Download 15. The Importance of Energy Changes and Electron Transfer in

16. CARBOHYDRATES
Carbohydrates
Types:
Monosaccharides: Cn(H2O)n
Oligosaccharides: A few monosaccharides are linked.
Polysaccharides: glycogen, starch, cellulose
Roles in biochemistry
1) Major energy sources
2) Oligosaccharides: processes in cell surfaces
cell-cell interaction, immune recognition
3) Polysaccharides: essential structural components
16.1 Sugars: Their Structures and
Stereochemistry
◈ Monosaccharides
- Building blocks of all carbohydrates
- Simple sugars
- Polyhydroxy aldehyde (aldose) or polyhydroxy ketone (ketose)
- Aldose: aldotriose, aldotetrose, aldopentose, aldohexose, aldoheptose
- Ketose: ketotriose, ketotetrose, ketopentose, ketohexose, ketoheptose
16.1 Sugars: Their Structures and
Stereochemistry
◈ Optical isomers (stereoisomers)
- Enantiomers: mirror images
- Configuration: three dimensional arrangement of groups around a
chiral carbon atom (D, L system and R, S system)
16.1 Sugars: Their Structures and
Stereochemistry
◈ Fischer-projection
- Convention of a two dimensional perspective of molecular structure
- By German chemist Emil Fischer
- Bonds written vertically represent bonds directed behind the paper.
- Bonds written horizontally represent bonds directed in front of the
paper.
16.1 Sugars: Their Structures and
Stereochemistry
- Most highly oxidized carbon is written at the “top”.
16.1 Sugars: Their Structures and
Stereochemistry
- What are the possible stereoisomers for an aldotetrose ?
16.1 Sugars: Their Structures and
Stereochemistry
- Diastereomers:
nonsuperimposable, nonmirror-image stereoisomers
L-threose  D- & L-erythrose
L-erythrose  D- & L-threose
- Epimers: diastereomers differ from each other in configuration at
only one chiral carbon
D-erythrose  D-threose
16.1 Sugars: Their Structures and
Stereochemistry
◈ Some of possible isomers
- Aldopentose: three chiral carbons, then 23 stereoisomers (four D
forms and four L forms).
- Aldohexose: four chiral carbons, then 24 stereoisomers (eight D
forms and eight L forms).
16.1 Sugars: Their Structures and
Stereochemistry
Aldose
16.1 Sugars: Their Structures and
Stereochemistry
Ketose
16.1 Sugars: Their Structures and
Stereochemistry
Cyclic Structures: Anomers
◈ Anomers
- Five or six carbon atoms as cyclic molecules rather than as open-chain
forms
- Result of interaction between functional groups on distant carbons
- Hemiacetal: interaction between C-1 and C-5, to cyclic form in aldose
- Hemiketal: interaction between C-2 and C-5, to cyclic form in ketose
- Anomeric carbon: a new chiral center carbon (α and β forms)
16.1 Sugars: Their Structures and
Stereochemistry
Cyclic Structures : Anomers
16.1 Sugars: Their Structures and
Stereochemistry
Cyclic Structures : Anomers
- Fischer projection of α-anomer and β-anomer of a D sugar
16.1 Sugars: Their Structures and
Stereochemistry
Cyclic Structures : Anomers
◈ Haworth projection formula
- Cyclic structures in perspective drawings
- Furanose: five-membered ring
- Pyranose: six-membered ring
16.1 Sugars: Their Structures and
Stereochemistry
Cyclic Structures: Anomers
- Pyranoses exist in solution in chair conformation. (cf. boat form)
16.1 Sugars: Their Structures and
Stereochemistry
Cyclic Structures: Anomers
- Useful shorthand
for structures
16.2 Reactions of Monosaccharides
Oxidation-Reduction Reactions
◈ Oxidation-Reduction Reactions
- Oxidation of sugars: provides energy when oxidized to CO2 and
H2O in aerobic process
- Reverse of oxidation of sugars: reduction of CO2 and H2O in
photosynthesis
16.2 Reactions of Monosaccharides
Oxidation-Reduction Reactions
◈ Oxidation reactions of sugars in lab
- Aldose: reducing sugars
- In cyclic form, compound produced by oxidation of aldose is a lactone.
- Vitamin C: an unsaturated lactone
Lactone
16.2 Reactions of Monosaccharides
Oxidation-Reduction Reactions
◈ Detecting presence of reducing sugars
- Tollens reagent: using Ag(NH3)2+, Silver is reduced.
