-glucosamines in β(1-4)
linkage.
4.5. POLYSACCHARIDES: HETEROPOLYSACCHARIDES. PEPTIDOGLYCAN:
• Component of the bacterial cell wall.
• β(1-4)-linked polymer of Nacetylglucosamine and Nacetylmuramic acid units.
• The polysaccharide is
joined to a tetrapeptide
(covalent bond).
• Lisozyme hydrolyze
the glycosidic bonds.
Peptidoglycan of the bacterial cell wall
Gram-positive Staphylococcus aureus.
4.5. POLYSACCHARIDES: HETEROPOLYSACCHARIDES.
GLYCOSAMINOGLYCANS (GAG):
• They are present in animal cells.
• The GAG are linear polymers constituted by disaccharides. One of the
monosaccharides
is
always
N-acetylglucosamine
or
acetylgalactosamine. The other one is usually uronic acid.
• They are negatively charged (sulphate or carboxilate groups).
• They promote molecule association (charge-charge interactions).
• GAGs recognize specific ligands (cell interactions).
N-
4.5.
POLYSACCHARIDES:
GLYCOSAMINOGLYCANS (GAG):
HETEROPOLYSACCHARIDES.
 Hyaluronic acid (it is not protein linked). It is highly hydrated by virtue of
strong interactions between water molecules and the polyanionic
complex. It is present in cartilage and tendon, vitreous humour (eyes),
extracellular matrix, mucosal surface and synovial fluid.
 Chondroitin sulfate. It promotes tension resistance in cartilage, tendon
and arteries. It is present in brain, kidneys and lung.
 Dermatan sulfate. It is present in skin, veins and cardiac valves. Its
concentrations increases when cells grow old.
 Keratan sulfate. Minor component of the proteoglycans. It is present in
bones, cartilage, horns and inter vertebral discs.
 Heparin. Natural anticoagulant substance. It is present in mastocytes
(type of leukocyte). It contains D-glucuronic acid.
4.5.
POLYSACCHARIDES:
GLYCOSAMINOGLYCANS (GAG):
HETEROPOLYSACCHARIDES.
4.5. POLYSACCHARIDES: HETEROPOLYSACCHARIDES. GLYCOCONJUGATES:
• Complexes of proteins or lipids with oligosaccharides and
polysaccharides (covalent linked). They are present in the cellular
surface and in the extracellular matrix. They are positive side of the
cytoplasmic membrane orientated.
 Proteoglycans: proteins linked to polysaccharides GAG.
 Glycoproteins: proteins covalently linked to oligosaccharides.
 Glycolipids: membrane lipids (sphingolipids) covalently linked to
oligosaccharides.
4.5. POLYSACCHARIDES: GLYCOCONJUGATES: PROTEOGLYCANS:
• A 'Nucleus protein' is covalently linked to one o more than one GAG.
a) GAG link to a nucleus protein by means of a trisaccahride. b) Proteoglycan containing
a transmembrane protein. C) Agrecan structure (it is present in the extracellular matrix
of cartilage).
4.5. POLYSACCHARIDES: GLYCOCONJUGATES: GLYCOPROTEINS:
• Proteins and carbohydrates are linked by means of N- or O-glycosidic
bond.
Oligosaccharides
O-linked
Oligosaccharides
N-linked
4.5. POLYSACCHARIDES: GLYCOCONJUGATES: GLYCOPROTEINS:
• Glycoproteins structure is more complex that proteoglycans
structure.
• The structure of the oligosaccharide is very important in the
identification of the protein to interact with.
• Localization:
 Positive side of the cytoplasmic membrane (i.e. glycophorin A in
erythrocytes).
 Proteins secreted by eukaryotic cells (i.e. immunoglobulins,
hormones, coagulation factors).
 Proteins from lysosomes.
4.5. POLYSACCHARIDES: GLYCOCONJUGATES: GLYCOPROTEINS:
• Biological advantages:
 Polarity and solubility of a protein can be modified.
Identify the final location of a just synthesized protein.
 Protection against enzymes digestion.
 Specific biological roles.
4.6. LECTINS:
• Class of proteins that bind carbohydrates with high specificity and affinity
(non covalent bonds)
• They participate in several processes such as cell signalling and adhesion.
They also control the intracellular location of the proteins that are just
synthesised.
• In animals, the contact between cells is mediated by lectin-sugar
interactions. Examples:
- Physiological roles such as leukocyte
movement (a).
- Microorganisms infection (b) and (c).
- Toxins mechanisms (d).
Specific interaction between lectins
and oligosaccharides.