- Glucose oxidase
16.2 Reactions of Monosaccharides
Oxidation-Reduction Reactions
◈ Deoxy sugars
- L-fucose: in ABO blood-group antigens
- D-2-deoxyribose: found in DNA
16.2 Reactions of Monosaccharides
Oxidation-Reduction Reactions
◈ Alditols
- When carbonyl group of a sugar is reduced to a hydroxyl group
16.2 Reactions of Monosaccharides
Esterification Reactions
◈ Hydroxyl groups of sugars
- Behave like all other alcohols
- React with acids and derivatives of acids to form esters
- Phosphate ester: intermediates in breakdown of carbohydrates
16.2 Reactions of Monosaccharides
The Formation of Glycosides
◈ Glycosidic linkage (R’-O-R)
- Not an ether: Because glycosides can be hydrolyzed to alcohols.
- Glycosidic bond: Hemiacetal carbon can react with methyl alcohol
to give a full acetal, or glycoside.
- Furanosides: glycosides from furanoses
- Pyranosides: glycosides from pyranoses
16.2 Reactions of Monosaccharides
The Formation of Glycosides
◈ Glycosidic bonds between monosaccharide unit
- α- or β-glycosidic linkage: Anomeric carbon of one sugar bonded
to any of –OH groups on a second sugar.
16.2 Reactions of Monosaccharides
The Formation of Glycosides
Wrong configuration??
16.2 Reactions of Monosaccharides
The Formation of Glycosides
◈ Oligosaccharides and polysaccharides
- Chemical nature: which monosaccharides are linked, particular
glycosidic bond formed.
- Some internal residues can form three glycosidic bonds to
branched-chain structures.
16.2 Reactions of Monosaccharides
The Formation of Glycosides
16.2 Reactions of Monosaccharides
The Formation of Glycosides
◈ Test for a reducing sugar
- Only if the end residue is a free hemiacetal: can be positive
- Can be negative for not enough reducing ends to detect
Maltose
16.2 Reactions of Monosaccharides
Other Derivatives of Sugars
◈ Amino sugars
- Compounds related to monosaccharides
- Amino groups (-NH2) or one of its derivatives is substituted for
hydroxyl group of parent sugar.
Found in
Bacterial walls
16.3 Some Important Oligosaccharides
◈ Disaccharides
- Linking two monosaccharide unit by glycosidic bonds
- Most important examples: sucrose, lactose, and maltose
- Other examples: Isomaltose and cellobiose
16.3 Some Important Oligosaccharides
◈ Sucrose
- Common table sugar
- Extracted from sugarcane and sugar beets
- The α C-1 carbon of glucose is linked to β C-2 carbon of fructose.
- Not a reducing sugar
- Hydrolysed to glucose and fructose
16.3 Some Important Oligosaccharides
◈ Other sweeteners
- Fructose: sweeter than sucrose, high-fructose corn syrup
- Saccharin: cause cancer in laboratory animals
- Aspartame (NutraSweet): cause neurological problems
Saccharin
16.3 Some Important Oligosaccharides
◈ Artificial sweetener: sucralose (Splenda: trade name)
- Derivative of sucrose
- Differences from sucrose
① Three of hydroxyl groups have been replaced with three chlorine
atoms.
② Configuration at carbon atom number four of pyranose ring of
glucose has been inverted.
- Not metabolized by the body
(no calories)
- Some report for its toxicity
16.3 Some Important Oligosaccharides
◈ Lactose
- Made up of β-D-galactose and D-glucose
- Can be either the α or β form.
- a reducing sugar
16.3 Some Important Oligosaccharides
◈ Maltose
- From hydrolysis of starch
- Consist of two residues of D-glucose in an α(1 → 4) linkage
- Differ from cellobiose in the glycosidic linkage
- Mammals can digest maltose, but not cellobiose.
16.4 Structure and Functions of
Polysaccharides
◈ Polysaccharide
- Many monosaccharides are linked together.
- Homopolysaccharide: only one type of monosaccharide
- Heteropolysaccharide: more than one type of monosaccharide
- Cellulose and chitin: β-glycosidic linkage, structural materials
- Starch and glycogen: α-glycosidic linkage, storage polymers
Potato starch
16.4 Structure and Functions of
Polysaccharides
Cellulose and Starch
◈ Cellulose
- Major structural component of plants
- Linear homopolysaccharide of β-D-glucose, β(1→4) glycosidic bonds
16.4 Structure and Functions of
Polysaccharides
Cellulose and Starch
◈ Starch
- Energy-storage polymers
- Polymer of α-D-glucose, α(1→4) glycosidic bonds
- In plant cells, as starch granules in cytosol
16.4 Structure and Functions of
Polysaccharides
The Forms of Starch
◈ Types of starch
- Amylose: linear polymer of glucose, helix with 6 residues per turn
- Amylopectin: branched chain polymer, α(1→6) linkage
16.4 Structure and Functions of
Polysaccharides
The Forms of Starch
- Test for starch:
starch-iodine complex
(dark-blue color)
16.4 Structure and Functions of
Polysaccharides
The Forms of Starch
◈ Hydrolyzing starches
- α- and β-amylase: attack α(1 → 4) linkages
- β-amylase: exoglycosidase, cleave from nonreducing end, produce
maltose
- α-amylase: endoglycosidase, hydrolyze anywhere, produce
glucose and maltose
- Debranching enzymes: degrade α(1 → 6) linkages
16.4 Structure and Functions of
Polysaccharides
Glycogen
◈ Glycogen
- Carbohydrate storage polymer in animals
- Similar to amylopectin
- Glycogen (every 10 residues) is more highly branched than
amylopectin (every 25 residues).
16.4 Structure and Functions of
Polysaccharides
Glycogen
- Glycogen granules in well-fed liver and muscle cells
- Glycogen phosphorylase: cleaves one glucose from nonreducing
end of a branch to produce glucose-1-phosphate
- Significance of the number of branch points
1) More branched polysaccharide is more water soluble.
2) More potential targets for quicker mobilization of glucose.
Glycogen
in animal
16.4 Structure and Functions of
Polysaccharides
Chitin
◈ Chitin
- Similar to cellulose in both structure and function
- Monomer is N-acetyl-β-D-glucosamine.
- Exoskeletons of invertebrates
- Cell walls of algae, fungi, and yeasts
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16.4 Structure and Functions of
Polysaccharides
The Role of Polysaccharides in the Structure of Cell Walls
◈ <Bacteria Cell Walls>
- Heteropolysaccharides are major components.
- Two monomers: N-acetylmuramic acid, N-acetyl-D-glucosamine
- Polysaccharides are cross-linked by peptides.
16.4 Structure and Functions of
Polysaccharides
The Role of Polysaccharides in the Structure of Cell Walls
- Cross-links in bacterial cell walls:
Oligomer is bound to
N-acetylmuramic acid,
forming a side chain.
16.4 Structure and Functions of
Polysaccharides
The Role of Polysaccharides in the Structure of Cell Walls
- Tetrapeptide forms
two cross-links to
pentapeptide, (Gly)5.
16.4 Structure and Functions of
Polysaccharides
The Role of Polysaccharides in the Structure of Cell Walls
- Peptidoglycan:
Result from cross-linking of
polysaccharides by peptides
16.4 Structure and Functions of
Polysaccharides
The Role of Polysaccharides in the Structure of Cell Walls
◈ <Plant Cell Walls>
- Consist of largely cellulose
- Pectin: heteropolysaccharide made up mostly of D-galacturonic acid,
a gelling agent in yogurt, fruit preserves, jams, and jellies
16.4 Structure and Functions of
Polysaccharides
The Role of Polysaccharides in the Structure of Cell Walls
- Lignin: major nonpolysaccharide component in plant cell walls
a polymer of coniferyl alcohol
16.4 Structure and Functions of
Polysaccharides
Glycosaminoglycans
◈ Glycosaminoglycans
- Repeating disaccharide
- A variety of cellular functions and tissues
- Heparin: natural anticoagulant preventing blood clots
16.4 Structure and Functions of
Polysaccharides
Glycosaminoglycans
16.5 Glycoproteins
◈ Glycoproteins
- Carbohydrate residues in addition to polypeptide chain
- Important examples: antibodies, antigenic determinants in erythrocyte
- Human blood groups: A, B, AB, and O types
16.5 Glycoproteins
◈ Proteoglycans
- Glycoproteins with extremely high carbohydrate content (85%-95%
by weight)
- Constantly being synthesized and broken down in eukaryotes
THE END!!